Author: Pam Knox

In my blog post last month, I mentioned the likelihood of having a very active Atlantic tropical season, especially because the ocean surface temperatures are so warm. But despite an early start to the season with the first three named storms (including Beryl, the earliest ever category 5 storm in the Atlantic Ocean), it’s been quiet for the last few weeks. The ocean temperatures continue to be very warm. What is preventing the development of tropical storms in such a warm environment? One of the main culprits now is Saharan dust that blows west off the African continent and affects the vertical structure of the atmosphere. This keeps tropical waves from developing the necessary circulation to strengthen into a powerful storm. In this post, we will discuss the impacts of the Saharan dust and how it is both good and bad for the environment.

What is Saharan dust and where does it go?

The Sahara Desert covers most of the northern portion of the African continent. It’s the world’s largest source of wind-blown dust supplied to the ocean and adjacent land. It is one of the driest places on earth and is covered with sand and rocks but very little plant materials. This means the dust from the Sahara is mineral dust with low organic content. Seven elements (Ca, Mg, Al, Ti, Fe, K, and Na) account for 98% of the total analyzed inorganic burden. The dust particles are often very fine, so they can travel a long distance from their source region on the continent.

Winds in that part of the world blow from east to west near the surface. You might know of them as the “trade winds”, which are often described in elementary geography classes as the winds that helped European ships travel west to North America. The trade winds form a band of westward-blowing winds from about 30 degrees south to 30 degrees north latitude around the globe. The strength of the trade winds changes over time, but when they are strong and a lot of dust is available over the Sahara, the particles can blow all the way across the Atlantic, covering large parts of the Atlantic and bringing low air quality and beautiful sunrises and sunsets to people in its path. This month has been particularly dusty, with satellite records showing this is the 2^nd dustiest July since continuous records began in 2002. Generally the dust plumes occur at a height of 5,000 to 20,000 feet where the trade winds are the strongest.

How does Saharan dust affect tropical storm development?

The air that carries the Saharan dust is usually very dry, which disrupts the usual moist conditions above the ocean surface and keeps thunderstorms from growing vertically. The vertical air movement would normally help initiate the decrease in surface pressure that helps storms grow. The dust particles also serve as nuclei to absorb even more moisture from the air, keeping the layer dry. The dust is opaque (which makes it visible from satellites) and shades the surface of the ocean, cooling it off and reducing its ability to energize storms.

The Saharan dust layer is most likely to occur in the period between mid-June and mid-August, but there are variations over time and location because of the strength and direction of the wind. Sometimes the winds even blow from south to north, bringing dust to Europe, although this is less frequent. Tropical storms can sometimes form in pockets of relatively dust-free air, as Hurricane Beryl did this year, but the thickest layers are very effective at shutting down storm growth.

How does the dust affect air quality and human health?

Saharan dust incursions into the Southeastern United States can often been seen in air quality measurements taken in cities around the region. Like any other dust particles or other aerosols like smoke from forest fires, the particles can trigger asthma, burning eyes, and other symptoms associated with bad air quality. The dust can be seen in lower visibility around the cities, deposits on horizontal surfaces like cars and plants either directly from the dust or from “dirty rain” which contains the dust and brings it down to the ground. It can also result in spectacular sunrises and sunsets due to the scattering of the sun’s rays by the particles (similar to those from volcanic eruptions). If you are sensitive to poor air quality and plan to work outside in your garden, you will want to monitor air quality carefully and avoid the times when the pollution is worst.

How does the dust affect plants?

Saharan dust has important positive impacts on both phytoplankton in the ocean and on the Amazon rainforest. Those areas are often missing nutrients that would allow plant growth and so additions of iron and phosphorus into those areas can improve soil or water fertility and plant growth. Unfortunately the dust can also contain bacteria or other organic material that can lead to undesirable growth of algae in the ocean. The dust is not acidic, so acid rain is not something we worry about with rain containing the Saharan dust, unlike rain from volcanoes or from coal-burning power plants. The dust can also reduce absorption of sunlight by plants if there is a large amount.

How can gardeners prepare for episodes of Saharan dust?

First, we need to recognize that while we have not studied Saharan dust impacts for long, it has been around for many years and is a natural part of the earth-atmosphere system. It has beneficial impacts on soil nutrients in tropical rainforests and gardens in the affected areas and helps reduce activity in the tropics early in the season. But with dust events decreasing in the next few weeks, we can expect the Atlantic tropics to start heating up again as the most active part of the season gets underway. Gardeners should monitor their plants for dusty conditions and should also keep track of air quality impacts if they have asthma or other breathing disorders that could be affected by the dusty conditions. Gardeners in other parts of the world should also be aware of sources of dust and other pollutants that could affect their gardens and their own health. The Garden Professors blog has discussed the impacts of dust on gardens in several of our previous posts so please search for them if you want more information.

Last month I discussed the forecast for the Atlantic tropical season and pointed out that it is likely to be an active one. As I write this, there has already been one named storm (Alberto, which went into Mexico but dropped a lot of rain in southern Texas) and two more areas of potential development are moving their way through the Atlantic (note TS Beryl formed on Friday, June 28 at 11 pm after this was written). Hurricane season has begun! This month I will discuss what a tropical storm is and how they form into hurricanes. I will end by discussing how tropical storms and hurricanes impact gardens and what you can do to prepare for them.

Where do tropical storms and hurricanes form?

While we think of hurricanes as hitting the southeastern part of the United States, they are actually much more widespread than that. The map below shows that tropical storms can form in both hemispheres and affect every continent except for Antarctica. Here in the United States we see them most often over the Atlantic Ocean but can experience storms on the west coast from time to time as well. The storms are not always called hurricanes, they can be called typhoons in the Western Pacific and cyclones in the Indian Ocean and Australia. To be considered a hurricane or one of these other storms they have to record a sustained wind speed of 74 mph or higher. Storms in the United States that are stronger than that are classified by the Saffir-Simpson scale into categories 1 through 5 depending on how strong the winds are. And of course the wind gusts in the storms can be quite a bit higher than the sustained winds, they are just more localized and last for only short periods.

What ingredients are needed for a tropical storm or hurricane to form?

The prerequisite conditions for hurricanes are: warm, deep ocean waters (greater than 80°F / 27°C), an atmosphere cooling rapidly with altitude, moist middle layers of the atmosphere, low wind shear, and a pre-existing near surface region of low pressure in the surface environment. But you might have noticed from the map that even if these conditions are in place a tropical cyclone is not likely to form if it is not at least 300 or so miles from the equator. This is because of the Coriolis force which acts on moving air on a rotating planet to push air to the right of the original direction of movement in the Northern Hemisphere and to the left in the Southern Hemisphere. Low pressure draws air into the circulation, but the Coriolis force helps it to spin up into a storm with a defined circulation.

The seeds of low pressure where storms form can come from atmospheric waves moving east to west off of Africa, sometimes from stalled fronts over the Gulf of Mexico or along the East Coast of the United States. These usually provide the initial trigger of storm development. But not all waves or fronts can develop into cyclones if the other conditions are not right. The location of typical development depends on the time of year, with early and late storms developing closer to the United States and most storms in the peak period from mid-August to mid-October forming from waves coming off the west coast of Africa.

You might wonder why there are almost no tropical storms in the southeastern Pacific or in the southern Atlantic Ocean. That is because the water is normally too cold to sustain storm development. Since ocean temperatures are warming over time we could see more storms there in the future, especially in the South Atlantic where temperatures are already warmer than the SE Pacific. The tropical season could also become longer as the ocean warms up to 80 F earlier in the year in the future.

Tropical storms and hurricanes move under the influence of winds midway up in the atmosphere which push along the core of the storm as it is growing or weakening. The stronger the core of the storm, the closer the link between the large-scale atmospheric pattern and the storm movement. In the map above you can see that most storms move in a curving pattern that begins in the east near the equator and moves west over time before recurving to the northeast in a clockwise manner. This pattern is caused by subtropical high pressure, called the “Bermuda High”, over the Atlantic but by other names in other parts of the world. The path of each storm is unique due to the weather pattern present at the time of the storm, and sometimes they can take some crazy paths if the weather pattern is unusual.

How do tropical storms become hurricanes?

Usually, a wave of low pressure over the ocean pulls in air towards the center to reduce the pressure gradient. As the air moves in, the Coriolis force causes it to start spinning. In the Northern Hemisphere this spin is counterclockwise. The air above the surface circulation starts to flow out of the storm and drops the pressure at the surface causing the storm to intensify as air rises near the center of the storm. This continues as long as there is a source of energy (warm water) below it and there is no jet stream high up in the atmosphere to disrupt the development of the circulation. When the sustained wind speed reaches 74 mph its designation is changed from Tropical Storm to Hurricane and it stays that way until the wind speed drops as the storm weakens over land or colder water.

What impacts do tropical cyclones have on gardens and what can you do to prepare?

Tropical systems have a variety of impacts depending on where they are and how strong they are. Thoughtful gardeners will consider all the risks that severe weather can have on their gardens and get ready long before the storms hit. The strong and gusty winds are the most apparent impact; they can cause damage to trees, buildings, and plants and can cause significant damage to gardens. It’s a good idea to walk through your property periodically to look for dead or diseased limbs that could become airborne missiles in strong winds (whether or not they are from a hurricane). Decorative items and furniture left outside can damage tree trunks as well as houses and gardens when they become wind-borne. So if a storm is imminent, scout your property to remove anything that could be potentially hazardous.

Another important impact is flooding rain. The amount of rain that falls from a hurricane depends in part on how fast it is moving, since a slow-moving storm can drop more rain on a particular spot than one that is moving through quickly. The storm does not have to be strong to produce a lot of rain either—some of the weaker storms have been great rain-makers. And it does not even need to be an organized storm. Wet tropical systems that are not fully organized into storms have the potential to produce flooding rain, as we saw in southern Florida just a couple of weeks ago with over 20 inches of rain in some locations. The remains of hurricanes can also cause floods far inland, especially if there are mountains to help the air rise. Hurricane Agnes in 1972 had damage from the Caribbean all the way to Canada because of the torrential rains that fell along the Appalachian Mountains as it moved north. Gardeners who live in areas where flooding is likely should plan ahead to divert rain into rain gardens away from their planting beds to reduce erosion and keep soil from becoming saturated.

Hurricanes can also cause other impacts too, especially if you are near the coast. Storm surge can drive sea levels up to 25 feet above mean sea level as the water builds a dome under the area of lowest pressure that moves along with the storm until it makes landfall. If you are in a coastal area, you need to consider what the elevations of your land and house are so you know how much the water might rise in a strong storm. Another impact is the strong storms that can occur in the spiral bands outside the main circulation. These storms can hold weak tornadoes as well as heavy rain and gusty winds. In Hurricane Ivan in 2004, we had a tornado in Athens GA at the same time that the main storm was making landfall along the coast several hundred miles away.

As gardeners, it is important to keep in mind that tropical storms and hurricanes are not all bad. The rain that comes from these storms may include 30-40 percent of the summer rain that is expected to fall in a tropical area, and if few storms come, drought is more likely.  But the damage is also like to stress your gardens (not to mention the gardeners!), so learning more about these storms and planning ahead to prepare for the damage they might bring is a good thing for every home owner in an area prone to tropical activity to do now, before the storms come.

We are entering the hottest time of the year for most of our readers except for those who live in the Southern Hemisphere or in tropical locations where there is not a big seasonal cycle. Heat can have a big impact on both gardens and gardeners, so this is a great time to look at a new product that is now available from the National Weather Service to alert people who spend time outside to the dangers of high temperatures. This new HeatRisk product will help you use the 7-day forecast to identify times when the heat will be the most severe—which will allow you to plan your outdoor work accordingly to avoid the worst dates and times of dangerous heat conditions. I will also provide some resources for how heat affects plants from The Garden Professors and briefly talk about one potential consequence of high temperatures on the upcoming Atlantic tropical season, which starts June 1 in the United States.

How does high heat affect gardeners?

Usually when we talk about heat, we are talking about high temperatures. But as they say, “it’s not (just) the heat, it’s the humidity.” High temperatures alone can cause problems for humans and animals because our bodies are built to work best in a narrow range of temperatures. If the temperature goes above that range (or below it), our physical systems experience distress and eventually will shut down. High humidity makes it worse because it makes our natural ability to cool off by sweating less effective because the water on the skin from sweat does not evaporate readily when the water content of the air is high. Many indices for the heat index factor in both temperature and humidity, and the wet bulb globe temperature (WBGT–more on this in a minute) includes temperature, humidity, wind speed, and solar radiation because all of these factors can affect the body’s ability to cool off.

If you are outdoors for a long time and start to experience dizziness or nausea or even worse become unconscious, then you are likely experiencing a heat-related illness and you need to get to a cooler area where you can recover right away. In the worst cases, a trip to the hospital may be needed when the body temperature is elevated above the safe range for human life. You can learn more about protecting yourself from high heat at Heat.gov.

What is the NWS HeatRisk map?

Fortunately, there are number of online tools available that can help identify days and times when the danger from high heat is most likely. The National Weather Service has just released a new experimental product called HeatRisk, which provides an interactive map that shows where the heat will be the most dangerous over the next few days. An example of the map is shown below. You can either zoom in on the map or click on your location to get a specific temperature forecast for that spot.

Another tool that may be useful is available across the United States from the Southeast Regional Climate Center at https://convergence.unc.edu/tools/wbgt/. Their tool produces hourly forecasts of WBGT based on National Weather Service forecasts for several days ahead at whatever US location you choose (not just the Southeast) that can determine when conditions are most dangerous for working or playing outside. The WBGT is often used by sports teams to determine if it is safe for players to practice outdoors and how often they need to take a break. The tool gives you the choice of several state regulations for threshold values for WBGT that should determine whether football or other practice is safe. The same information can be used to decide if outdoor workers need extra water breaks in the shade or when gardeners should come in, cool down, and rehydrate.

How does high heat affect plants and gardens?

Plants respond differently to heat than humans and other animals do because they don’t sweat. High heat can cause the plants to close the stomas in their leaves to retain moisture but a long enough period of high temperatures and dry conditions with little soil moisture leads to wilting and eventually, death of the plants. I am not an expert on this, but fortunately Jim Downer and other GP authors have written several blog posts in the past that address the effects of heat stress on plants. John Porter has also written on the effects of high temperatures on pollination and ripening. Check out some of these posts, especially Jim’s post on abiotic disorders, to see what effects high temperatures may have on your garden.

What do we expect from this year’s Atlantic tropical season and how is it related to high temperatures?

This year we have seen a lot of record high temperatures across the globe due in part to greenhouse warming. Sea surface temperatures in the northern Atlantic Ocean have been especially high, most likely due to a combination of greenhouse warming and the absence of aerosol particles in the atmosphere due to the switch to cleaner fuel for ocean vessels a couple of years ago. This change allowed more sunlight to heat up the ocean surface. These ocean temperatures are so much higher than normal that the temperatures are closer to August values than what we usually expect in late May. Since hurricanes feed and grow over water than is warmer than 80 F, it means that the atmosphere over the main development region for Atlantic tropical storms is stoked and could contribute to both a larger number of storms than usual and more rapid development for any storms that do develop. It’s no wonder that the forecasts for the number of named storms in the Atlantic this year is one of the highest ever predicted. So if you are anywhere within reach of an Atlantic storm (which is most of the eastern US but also includes most of the Caribbean, Mexico, and potentially even Central or northern South America and parts of Europe), you should be prepared for tropical activity well in advance of any storms that might come your way this year.

Enjoy the hot weather but treat it with respect

If you like hot weather as much as I do, you are looking forward to the warmer weather we will see over the next few months. But if the weather gets too hot, as it is now in India, Mexico, and other places, heat-related illnesses and even deaths will become more likely. In 2023, the United States set a new record for the number of heat-related deaths. Climate change will make devastating heat waves more likely in the future, so make sure you are prepared. If you understand how heat affects your bodies, pets, and gardens and know how to take care of yourself, you will be better equipped to enjoy the dog days of summer this year and in the future as the earth’s temperature continues to rise. Be safe and enjoy the summer heat!

As I write this, about half of the lower 48 United States has passed the median date of the last freeze according to the National Centers for Environmental Information. Here in the Southeast, we are well into the planting season even though our usually early planting for crops like corn was delayed due to very wet soil. The rest of you may have to wait for a few more weeks before you can put any heat-loving plants into the ground. As we enter the growing season for the majority of the country I thought it might be helpful to take an updated look at what we expect this summer and fall to give you an idea of what conditions you might experience.

What factors will control the climate this summer?

In the Southeast most gardeners say that you should not plant summer crops and flowers until after Easter, although since Easter has a variable date that can sometimes be a problem when it is unusually early. In western Michigan where I grew up my grandmother always told me to wait until after Memorial Day. What rule of thumb do you use? If you look at the map below, you can see why! Wherever you garden you need to know the specific weather and climate to expect in your location. That includes things like the plant hardiness zone, how much rain to expect, and the specific microclimates within your garden (especially if it is a large one). That will help you pick the plants and trees that will do best in your location.

Every year is unique in terms of what temperature and precipitation patterns occur, but in many parts of the country we can get some indications of what might occur due to large-scale climate patterns that are occurring across the world. Of course these also affect the weather in other places from Europe to Australia and points in between, but I am going to focus on the US in this post.

The biggest patterns that are going to be affecting the climate this summer include 1) rising temperatures due to greenhouse warming, 2) the predicted transition from a strong El Niño to a La Niña later this summer, and 3) unusual warmth in the Atlantic Ocean which will affect the development of tropical storms and hurricanes in this year’s Atlantic Tropical Season.

Impacts of greenhouse warming trend

As temperatures rises around the globe we can expect both daytime high temperatures and overnight low temperatures to increase in temperature. In most areas the minimum overnight temperatures are rising faster than the daytime highs. This is due to a combination of increased humidity caused by increases in evaporation and more water-holding capacity of the air and heat-trapping in urban areas due to pavement and buildings. You can determine trends in temperature and precipitation for your location using the “Climate at a Glance” tool for anywhere in the continental US, including maximum and minimum temperature. The increased humidity will increase the likelihood of fungal diseases in plants that are susceptible so you will want to watch carefully and be prepared to treat them. You should also watch for protracted hot, dry spells and increased water usage, which might require you to water more often. But keep in mind that while the average temperature might be warmer, there will still be ups and downs with the daily weather.

Impacts of El Niño swinging to La Niña

We are currently in a waning El Niño (EN) after experiencing a strong EN over the winter. The winter weather pattern showed a very clear EN pattern over most of the country (and other parts of the world for that matter) with unusual warmth and dry conditions in the northern US and wet, somewhat cooler, cloudier conditions in the southern part of the country although that was tempered by the long-term temperature trend upward. The current EN is expected to disappear rapidly over the next few months and swing to the opposite phase, La Niña (LN), by mid to late summer as shown below. I also discussed this back in February. This LN will likely control our weather for a good part of the rest of 2024 and into the spring of 2025.

How will this affect the growing season weather in the US? While the correlation between LN or EN and summer weather is less strong than the winter correlation, we do expect to see some lingering effects of EN for the next few months before LN kicks in. That means wetter conditions are likely to continue in the southeastern US for the next few months before dry and sunny conditions move in later this summer or fall. The timing of when that transition occurs depends on how quickly the transition from EN to LN occurs. It is changing right now, but it’s still too early to tell how soon it will affect our summer weather. In the northern US we will probably see more seasonal weather for the next few months but next winter is likely to be much colder and wetter than last year. Again, the transition should occur later this summer but could wait until late fall to really become apparent. Some areas like the Central Plains are not very predictable by the phase of EN or LN so we are less certain about what you will experience if you live there. You can see the lack of certainty in the May-July temperature map below.

The warm Atlantic and what we expect from the Atlantic tropics this year

Last year, we had 19 named storms plus three other unnamed storms that were close to tropical status. This is in spite of the El Niño, which usually suppresses development of tropical systems because of strong winds aloft that keep tropical storms from developing the vertical structure they need to grow. Most of those storms stayed over the Atlantic Ocean where the water temperatures have been at record-setting levels for over a year. They are still at record-setting levels now and are even hotter than last year at this time. This year with a La Niña there is not much to keep storms from developing so I expect to see more storms, especially in the western Atlantic and in the Gulf, where they are more likely to come onshore and do damage or drop a lot of rain along their paths. I have seen predictions of as many as 33 named storms this year, although that would be a record and climatologists don’t generally like to forecast record values. A more conservative value of mid-20s for named storms seems more likely, although this is still a lot more than we usually get. What you actually experience depends critically on the path that the storms take, which cannot be predicted until after the storms form. So you could get hit directly by strong winds and heavy rain or you could be in the area outside the path with clear skies, sinking air, and no rain at all.

No matter how many named storms we get, those of you who live in the eastern US where a hurricane or tropical storm (or their remnants, which can also carry flooding rain to places far away from the tropics) can travel should be watching carefully when the storms start popping later this spring or early summer. Conversely if you live in the western US, you may see less activity this year than last year since when LN is strong, the Eastern Pacific Ocean (EPO) storms have a harder time developing due to the colder ocean water in the EPO associated with LN. But with rising global temperatures we are in uncharted territory so surprises are always possible.

What NOAA’s Climate Prediction Center predicts

The combination of all of these factors (and other climate influences as well) is collected into NOAA’s Climate Prediction Center maps. I have shown the one for May through July 2024 below. It shows the likelihood of wet conditions in the Southeast and drier conditions out west associated with the lagging EN conditions. You can see maps for other time periods at Climate Prediction Center – Seasonal Outlook (noaa.gov).

For gardeners, if you are in the region where frost is still likely, you should hold off on planting tender vegetables and flowers at this point or at least start them inside. If you are in the southern reaches where frost is no longer likely then you can (and probably are already) plant the summer flowers and vegetables you are craving to set out, as long as no cold outbreaks are predicted. If you live in an area that is affected either directly by tropical storms and hurricanes or indirectly by heavy rains that remain after the storm has weakened now is a good time to clear out dead limbs and other potential flying debris, think about drainage in your gardens in case of heavy rain. Please make your hurricane plans in case one tracks over you (you can find one for Georgia at https://gacoast.uga.edu/wp-content/uploads/2020/08/ResidentsHandbook.pdf but most of the information there is relevant to large parts of the country). If you live in other parts of the country like the western states, you could see dry conditions and potential wildfires return to those areas so you should prepare for those conditions. By late fall, the La Niña should be well established and dry conditions are likely to occur in the southern tier of the US while cold and wet conditions are more likely in the northern states.

In the last week, I’ve driven all the way from western Virginia, where the redbuds are blooming, to Tallahassee, FL, where red clover is everywhere. As I drove through the mountains north of Charlotte NC, I noticed some signs indicating that strong gap winds may blow down the valleys when atmospheric pressure patterns align to produce strong pressure gradients that drive the wind. I have discussed wind before in previous posts (“Who has seen the wind?” and “Does wind chill affect plants?”) so you can find the basics of what causes wind and some of the different kinds of local winds by going to those posts. In today’s article I want to share some different local names for winds and other local weather and invite you to share your own local weather names. Note that this is not a complete list, but I will provide links at the end that prove a bigger sample of all the names that are used around the world to denote different kinds of weather, especially wind.

Local weather names based on topography

Local mountains and valleys can cause a big variety in the types of winds we observe. Generally, these winds can be classified as katabatic winds blowing downhill and anabatic winds blowing uphill. The direction depends on the time of day due to heating but also to large-scale weather patterns that direct the flow of air. Local winds can also occur due to changes in the heights of the ridges so that where the ridges are low, air can spill over the mountains in the gaps between peaks. Winds blowing downslope can also accelerate as they move to lower elevations, increasing their strength. Those winds can be very strong because of the funneling effect of the terrain leading to warnings like the ones I saw on Interstate 77 in the northern North Carolina Mountains. Some of these local winds in other parts of the world are called the Viento Zonda (or Zonda wind) in Argentina, the Williwaw in the Alaskan Panhandle, Karaburan in Central Asia, Chinook wind along the Front Range of the Rocky Mountains in the USA, Mistral in France, and the Warm Braw in the Schouten Islands north of New Guinea. You can read more about each of these by looking online at Wikipedia or other sites (link below). Winds which are affected by topography can provide good sources of steady wind for wind farms.

One of the most interesting large-scale topography-driven winds is the Tehuantepecer in southern Mexico which begins in the Gulf of Mexico (after coming south across North America) as a north wind that crosses the Mexican isthmus and blows through the gap between the Mexican and Guatamalan Mountains. It is so strong that it can be felt as much as 100 miles out to sea. This happens several times a year, especially in winter when the wind is more often from the north, and is amazing to see on https://earth.nullschool.net/ when it happens. In fact, as I am writing this on Thursday (March 28, 2024) it is happening today! How cool is that?

Local weather names based on changes of air mass

Some winds are named for abrupt shifts in atmospheric temperature and humidity when air from a different source region moves in. They can be small-scale changes due to outflows from individual thunderstorms like gust fronts or can be larger-scale changes due to wind blowing an air mass with colder, hotter, or drier characteristics into the area.

Some of the winds associated with drier and dustier conditions occur near desert locations as the wind shifts to bring in air from the desert regions to replace the air that was already there. One of the most common terms for one of these is haboob, which originated in Sudan but is now used in the western U. S. (Haboob basically means “dust storm” in Arabic but sounds a lot more exotic). A haboob is associated with a wall of hot, dusty air that moves into the region from the desert, bringing low visibilities to the region (often resulting in car accidents as drivers caught unawares can be blinded by the sudden change in conditions). Other dry winds include the Khamsin in Egypt and the Red Sea region, the Scirocco in North Africa and the Mediterranean, and the Harmattan in West Africa.

Cold winds include the Blue Norther, a fast-moving cold front that moves in from the north that can send temperature plummeting by 20-30 degrees in a few minutes, the Bora in the Adriatic region, the Khazri in the north Caspian Sea, the Montreal Express in New England, and the Norte in Mexico. In the Southeast US, we have what we call the Wedge, which is a shallow layer of cold air that moves south along the eastern slope of the Appalachian Mountains under northeast flow, bringing clouds, cold weather, and the chance of ice storms to the region in spring. The Wedge is partly due to topography as well, since the cold air is so shallow that it can’t move west over the Appalachian Mountains and thus is forced down to us in parts of the Carolinas and Georgia. Hot dry winds include the Brickfielder in southern Australia, the Leveche in southern Spain, and the Diablo and Santa Ana winds in California, which are also affected by air moving down from the mountains into coastal areas of the state when high pressure dominates the Southwest.

Local winds are associated with thunderstorms

In addition to the wind names associated with topography and change of air quality there are also some names that are tied to smaller weather events like hurricanes and thunderstorms. Those include the Kalbaishakhi in India and Bangladesh, the Bayamo on Cuba’s southern coast, the Pampero in Argentina and Uruguay, the Cordonazo on the west coast of Mexico, and the Borasco in the Mediterranean. Strong winds associated with thunderstorms can cause tremendous damage to gardens, trees, and buildings and can cause problems with flights and road transportation. Since it is spring and we are entering severe weather season for a lot of the US, it’s a good reminder that it does not need to be a tornado to cause significant damage—straight-line winds can be just as severe.

Knowing your local climate is important for gardeners

Anyone who lives for a long time at a location will start to recognize the local weather and climate patterns that govern your local garden conditions. If you are really dedicated, you can even measure these variations. If you know that in certain seasons, you are more likely to experience very dry dusty air, you might consider plants that can survive those conditions with less care. If you live in an area that is subject to frequent strong local winds, you will need to plan your garden to place more wind-resistant plants where the air flow is the strongest or else construct a wind shelter to keep more sensitive plants safe. Buildings can also affect the wind flow and can cause “wind tunnels” where the air is constricted and blows faster in the narrow passage.

Note: For those of you who wonder about the title of this post, Maria (sometimes listed as Mariah) is a fictional wind popularized in “Paint Your Wagon” (Lerner and Lowe, 1951) and by the Kingston Trio (1959). The name may have originated with the 1941 book “Storm” by George R. Stewart according to my colleague Jan Null of Golden Gate Weather Services.

Sources of more local wind information

Here are some websites that have listings of additional local winds, although none of them is a complete list, I am sure.

Wikimedia (with links to most of the individual winds): https://en.wikipedia.org/wiki/List_of_local_winds.

GG Weather: https://ggweather.com/winds.html

U. K. Met Office: https://www.metoffice.gov.uk/weather/learn-about/weather/types-of-weather/wind/wind-names

If you have a local name for the wind or weather in your region, please share it in the comments!

In this blog I’ve talked several times about El Niño and La Niña and how they affect climate across the Northern Hemisphere as well as their impacts on the rest of the world. We are currently in a strong El Niño with sea surface temperatures in the Eastern Pacific Ocean (EPO) that are much warmer than the long-term average. But underneath the surface the ocean currents are starting to change and the El Niño is expected to swing quickly into the opposite phase, La Niña. That will affect us in North America but also other parts of the world since both El Niño and La Niña are linked to global atmospheric patterns. Since a La Niña Watch was just issued by NOAA this week I will be talking about the changes we can expect to see over the next few months and how those changes will affect gardens and gardeners.

Review—What are El Niño and La Niña?

El Niño and La Niña are opposite phases of a large-scale atmosphere/ocean pattern that is driven by temperatures in the EPO. The pattern affects climate in many places around the globe. It is the biggest driver of seasonal climate in the Southeast and Pacific Northwest as well as in some other countries, especially in Northern Hemisphere (NH) winter when it is usually the strongest.

In September 2023 I discussed the likely impacts of the El Niño that was growing at that time and how it would affect your winter gardens. The conditions I expected have mostly been observed, although there are some local differences that are not surprising considering that each event is unique. Northern states have been incredibly warm with little snow, while in the Southeast we have had a lot of rain and cooler (although not frigid) temperatures due to wet soils and a lot of clouds blocking the sun. California is getting hit by one atmospheric river event after another, so they are also very wet and are even seeing a lake in Death Valley. I imagine they will have quite a bloom of spring flowers when it gets warmer because of the ample moisture. What have you experienced in your area? Did my earlier column get it right?

How is this season different than a typical El Niño winter?

Even though we have had the swings of El Niño and La Niña (collectively called El Nino Southern Oscillation or ENSO) for thousands of years as evidenced by layers in ice sheets in Peru and ocean sediments, there are other changes that are not cyclical. The rise in global temperature over time is showing up as a warming trend in all seasons but especially in winter. That does not mean we don’t see other swings in climate over time because ENSO and other atmospheric cycles are still occurring, but they are superimposed on the slowly rising temperature associated with increases in greenhouse gases so the cold outbreaks aren’t quite as severe and the warm spells last longer.

This year one of the most notable things we are seeing in global climate is the unbelievable warmth in the Atlantic Ocean. Temperatures there now are at values consistent with June or July temperatures! This is the energy that will feed tropical storms later in summer (more on that in a minute). Scientists are still not sure of all of the factors that are contributing to these record-setting conditions, but they may include the eruption of Hunga Tonga, the elimination of sulfur emissions from modern cargo ships, and changes in the global ocean circulation.

In addition, in spite of one cold big outbreak this winter across the eastern United States, most areas have been warmer than normal resulting in an early spring that has brought honeysuckle leaves to my yard more than a week early. You can follow the “green wave” north and see when it gets to your area or verify that it’s already there at the National Phenology Network site. I am concerned about the possibility of having another late frost like 2023 that could impact the peach and blueberry growers in the Southeast since our average last spring frost date is early to mid-March for most of the commercial peach region. There has been enough cold weather for most of the fruit-producing plants to have reached their required number of chill hours, which means the warm weather is making them ready to bloom. While I don’t see another cold outbreak on the horizon for the next few weeks, we have had frosts in the Southeast into April before so we are not out of the woods yet.

When will La Niña begin?

Climatologists predict that El Niño will weaken through spring and we will swing back into neutral conditions by the April through June period. From there most models predict we will move into a La Niña by the June through August period. By NH fall (September through November) there is a 77% chance we will be in La Niña conditions. This has implications for the summer and especially for the Atlantic tropical season since in neutral and La Niña years the number of tropical storms that occur in the Gulf and Atlantic is higher than in El Niño years. Last year despite El Niño we had 20 named storms, much more than the average of 14 events. This was in part due to the unusually warm water. Most of those stayed over the Atlantic Ocean rather than make landfall due to the presence of a strong jet stream high in the atmosphere which disrupted the development of storms farther to the west and prevented a lot of damage to us in the United States. In spite of that, we still had Hurricane Idalia and Tropical Storm Ophelia, both of which caused a lot of damage to infrastructure and agriculture.

With La Niña fully in place by fall, there will be little to stop the development of tropical storms in the Gulf and Atlantic Ocean except for Saharan dust and unfavorable weather patterns in the United States that could at least shunt any storms away from land. Some early unofficial predictions are for 25 or more named storms to occur this year, although the official predictions are still a few weeks away.

Next winter, we can expect La Niña to control a lot of our climate. That means warmer and drier conditions across the southern part of the United States while cold and wet conditions return to the northern states. Here in the Southeast, that means soil could be pretty dry in spring 2025 leading to issues with planting although it will be easier to drive heavy equipment into the fields than I expect will happen this year.

What does all of this mean for gardeners in the United States?

Because of the recent warm conditions associated with rising temperatures and enhanced by El Niño in northern parts of the country, spring is coming early to many places. That can be a good thing if you like flowers and don’t like snow, but it does mean that your early flowers will still be susceptible to frost damage if we get another cold outbreak later in March or even into April or May for northern states. So you will need to be prepared to protect the tender plants if a frost or freeze occurs.

The end of El Niño and the eventual rise of La Niña also has implications for areas that are affected by tropical systems. This includes the Gulf and East Coasts and areas downwind of those locations but can also include parts of California and the Southwest which can see impacts from tropical systems in the EPO west of Mexico. Rainfall could be hit or miss in the late summer depending on where the storms go. You should prepare well in advance of June 1, the official start of the season, because the warm ocean water could allow tropical storms to develop in May ahead of the “official” start. That means making sure you remove damaged limbs or other objects that could become wind-borne debris, make sure you have adequate drainage for heavy rain, and keep an inventory of your belongings that could be washed or blown away in a storm. Have a family plan to keep in touch and evacuate if you need to, including pets and livestock. You can find a lot of good information on preparing for and recovering from natural disasters in this University of Georgia handbook, even if you are not in Georgia or the Southeast.

While the current warm weather makes gardeners eager to get out in their plots and get started, it’s probably too early to start in most of the country except the most southern areas. But you can dream and start planning for the warmer weather soon to come!

Over the course of this winter there have been several days when the temperature plummeted after the passage of an Arctic front as strong winds blew frigid air into parts of the United States. It can happen in other parts of the world, too. When this happens, the National Weather Service (NWS) often issues Wind Chill Warnings urging people to bundle up before they head outside into the dangerously cold weather. From time to time I am asked if plants also experience wind chill. This week let’s explore how wind does and does not affect plants.

What is wind chill?

The Oxford Dictionary defines wind chill as “the cooling effect of wind blowing on a surface”. That is partially correct as a general statement of how the wind makes you feel, but it is lacking as a scientific definition. A better one is “a quantity expressing the effective lowering of the air temperature caused by the wind, especially as affecting the rate of heat loss from an object or human body or as perceived by an exposed person.” The wind-chill index is a calculated index that tries to quantify how a strong wind can remove heat from a human or animal body. A low wind chill indicates that heat is being rapidly removed from the body, potentially resulting in the lowering of internal temperature and the chance of frostbite to extremities like fingers, toes, and noses if directly exposed to the frigid wind.

“Wind chill” is used by the NWS as a way to provide a warning message to people who might be working or playing outside to make sure they are well protected from direct contact with the wind. As a calculated index wind chill cannot be measured directly although there have been some lab experiments in temperature-controlled laboratories that have tried. The formula for calculating wind chill has been changed in the past as science has improved our understanding heat transfer by wind.

How does wind chill work?

When wind blows across a surface, it causes a transfer of energy between the wind, which is at one temperature, to or from the surface which is most likely at a different temperature. If the temperature of the wind is lower than the surface, then heat (which is just a measure of the energy of the molecules at the surface) is stripped from the surface and transferred to the wind. That lowers the energy of the surface and cools it off (note that this is different than evaporative cooling, which is cooling due to evaporation of water from a surface). The faster the wind blows, the quicker the energy is stripped away. In summer, when the ambient air temperature is high, this cooling effect from a breeze off a cooler water body like the ocean may feel pretty good. But in winter, when temperatures are already icy, it may cool off the surface (and by connection, the body beneath the surface) to dangerous levels resulting in frozen cells that are the hallmark of frostbite or hypothermia if the core body temperature is affected.

Plants do not generate internal heat and so are generally the same temperature as the air. Because of this, there is no transfer of heat energy between the air and the plants and so the plants would not experience “wind chill.” However, anything that has an internal source of energy, including humans, animals or running engines, could experience a chilling effect as heat is stripped away from the surface by the wind especially if the warm surface is exposed directly to the cold wind. That is why it is important to wear layers to provide protection from the wind when the wind chill is expected to be extreme since it keeps heat from being removed from the skin. Ranchers help protect their livestock in blizzards by creating wind breaks that reduce the wind speed and so limit the impacts of wind chill on their cattle. Newborn calves may be especially susceptible to wind chill on their ears and may be outfitted with earmuffs to protect them from frostbite.

What other impacts does wind have on plants?

Even though plants do not experience wind chill, that does not mean that there are no impacts from the wind. In addition to transferring heat, wind can also transfer moisture from the plant to the air, desiccating the plants if the humidity of the air is low. This can happen at any temperature as long as there is not much water vapor in the air, but we generally think of it happening at high temperatures because the difference between the water content of the plants is much higher than the water vapor in the air. The difference in humidity, like the difference in temperatures, leads to transfer of water out of the plant and into the air blowing over it. It’s not all bad, though–wind blowing through the plant canopy can be a good thing if it keeps humidity levels near the plant low enough to prevent the development of fungal diseases that thrive on very moist conditions, so gardeners should consider wind conditions in their garden when they determine where to place their plants and what to grow.

Winds can also cause damage to trees and garden plants. I recently read an article that indicated nearly all tree trunks break at the same wind speed of around 90 mph (42 m/s), regardless of species or size. According to the authors, “In a strong wind, a tree may break through one of three mechanisms. Uprooting can occur in rain-moistened ground, or if the tree’s roots are rotten. Alternatively, if the roots can hold, then it becomes the tree trunk that is at risk from breakage – either through torsion or, more commonly, bending.” Of course, damage to tree limbs can cause breakage at weak spots as well at lower wind speeds.

Wind also has beneficial impacts on plants. The gusty nature of wind causes variable forces on tree trunks and plant stems that increase their strength by forcing them to resist the wind. That is one reason why you might not want to stake young trees too tightly, because they need to be able to move in the wind to form strong tapered trunks. Wind that blows all the time in the same direction causes trees and plants to grow in distorted (and sometimes beautiful) ways as their shapes are formed by those constant winds. Wind also provides a way to spread pollen from one tree to another, helping to spread genetic material through a wide area for reproduction.

If you are interested in reading more about how wind affects gardens, check out my earlier blog post at https://gardenprofessors.com/who-has-seen-the-wind/.

This year is almost certain to be the warmest on record for the earth as a whole, although there are still a few days in December that could slightly affect the final numbers. As we close out 2023 I want to spend a few minutes reviewing the weather and climate of the past year, both the average conditions and some of the extremes we saw. While this is skewed towards the United States, I did include some events happening in other parts of the world for our non-US readers. I will also take a peek at what is likely to happen in 2024.

What were the average climate conditions in 2023?

Since the year is not quite over I can’t provide a final average for temperature or precipitation for the complete 365 days, but there are some websites that allow me to look at all but the last few days. The images below are from the High Plains Regional Climate Center for January 1 through December 27. They show the temperature departure from normal and the percent of normal precipitation for the continental United States. (You can see the global temperature statistics for January through November 2023 at the National Centers for Environmental Information.) In most parts of the U.S. the temperature was warmer than the 1991-2020 normal; the exception was the western mountains, where temperatures were colder than normal. Keep in mind that the normal period being used for comparison (1991-2020) was a period that was quite a bit warmer than the long-term temperature average in the United States, so this map underestimates how warm this year was compared to most of the 20^th century.

Precipitation was more variable than temperature, as it usually is. The driest areas this year were in the southwestern Gulf of Mexico states, particularly Louisiana and Texas, and in the Pacific Northwest.  It’s not surprising that these were also areas with significant droughts, including a lot of the Corn Belt which also saw very dry conditions during the growing season. By comparison, California and New England experienced multiple storms bringing significant rain to those areas, including Tropical Storm Hilary (the first tropical storm to hit California in 84 years) in mid-August. In the Southeast, Hurricane Idalia did almost $5 billion in damage in late August from heavy wind and rain, half of that in Georgia alone. But that did not stop a flash drought from developing there in fall with the almost complete cessation of rainfall for up to 60 days.

What extremes did we see in 2023?

The averages show the overall conditions that occurred this year but don’t begin to capture the extremes in temperature and precipitation that occurred. In the United States alone there have been 25 billion-dollar weather disasters so far this year, including the tropical systems mentioned before along with numerous rounds of severe weather across the country and the devastating firestorm in Maui in August. In other parts of the world, many regions experienced their warmest September-November period since records began in 1880. Significant heat waves occurred in Texas and Mexico as well as Europe, Chile, and Canada, where widespread forest fires that flared up blanketed Canada and many parts of the eastern United States with poor air quality and low visibility in the summer.

Floods and droughts occurred around the world this year. The Mississippi River dropped to record-low water levels for the second year in a row due to the drought in the Midwest. On the other extreme, notable flood events occurred around the world, include floods in Ghana, the Horn of Africa, Pakistan, Bangladesh, and Chile. Storm Daniel brought unprecedented rain to Libya, breaking dams and causing tremendous damage in September. Nine separate atmospheric river events caused tremendous flooding in California early in the year, significantly reducing drought conditions there and contributing to the reduction in drought area in the United States in the first half of 2023.

While El Niño usually means that the Atlantic tropical season is quiet, this year was unusually active with 20 tropical storms and hurricanes. This is in spite of the presence of a jet stream aloft due to El Niño that usually keeps storms from developing. Most of those storms stayed out to sea, so impacts on the United States were limited (except for Idalia and Hilary out west). In other countries, Hurricane Otis hit western Mexico near Acapulco in October, bringing catastrophic damage to an area that almost never gets hits by tropical storms. Cyclone Lola devastated the northern part of Vanuatu in late October as well.

The global temperature will set a new record for warmth in 2023

The New York Times provided a sobering look at monthly temperatures for each month going back to 1850 (above). It shows that 2023 had several months that were the warmest on record for those months, due to the expansive area of warm ocean water associated with El Niño in the Eastern Pacific Ocean along with record-setting sea surface temperatures in the Atlantic that contributed to a very active tropical season in spite of being an El Niño year. The impacts of this warmth are being seen in dropping sea ice coverage, more and stronger heat waves, and increases in wildfires in forested areas. Some people argue that the warming trend appears to be accelerating in recent years, a concern that urges us to consider how we can reduce greenhouse gas emissions and slow down the increasing temperature trend.

What do we know about 2024 so far?

The current El Niño is expected to continue through the next few months before it weakens and turns back to neutral conditions around the April-June period. A La Niña could occur later in 2024, which means that next year’s winter could be warm and dry in southern parts of the Northern Hemisphere and colder and wetter along the northern border of the US and up into Canada. Pending on how long the El Niño lasts, the warm ocean temperatures could contribute to another record-setting warm year in 2024 although it’s too early to be sure. It also depends on shorter-term weather events like more frequent occurrence of cold weather due to a shift in the weather pattern in January to more variable conditions later this winter, as many forecasting models think is likely. Meanwhile, neutral conditions or La Niña conditions later in the year could mean that Atlantic tropical activity increases to an even more active level than last year.

Thank you, gardeners, for another great year!

Most of the United States as well as the rest of the world experienced a warmer climate again in 2023, so gardeners will continue to need to choose plants that are appropriate for their warming climate zones. Extreme conditions, including devastation by individual storms as well as natural climate variability, will continue to affect home gardens through water stress caused by drought and extreme heat as well as damage caused by floods, high winds, and freezing temperatures. Building a resilient garden that can withstand these extremes will allow your garden to thrive through whatever conditions the atmosphere throws at it.

I want to end this year by thanking you all again for your loyal readership and your thoughtful questions and comments on many topics. I encourage you to share your 2023 garden challenges (weather or otherwise) in the comments along with your plans for how you plan to address them in 2024 and beyond. I look forward to reading them! We will see you again in the New Year.

Some lists of top weather and climate events for 2023 (mostly videos):

Weather Nation: Looking Back at the Top 10 Weather Events of 2023

Pattrn: 2023: Year of Extremes

NBC News: The biggest climate stories of 2023

Climate.gov: Climate Highlights of 2023

Atmos Earth: Your 2023 Climate Wins, Wrapped

This month has been an exciting one for climatologists around the United States with the November 14 release of the Fifth National Climate Assessment (NCA5), a massive project that is undertaken every four years to capture our current understanding of climate change based on recent research. I was a chapter author for the Southeast and spent the last two years working with over 700 authors around the United States to gather and document how the climate is changing and how it is affecting all of us. This week I will explain how NCA5 was put together, what it says about climate, and what gardeners can do to help reduce the future impacts of global warming and other climate changes. But this month was also exciting because USDA just released an updated Plant Hardiness Zone map, just a few weeks after my post in October about how the 2012 map was outdated. I guess they were listening (just kidding!). I will discuss that briefly at the end of this post, too.

What is the National Climate Assessment?

The National Climate Assessment (NCA) is a report mandated by Congress to compile the latest scientific findings on how climate is changing so that we can respond to reduce its future impacts. It is published every four years, and the last one (the 4^th NCA) was released on the day after Thanksgiving in 2017. While the underlying message has not changed, each assessment focuses on the newest scientific research that has been published since the last assessment was done. The document is divided into chapters so that the authors of each chapter could concentrate on that topic.

NCA5 starts with a review of the general scientific principles of how the climate is changing. That is followed by seventeen chapters focused on national topics such as agriculture, water, energy, and transportation as well as specific groups that are being especially affected by climate change such as indigenous peoples. Following the national topics, chapters address changes that are happening in ten different regions of the country . These address how we need to reduce future greenhouse gas emissions that are driving the warming of the earth as well as how we can adapt to the changes that are already happening now and may get worse in the future.

How was NCA5 produced?

There is a long process involved in producing a national climate assessment. Teams of scientists from an array of disciplines were chosen as authors for each chapter to write the initial text of the document. To keep the authors on task and within tight word limits, there were lead chapter authors and technical advisors who moderated group meetings where the key messages for our chapter were identified. Initial figures to include in each chapter were drafted by a graphics team or requested from scientific journals. After the first draft was complete it was first reviewed by federal agencies to make sure that their concerns were addressed and then by the public, who provided many additional comments. All of these comments were provided to the chapter authors so they could refine their text and figures for the next draft. In all, the document when through six different reviews and all comments were addressed.

Where can I read NCA5 and learn more about what it says?

The NCA contains a vast amount of information in its 32 chapters, five appendices, and special topics, so it is hard to summarize. I encourage you to explore the document online to see what it says about your region and special topics of interest like agriculture, land, and ecosystems. A good starting place is the introductory website https://www.globalchange.gov/our-work/fifth-national-climate-assessment, which explains how the report was written and provides links to read the report, attend a webinar on an individual chapter, and see where the figures came from. I also encourage you to explore the excellent interactive atlas developed in conjunction with the report. Many other resources such as podcasts are available, too.

What are some steps that gardeners can take to respond to climate change?

There are two approaches that gardeners (and all of us) need to take to respond to the challenges of a changing climate. We are already dealing with the consequences of trends towards warmer temperatures and more extreme swings in the water cycle such as increases in floods and droughts. Gardeners are adapting to these changes in climate by planting different plants that are better suited to the warmer climate and changing how they manage their gardens using rain gardens, drip irrigation, and other techniques. Adaptation is a key approach that gardeners will continue to need to follow as the climate continues to get warmer and more variable.

In addition all of us, including gardeners, have a responsibility to cut the emissions of additional fossil fuels which are driving most of the warming (mitigation is reducing the inputs to prevent future harm). This will reduce the impacts that our world will have to navigate in the future. Even a small decrease in the emission of greenhouse gases now can prevent the worst outcomes. A prime target for gardeners is the elimination of gasoline-powered equipment like blowers, mowers, and trimmers. These small tools have highly inefficient engines that emit a lot of greenhouse gases as well as air pollutants (and a lot of noise, too). Switching to electric tools and vehicles, composting, adding solar power to houses and businesses, and conserving energy and water (which often uses energy to purify it) through carefully chosen plantings as well as through other methods can also help reduce future warming.

What about the new USDA Plant Hardiness Zone map?

I was surprised last week that the USDA had just produced an update to the 2012 map that I discussed last month. In that post, I noted that the 2012 map was already outdated due to the increasing temperatures we have seen in the 21^st century. The new 2023 map uses data from 1991-2020, the current 30-year normal period, to identify the current plant hardiness zones for the United States. You can see the new map and zoom to your city at https://planthardiness.ars.usda.gov/. By comparing it to the 2012 map, you will see that more than half the country has increased by half a zone, which correlates to about a 5-degree F increase in the average lowest minimum temperature a location experiences each year. I asked USDA for a map that showed the changes of zone and was provided one by Chris Daly of the PRISM group that put together the 2023 map (below). Areas in tan experienced a half-zone change since the 2012 map. (There are a few areas in the Mountain West where the zones got colder, as shown in green, but these are mostly linked to new datasets that were available for the analysis rather than any changes to the local climate there.)

Science has made it clear that the earth’s climate is changing and that most of the warming we are experiencing is due to burning of fossil fuels. We must learn to adapt to these changes and make sure that all groups can be protected from the worst impacts of the more extreme weather we are likely to experience. But we can also make changes now to reduce those future impacts, and I know gardeners will be part of that solution.

UPDATE: As of 11/15/2023, the USDA has published an updated Plant Hardiness Zone map that covers the 1991-2020 period, which includes a lot of the warmest years on record for the US. This map shows more detail than the old map and generally increases the zones in most areas by maybe a half-category. It also now includes Canada and Mexico. You can see it and read about it at USDA Plant Hardiness Zone Map | USDA Plant Hardiness Zone Map.

One of the first questions a gardener should ask when they are considering adding new plants to their garden is whether the plants can survive and thrive in the weather and climate conditions in their yard. One of the most useful tools for this is the USDA Plant Hardiness Zone designation. It provides a quick snapshot of the coldest weather the location is likely to experience, a key factor for how well the plants will survive in that area.

What are plant hardiness zones?

Plant hardiness zones are based on the average annual minimum winter temperature at a location. For simplicity the zones are based on 10-degree Fahrenheit ranges. Each zone is further subdivided into “a” and “b” categories for the colder and warmer halves of the range. You can see the temperature ranges listed on the USDA Plant Hardiness Zone Map website, which also includes a link to an interactive map that will help you determine what zone your location is in. My home in Athens GA is listed as being in zone 8a, which has an average annual minimum temperature range of 10-15 degrees F. Linda provided good descriptions of how to use the zones in this blog in 2019 in A Gardener’s Primer to Cold Hardiness, Part 1 and Part 2.  

How accurate is the USDA Plant Hardiness Zone map?

The latest official version of the map was published in 2012 and showed that most areas had experienced a half-zone change to a warmer zone from the previous map because of rising temperatures. There has been no new map since that time but as temperatures have continued to rise it seems pretty clear to me that the current map is outdated. And in fact, even back in 2012 shortly after it was published, Bert Clegg posted an article in this blog showing that the 2012 map was likely already outdated when it was published because it was based on a 30-year average in an era when temperatures are rising and minimum temperatures are rising much faster than maximum temperatures due to increases in humidity and urbanization.

We need to be a little bit careful with this comparison because the average minimum temperature is not the same thing as the average annual minimum temperature. The average minimum temperature is the average of all the daily minimums in a specified time period, while the average annual minimum temperature is the average of the single lowest daily temperature that occurred each year. You can have a fairly warm winter which still experiences an extreme cold outbreak that has a very low minimum temperature on one or two days. In fact, December 2022 had exactly that situation with the fiercely cold outbreak that occurred right around Christmas across a lot of the eastern United States. The extremely cold air was barely seen in the winter average temperature at all since February 2023 was extremely warm for most of the month and washed out the impact of the extreme cold since it occurred over just a few days in the average. But it certainly caused a lot of damage to plants that were exposed to the frigid air on those few icy days! If the 2012 map was outdated when it was published, it is surely more out of date now after an additional decade with some of the warmest years on record.

How will the plant hardiness zones change in the future?

As global warming continues, the average annual minimum winter temperature is expected to continue to rise. This will result in a northward movement of plant hardiness zones over time. For example, areas that are currently in Zone 6 may become Zone 7 or 8. The rate of change will depend on how fast the earth warms and that depends on how much and how quickly humans respond to minimize greenhouse warming. It would not surprise me if our hardiness zones in most parts of the United States now are at least a half-zone warmer than what is shown on the 2012 map and it could be even greater in some locations. Not all areas of the country (and the world, for that matter) are warming at the same rate, and areas closer to the poles tend to be warming more quickly because of the loss of snow and ice in winter, especially in the Northern Hemisphere.

How will the shift in plant hardiness zones affect gardens?

This shift will have a significant impact on gardening and agriculture. Plants that are not adapted to warmer temperatures may struggle to survive. For example, some fruit trees that are currently grown in Zone 6 may not be able to produce in Zone 7 because they require a certain amount of cold weather to set a good flush of blossoms that form the fruit. Warmer winter temperatures will increase the chance of insect pests and diseases surviving over the colder months, leading to more problems in the next growing season. The last spring frost is likely to come earlier and the first fall frost later in the year. This might make some gardeners happy, since they can get out and start planting earlier, but has implications for pollination since the pollinators may not be able to adapt to the changes in the timing of flowering. That would result in less fertile crops and potentially lower yields of vegetables and other crops.

Gardeners and growers will need to adapt to the changing climate by selecting plants that are suited to warmer temperatures. You may already be doing this by choosing varieties and species for your gardens that are listed as being suitable for a warmer Plant Hardiness Zone than the 2012 map suggests. Gardeners may also need to change their planting practices, such as planting earlier in the spring or providing more shade for plants. In addition, changes in precipitation (which are not included in the Plant Hardiness Zones)  also affect what kind of plants you need to put in your garden since drought is likely to increase in warmer conditions at the same time that individual storm events may drop more rain than in previous years.

Of course, this does not negate the effects of local climate variation across your plot of land. Variations in shade, soil, and drainage will continue to affect variations in the microclimate across your garden, as I discussed in my first blog post, The Weather Where You Are. However, the local variations will occur on top of the changes to the overall plant hardiness in your region and global temperature increases are likely to cause much bigger changes to your local climate in the long term.

National Arboretum in October, DC Gardens, Commons Wikimedia.