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Have you ever stopped while you were gardening to look at the clouds? Clouds, like flowers, come in a variety of shapes and sizes that can form beautiful patterns in the sky. But clouds are not just pretty, they can also be used to make predictions about the weather in the coming days. In this week’s post, we will look at the different types of clouds and how they relate to coming weather. You can use that to prepare for your garden work by knowing when it will be sunny and predicting when rain is coming.

How are clouds classified?

Clouds were first classified by Luke Howard in 1803 in his “Essay of the Modifications of Clouds”. Howard classified the clouds by their shape, using Latin names for wisps (cirrus), lumps (cumulus), or sheets (stratus) to describe how they looked in the sky. Clouds that are precipitating either rain or snow are called nimbus clouds. Some clouds are a combination of types, such as stratocumulus clouds, which look like a layer of lumpy clouds, or cumulonimbus clouds, which are the tall thunderstorms that form in summer. You can find a great gallery of cloud photos at the Cloud Appreciation Society. Wikipedia has an exhaustive list of cloud types online as well.

Clouds are also classified by heights. Most clouds form in a single layer of the atmosphere, but you can often see multiple layers of clouds when you look at the sky. These layers may be caused by different mechanisms. The shape and height of the clouds provide clues to what is going on in the atmosphere and are especially related to the presence (and sometimes absence) of warm and cold fronts that are harbingers of precipitation and a change in wind and temperature conditions.

To estimate how high the clouds are, you can use a literal “rule of thumb” that works well for cumulus-type clouds. Hold your hand out towards a cloud. If the individual cloud is the size of your fist, then it is probably a low cloud. If the lump of cloud is the size of your thumb, then it is probably a mid-level cloud. And if it is the size of your little pinky joint, then it is probably a high cloud. Note that the clouds are the same size, but the difference in height makes them look like they are different sizes.

For stratus clouds, you can estimate the height by how transparent it is. A cloud layer that allows you to clearly see where the sun or moon is located is probably a high cloud. A layer that lets you see where the sun is but shows it with blurry edges (we sometimes call this a “ground glass” appearance), then it is probably a mid-level cloud. If it is so thick that you cannot see where the sun or moon is, then it is probably a low-level cloud. But please be careful not to look directly at the sun, since it can damage your eyes!

How do clouds form?

Clouds form where moist air cools off to the point that the water vapor condenses into small drops that become visible to us. Since the atmosphere generally cools off as you go up, the clouds form where the air is rising. The rising motion can be provided by heating from the earth’s surface, lifting of the air by a mountain, or large-scale upward motion caused by warm and cold fronts, where masses of air at different temperatures interact to create areas of rising air. The highest clouds usually form in the coldest air and form as ice crystals, leading to their wispy appearance. Lower clouds appear in warmer air where the water condenses as liquid droplets, leading to their more robust appearance.

How are cloud shapes related to atmospheric winds and structure?

The most interesting weather (at least to me, as a meteorologist) is where there are differences between masses of air at the surface and above the surface that are interacting. Boundaries between these air masses are called “fronts” and are named because they act like battle fronts between enemy armies. In a cold front (left side of diagram above), cold dense air near the surface pushes beneath a layer of warm, humid air, causing it to rise and cool. Clouds ahead of cold fronts tend to be relatively tall and energetic and form cumulonimbus clouds that can drop a lot of rain in a short time but generally tend to move through an area quickly unless the front stalls due to other atmospheric dynamics.

Warm fronts are large masses of warm, humid air that are pushing over the top of a layer of colder, more dense air (right side of diagram above). As the warm air rises, it slowly forms clouds as the layer cools to the temperature of condensation. Since the entire layer is rising, the clouds that form are often sheets of clouds rather than individual clouds. The higher clouds indicate that the moisture from an approaching warm front is present high in the atmosphere and shows that a warm front is likely to be approaching, signaling a change in the weather from cool to warmer conditions that could also drop rain as the warm front gets closer to your location. As the warm front gets closer, you should see the high clouds replaced by mid-level clouds like altostratus and then lower clouds like stratus clouds and nimbus clouds if they start producing rain. This usually happens over a day or two depending on the speed and strength of the surface warm front.

If you see lumpy cloud forms, then they are most likely related to the rising motion of air due to columns of warm air rising from the surface (“thermals”). Air between the columns is sinking, which leaves clear spots between the clouds. Fair-weather cumulus clouds are the tops of these thermals, when the rising air cools down enough for the water vapor in the column to condense, forming the cloud. These types of clouds usually form when the ground is heated, most often by sunlight during the day but sometimes by pavement or wildfire as well as mountainous areas. They usually form on days when you are in a mass of warm, humid air that is far from a frontal boundary, although if they grow taller, then a cold front is likely to be approaching.

If the air is very hot and humid and the surrounding air is cooler than the rising column, then the clouds can grow vertically to great heights before they hit a layer that is warmer than the rising air and stop growing. These tall clouds are called cumulonimbus clouds because they often drop heavy rain as they develop. These often form along and ahead of cold fronts and indicate that the wind is likely to shift from a south wind to a colder wind coming from the northwest (in the Northern Hemisphere). In the worst cases, the rain can cause erosion or damage to fragile plants when it is very intense. Plants that live in rainy areas evolve to have such things as pointed tips or shiny leaf surfaces that shed the water quickly. Cumulonimbus clouds are also sometimes associated with high winds, hail, and tornadoes, all of which can damage garden plants and trees as well as harm humans and animals.

How can you predict the weather by watching the clouds?

As you work in your garden, take a look at the clouds above you. If they are high and wispy, then moisture high in the atmosphere may indicate that a warm front and a chance of rain is likely in a couple of days, especially if they get lower and denser over time. The picture of daisies at the beginning of this post shows high, wispy clouds that could indicate a warm front is approaching. The glowing ring of light that appears in cirrostratus clouds in the picture just above this paragraph may also be a sign that there is moisture on the way, leading to the saying that “a ring around the moon means rain in 48 hours”.

The chance of rain may affect your plans to spray your gardens or lawns for pests or fungal diseases, since many garden treatments have weather-related requirements for when to use them. Some work better when applied to wet leaves after rain falls, but others need a period of dry weather for the chemicals to be most effective. Make sure you read the labels to know what kind of weather they need. It may also tell you that it would be a good idea to mow the grass before it rains in the next 24-48 hours.

If you are already in hot and humid conditions and you see cumulus clouds getting taller and more numerous over time, a cold front may be approaching. This could indicate a period of strong winds, heavy rain, some possible lightning, and sharply cooler temperatures which could be either a curse or a blessing depending on just how hot it has been. The very shallow clouds in the picture above (cumulus humilis) are most likely seen after the cold front has passed and there is minimal lifting to cause clouds to form.

Don’t forget to look up

A good gardener should always be keeping an eye on their garden but should also be watching the environment around it to see how the conditions might be changing in the future. Who knows what delights you will see if you just look around you? But don’t forget to look up, too, because the sky is full of wonders and can inform you about the future as well as strike you with awe.

If you’ve been paying attention to the weather across the United States this past week, you may have noticed that most of the eastern U. S. is experiencing extremely hot temperatures, especially when you factor in the effects of humidity. At the same time, in the western U. S., it has been snowing in the mountains, even though it is almost July! In this week’s blog post we will look at why this pattern of hot and cold conditions occurs so often and what is causing it.

Heat in the East

We have talked about persistent areas of very high temperatures several times in past blog posts (for example, here and here). Those areas most commonly form in summer under stagnant areas of high pressure that have sinking motion in the middle of the high. The sinking motion of the air keeps clouds and rain from developing, leading to very hot and dry air being trapped near the earth’s surface, raising temperatures and reducing wind speeds. Often those areas also include a lot of humidity, which makes the temperatures more oppressive because sweating does not cool you off efficiently if the humidity is high, especially if winds are also light.

When this happens, the National Weather Service will put out heat advisories or other statements reminding people to be careful to keep cool and stay out of the hot sun as a way to prevent heat-related illnesses like heat stroke. This is also important for gardeners, who like to spend time outside tending their plants and gardens. You can monitor the atmospheric conditions associated with heat to identify times when it would be better to stay inside where it is cool by using the National Weather Service’s HeatRisk map or the Southeast Regional Climate Center’s Wet Bulb Globe Temperature forecast tool (which is for the entire country, not just the Southeast).

Note that heat domes don’t always occur in eastern parts of the country. A deadly heat dome occurred in the Pacific Northwest (PNW) in late June 2021, breaking numerous high temperature records and killing 136 people in Washington alone in the period from June 26 through July 6, 2021. In that case it was the West that was under unusually high temperatures while the Eastern U. S. was cooler than usual. They also occur in other parts of the world, and in fact this week Europe is also experiencing very high temperatures due to another region of high pressure parked over them.

It is also interesting to note that in winter, areas of high pressure are often the coldest areas of the country due to the lack of cloud cover, which allows heat from the earth to escape to space, leaving colder conditions at the surface at night when no sun is there to heat the ground.

Cold and snow in the West

On the other end of the country, cold air was trapped on the other side of the frontal boundary between the eastern high-pressure center and the low-pressure trough that was present in the western U. S. The cold air was so intense, especially at higher elevations, that snow fell in some mountainous areas of western Montana and surrounding states. The Going-to-the-Sun Road in Glacier National Park had to be closed from June 20 to June 23 due to the heavy snow.

What connects these two extremes together?

The common connection between the heat in the East and the snow in the West is the large-scale atmospheric wave that is linked to both the low pressure in the west and the high pressure in the East. The atmosphere is a fluid and is constantly adjusting its pressure fields by forming waves with ridges of high pressure as well as troughs of low pressure. Sometimes these patterns get locked in place for a few days (or even longer in rare cases), which leads to more extreme effects, but they usually move on in a few days, causing the weather to go back to normal conditions or even flip-flopping to the opposite pattern. In fact, we discussed atmospheric waves back in July 2021 following the PNW extreme heat event in this blog.

The surface weather associated with these areas of high and low pressure are what cause the big changes in observed conditions that gardeners and farmers have to deal with since they can have big impacts on flowers and crops. The atmospheric wave pattern can lock in place for a number of reasons including conditions in other parts of the earth such as unusually cold or warm water in the ocean or droughts or floods in other areas. Fortunately, these stationary patterns usually shift or break down after a few days, leading to big changes in local weather conditions that might be more welcome.

How do cold and hot outbreaks affect gardens?

Most garden plants are fairly resilient to the changes in temperature, humidity, winds, and cloudiness that come with the shifts in the atmospheric waves and their associated surface weather. 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 stoma 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. Of course, most gardeners are watering their gardens to avoid this worst case!

Cold spells in summer do not cause as much damage as frost and snow in spring and fall because they just don’t get cold enough to damage the plants (unless you are in the mountains and the temperatures drop below freezing), but they can slow the plants’ growth and reduce their flower or fruit production. Food crops that depend on a significant number of growing degree days (a measure of the accumulation of heat over time based on daily temperatures) will grow more slowly, resulting in late harvest of whatever crop is being grown. In the worst cases, it could delay harvest so late in fall that fall frosts become a consideration, but most home gardeners do not need to worry about this as much as commercial farmers because they are not farming hundreds of acres of crops, just their own patch of land.

I encourage you to visit some of the links in the article above to learn more about heat domes and atmospheric waves as well as their impact on your garden plants. You can also use the search field to find additional sources of information about any topic of gardening that you might want to learn more about, especially one that relates to the science of gardening. It’s a great resource!

I am traveling in Colorado this week, so my thoughts naturally turned towards the mountains. Mountains affect gardening in a number of ways, many of which include a weather or climate component. They also provide some special challenges for gardeners because of the harsh conditions and short growing seasons that are often found in and near mountainous terrain.

How the mountains affect weather and climate

It is said that mountains create their own weather, and there is a lot of truth to that. Mountains interact with the atmosphere in several ways, and that interaction can change both the atmosphere itself and the conditions on the surface of the mountain that is being affected by the air.

One way that mountains affect the atmosphere is by providing a physical barrier to the flow of air. Obviously, air can’t flow into a mountain slope, so it must either go up the slope, around the mountain crest, or through the valley between peaks. Air that is pushed up the mountain usually results in the formation of clouds on the upwind side, since rising air cools as it goes up and eventually the moisture in the air condenses to form cloud droplets, and eventually rain or snow, called “orographic precipitation”.

On the downwind side of the mountain, the air typically sinks and dries out, since the moisture has been wrung from the air by the rain or snow left on the upwind side. That can lead to a “rain shadow” effect with low precipitation amounts downwind of a single mountain or a range of mountains such as the Rockies. Flow over the mountains can also result in the development of lenticular clouds downwind as the air rises and sinks in waves produced as the wind moves over the peaks. You can see how rainfall affects climate across the United States at NOAA’s “The highs and lows of climate“.

If the air is diverted sideways around the mountain it can block the wind from hitting some locations downwind of the peak. Our fearless leader Linda told me she experienced this just a few weeks ago when a strong low-pressure center moved into the Northwest, bringing strong winds to the region. However, Linda noted that in her location, those winds were blocked by Mount Rainier, resulting in much lower local wind speeds due to the shelter from the massive mountain.

Wind flow up and down the mountains due to temperature variations

Mountains can heat or cool the air around them, depending on the time of day and the season. In summer, the peaks warm up and provide a heat source that helps lead to the formation of thunderstorms. You can see this almost every summer day in the western US with storms that develop over the mountains as the sun warms them. Those storms then move out over the prairies, leading to scattered rain or even virga, rain that evaporates before it falls to the ground. At night or in winter, the air near the peaks cools quickly and the denser air flows down the mountains into the valleys, resulting in katabatic winds that can cause freezes in low-lying areas when the coldest air reaches the lowest elevations. In this situation, the valley floors may experience frosts while areas on the slopes remain above freezing because the dense air drains through them relatively quickly. The winds can also increase evaporation rates, limiting the amount of available moisture and causing water stress on garden plants.

Temperature variation with elevation and orientation

The surrounding atmosphere also affects conditions on the mountain terrain. Atmospheric temperatures decrease with height, so as you go up in elevation on a mountain, the temperature will drop. This can lead to cooler climates and shorter growing seasons due to the increased likelihood of frost with the colder temperatures. This limits the types of plants that gardeners can grow because the climate of that location has limited suitability for plants that grow well on the flatlands.

Another aspect of mountainous terrain is the number of microclimates that are present in the rocky, uneven landscape. Mountaintops and sides offer a range of microclimates, from sunny, well-drained slopes to shady, cooler areas, influencing plant growth in different locations. North-facing slopes get little direct sunlight and can pool pockets of cold air that result in frosts every month of the year, while sunny south-facing slopes could be much warmer and more suitable for a variety of plants. Any mountain gardener has to be especially aware of the local microclimates in their area and account for them when choosing what and when to plant.

Of course, there are other characteristics of mountains that can also affect garden success. Soils are often shallow, rocky, and low in organic matter. Lower pressure and humidity may cause problems with plants’ ability to thrive in the harsher conditions. Sunlight can be very intense and shade from taller plants may be limited. Some areas may experience extensive periods of snow, which can provide insulation but can also damage plants when it slides downslope.

Gardening in the mountains can provide challenges for many gardeners due to the difficult environment that the mountain air provides, but it can also allow gardeners to create unique collections of alpine vegetation that can provide enjoyment for years to come, all set in a diverse and scenic landscape. Be sure to look for additional information on alpine or mountain gardening on the web to make sure your garden is well-suited to your local conditions.

In the last few weeks NOAA has declared the end of the weak La Niña that has been present in the eastern Pacific Ocean and the return to neutral conditions. I want to take time today to discuss what it means for our summer growing season in the United States. I will also provide some links to guidance for how it might affect conditions in other parts of the world for our non-US readers.

Why do we keep track of La Niña and El Niño?

We have discussed El Niño and La Niña (collectively called El Niño Southern Oscillation or ENSO) many times in this blog. You can read more details here, here and here, for example. Over the past winter, the Eastern Pacific Ocean has been colder than normal; this is considered a La Niña event. We were waiting for it all fall but it was not officially declared until early January, and it was considered a weak event with cold conditions that barely met the criteria to be called a La Niña. A couple of weeks ago NOAA noted that the ocean temperatures no longer met the threshold to be called a La Niña and declared that we are back to neutral conditions with ocean temperatures returning to near-normal conditions. Where it goes from here is still uncertain, but the prediction is that we will be in neutral conditions for most of the summer and perhaps all the way through the rest of 2025.

We track ENSO carefully because it is an internal oscillation of the atmosphere-ocean circulation that has big implications for variations in climate across the globe.  If we know what phase of ENSO we are in, we can make some statistical predictions of what the climate is likely to be at our locations as well as others around the world. This is because the ocean temperature is linked to vertical cloud growth, which can act like a rock in a river, diverting the high-elevation winds north or south. Those winds push around the storms that bring clouds and precipitation to the regions where they are located, so that gives us some knowledge of what weather we might generally expect to experience. Of course, each El Niño and La Niña are different because the atmosphere is constantly adjusting in relation to other factors like droughts and floods, heat spells and cold outbreaks, so every event is unique, but statistically there are known relationships that give us some confidence in how our coming climate for at least the next few months might be.

What do we expect this Northern Hemisphere summer?

The strongest statistical relationships between the ENSO phase and climate conditions occur in the winter months, so the climate pattern this summer will be affected in some ways by the neutral conditions but will be more affected by other factors. The biggest impact will come during the Atlantic tropical season, since neutral years are linked to higher than average numbers of named storms. And sure enough, this year the early seasonal forecasts are for more storms than usual. The number is likely to be a little lower than last year because the Gulf and the Atlantic Ocean are not as warm as they were last year, although they are still warmer than average. Tropical cyclones in the Eastern Pacific Ocean could be fewer than normal because that is typical in a neutral year, but the water there is pretty warm so that would likely lead to more storms.

NOAA’s seasonal forecasts

NOAA’s Climate Prediction Center has issued their 3-month forecasts for the early summer (May-July) and late summer (August-October). The maps are shown below. For the early summer period of May through July, most of the country is likely to be warmer than normal with the exception of the Northern Plains, which has equal chances of above, below, and near normal temperatures. Note that the depth of the color shows how high the probability of that condition is, not how hot it will be. The rainfall this summer is expected to be drier than normal in the western half of the United States except for southern Arizona and New Mexico, which has a chance of experiencing a wetter than normal monsoon. The northern Gulf Coast and the Eastern seaboard is all expected to be wetter than normal, at least in part because some of that rain will be coming from the tropical storms that are expected to occur during the hurricane season from June through November.

For later summer (August through October), all of the country is expected to be warmer than normal, with some areas more likely than others. Rainfall continues to show a wetter pattern than usual in the Southeast and in the monsoon region but is likely to be drier than normal in the northern Plains.

You might ask why the maps show a tendency towards above normal temperatures. That is mainly due to the rising temperatures associated with global warming. As temperatures continue to rise due to the greenhouse gases being emitted into the atmosphere, on the average each year will be a little warmer than the last, although of course there are year to year variations due to ENSO and other variations like ocean temperatures and drought that affect individual years’ statistics. But in the absence of detailed information about the coming year, it is a better bet that we will get a warmer than average year than a colder than average one.

How can gardeners use that information to plan this year’s gardens?

If you are just starting to plan or plant this year’s garden, you can use the information from the CPC to help determine what kinds of flowers and vegetables to put in. If you know that the summer is likely to be warmer than usual, then you can purchase plants that love the summer heat! Be prepared to water them, though, unless you are planting succulents or cacti. This will be especially true if you are also predicted to be drier than normal over the summer, because your gardens are likely to demand a lot of water if there is no rain to quench their thirst. You can help keep moisture in the soil by adding arborist chips as mulch over the surface of the soil to minimize the loss of soil moisture from the root zone.

Since neutral conditions, where neither El Niño nor La Niña are present, are associated with more tropical storm activity in the Atlantic, if you are living in an area that is normally affected by those storms, you should prepare ahead of time by storm-proofing your garden and home and watching the forecasts carefully once we start to get into the active season. It is never too early to prepare!

If you live in another part of the world, there are also climate patterns associated with La Niña and El Niño that you can use to make similar garden plans. You can find them at IRI – International Research Institute for Climate and Society | ENSO Resources and Global impacts of El Niño and La Niña | NOAA Climate.gov.

I hope you have a great growing season and enjoy the fruits and the scents and sights of your garden if you are in the Northern Hemisphere or planning for the next growing season if you are in the Southern Hemisphere far from the equator. We love to see your photos on our Facebook page!

There are many popular songs about fire. Those of you who are fans of Bruce Springsteen will recognize these lyrics from “Dancing in the Dark”. They popped into my head when I was driving home from Asheville NC to Athens GA this past weekend and noticed plumes of wildfires punctuating the air along the highway. That inspired me to write this post on wildfires, which are affecting the Southeast this spring but also affects many areas of the United States and the world too, especially when those areas are in drought. In this post I will discuss how wildfires start, how the local environment may help them spread, and what you can do to protect your properties and gardens from the impacts of wildfires in your communities.

Wildfires versus prescribed burns

Fires in the environment can be caused by natural events like lightning or can be sparked by human sources. Some fires are set on purpose to clear land and reduce fuel loads so wildfires are less likely to occur and some are caused by ignition sources like sparks from dragging chains, a carelessly tossed cigarette butt, or an untended campfire. Some are set deliberately to cause damage and chaos by arsonists or are the result of careless children or adults. According to Earth.org, “40% of wildfires that affect British Columbia in an average year are human-induced. In the US, the amount is more than double, with nearly 85% of the nearly 100,000 wildland fires that affect North America every year caused by human activities, according to data from the National Park Service.”

The fires that are set to reduce fuel loads and remove overgrowth from land are called “prescribed fires” and they are regulated by most states. Farmers sometimes use controlled burns to remove cover crops and prepare for spring planting. Those who want to set a prescribed fire usually have to file a form or follow a procedure to indicate what they are going to burn and when and what the weather was at the time of the burn. They are also expected to file all the necessary permits and notices for smoke and fire hazards. In some states you must be certified to conduct a controlled burn. If the weather is too windy or the humidity too low, they are generally not allowed because the chance of a fire getting out of control is high in those atmospheric conditions. There have been instances of prescribed fires escaping their planned burn areas or causing significant hazards, including a number of deadly multi-car accidents when the aerosols from the fire attracted enough water vapor to form a “superfog” that moved across a busy highway and caused visibility to fall to near-zero feet which blinded drivers speeding down the roads. “Superfog” is especially dangerous if it occurs overnight when the humidity is the highest which causes it to be more dense.

What are the causes of wildfires and how do they spread?

The number one cause of wildfires from natural causes is lightning strikes, especially in areas of drought when the vegetation is very dry and moisture is scarce. Volcanoes can sometimes cause fires by dropping hot embers onto flammable land cover and buildings. Strong winds can quickly spread the fires to new areas downwind and provide a source of oxygen that helps them continue to burn. In areas with a lot of fuel like drought-stricken national forests or open grasslands the fires can rage out of control and cover large areas in short amounts of time.

How to know when you are threatened by wildfire

When conditions are ripe for wildfires, government agencies such as the National Weather Service and state forestry departments will often put out watches and warnings to notify people in areas that are vulnerable to wildfires to be aware of threatening conditions and prepare to evacuate if necessary. If you live in an area that is prone to wildfires, you should make a plan for how to get out quickly and safely and share it with your family and colleagues. If an evacuation order is sent out you should be prepared to move quickly to safety carrying necessary documents, medicines, and valuable property with you in a “go bag” which is assembled ahead of time.

What you can do to your gardens and properties to minimize damage from wildfires

The best way to minimize damage from wildfires is to design your homes with fire-proof or at least fire-resistant materials. In the landscape around your home, plant appropriate plants and keep combustible material at least 30 feet from your home. Create horizontal and vertical breaks in the landscape to slow the spread of flames. Make sure that you have removed low-hanging dead branches and removed any dry shrubs, pine needles, dead grass, and vegetation. You can find additional information on creating fire-safe landscaping here and here. If you live in a fire-prone area you should also be prepared to stay and defend your home if it is too late to evacuate, but this should only be done if you have no other options. If you are able, evacuation is always the best option.

After the fire – other hazards

Hazards caused by fires continue even after the flames are out. Burned-out trees have lower strength and may be prone to dropping limbs or even falling over, creating hazards to anyone or anything beneath them. Ashes may have toxic components that can be carried long distances by the wind or by vehicle tires. In one of my favorite books, The Control of Nature, author John McPhee discusses the debris flows that can occur out west when heavy rain falls on recent fire scars, leading to the destruction of buildings and blockage or destruction of streets and other infrastructure by mud and boulders.

Wildfires are a hazard which can affect any place that has flammable vegetation. Take the time to understand what your risk from wildfire is, plan your landscaping accordingly and you will be better protected from this dangerous natural (and sometimes human-made) disaster!

“Put rocks in the bottom of pots for drainage” is one of the most pervasive gardening myths, because it makes intuitive sense (as discussed in this earlier post). Understanding the science behind capillary barriers (what gardeners call perched water tables) is not only more mentally satisfying than the faulty belief but it can help you avoid other gardening practices and products that inhibit water movement within the soil (see earlier posts here, here, here, and here).

Science is not static however, and new research can change our understanding of soil-water relations. A new article was recently published in PLOS One that contradicts the well-established science about layered soils and other media impeding drainage. It’s important to give these contradictory viewpoints careful scrutiny before any deciding whether they represent a true paradigm shift or if they are fundamentally flawed. This article falls into the second category, primarily due to flawed experimental design.

The methods sections of scientific articles are, unfortunately, the least exciting. My approach to reading scientific articles is to read the abstract first and then the conclusion. (Kind of like having dessert before the salad course.) Then I read the introduction and discussion and finally the methods and results. Getting the big picture first prepares me for the nitty gritty details of how the work was done.

Red flags appeared as I read the introductory section: the author compared research on textural barriers between different soil types with their research, which was organic potting media overlaying drainage material. Differences in adjacent soil textures cause perched water tables, or as soil engineers like to call them, capillary barriers. Their presence is a well-established fact. But even given the faulty comparison there is substantial research showing that organic matter can also create capillary breaks with the underlying soil. This means there is a very large body of literature that supports the presence of capillary barriers between soils of different textures and between soils and other materials.

Eventually I got into the methods section. While reading about how to dissect methodology is not the most exciting thing in the world, learning how to do it is exciting – you never know what you will find. Ideally, methodological errors are found during peer review but sadly peer-review is not always of the highest quality. In any case, a careful reading of the methods section of this article generates more questions than it answers.

Without going into excruciating detail about experimental design (there are textbooks written on this), it’s important to understand what you will find in a rigorously designed experiment. There will be an underlying hypothesis (or research question) to explore, a well-designed experimental methodology with sufficient replicates and controls, and appropriate and objective statistical analyses. Paramount to all of this is that variability must be controlled, which means all replicates must be treated identically except for the variables being studied (in this case different drainage materials and their overlying potting media).

Below are many of the problems I identified, along with a brief explanation of why.

• “For trials with sand, a piece of fibreglass mesh (16 x 18 mesh count, approximately 1 mm spacing) was placed over the drainage hole to prevent the sand escaping.” Mesh should have been placed in ALL treatments, regardless if they were needed or not. The presence or absence of mesh has now become an uncontrolled variable (an unforced error to use sports terminology).
• “For each potting medium, trials were conducted with a layer of each drainage substrate to depths of 30 mm and 60 mm…” Weights of the materials (drainage as well as media) should have been made for each treatment replicate so that there are no differences in material weights among the replicates for each treatment. Each drainage substrate should then be shaken to eliminate any large gaps before adding the media on top. Differences in weights of the drainage material is another uncontrolled variable.
• “For each of the three potting media, a baseline moisture level was defined according to the mass of a fixed volume of medium.” These baseline moisture levels need to be reported in the methods. Furthermore, it would have been better to use fully hydrated media, as the wetting time for the different media were likely different.
• “Each filled container was irrigated from above using a watering can with a rose until the water level reached the top of the pot.” There should have been a fixed volume of water added to each container. Differences in how much water was added is another uncontrolled variable.
• “The containers were monitored until water was no longer visibly draining, which took between one minute and three hours.” The differences in time is another uncontrolled variable, as they do not appear to have been used in any of the data analysis. Among the questions one could ask is if there were differences in drainage times within the same treatment?
• “The saturating and draining process caused all the potting media to compact.”  Given that the media compacted to different levels, this introduces another uncontrolled variable. A fully hydrated media could have been compacted uniformly. Furthermore, adding water to a dry medium results in an unknown amount of water running down the inside of the plastic containers (which do not bind water), which is a loss unrelated to the experimental goal. Media hydration time varies among media. Unintended water loss is another uncontrolled variable.
• “The compacted depth was measured from one representative sample for each medium with each depth of drainage substrate…” This is a fatal methodological flaw. The depth should have been measured for each container and the average should have been calculated statistically. Furthermore, how was the “representative sample” chosen?

I did read the supplemental files (linked at the end of the article) which I hoped would contain much of the missing information I noted above. They did not – and again I had more questions arise than were answered.

The article was supposedly about drainage – and I expected there to be data generated that looked at drainage times and water loss. If a fixed volume of water was added to each container of fully hydrated substrate, it would have been easy to measure water loss over time. (You can do this at home using university extension information, such as this website from Iowa State University).  Instead, the article focuses on water holding capacity -which really doesn’t look at whether or not a perched water table exists -and on developing models.

If you’ve made it this far through my post, congratulations! Here are the takeaways:

• There were numerous design flaws and methodological errors, which introduced uncontrolled variables and created high levels of uncertainty. The data cannot be analyzed statistically and any discussion of the results is pointless.
• This is a good example of insufficient peer review. Had this been submitted to discipline-specific journal (like a journal in the soil or compost sciences), many of the problems I found would have been flagged. But the journal is a general interest journal, meaning that peer reviewers may have had little to no expertise with the science.

The cautionary tale here is don’t be a victim of SSS – single study syndrome. One contradictory article does not dismiss decades of peer-reviewed research and publications unless it meets a very high bar, which this one does not.

It’s that time of year again (unless you live in the Southern Hemisphere). Gardeners everywhere north of the tropics are thinking about what to plant and when to plant it. Everyone has their own method for determining when to put seeds in the ground outdoors. Many of them are tied to particular calendar dates or to holidays like Easter or phases of the moon. A few are even tied to sayings handed down from grandparents. Some of these methods have some usefulness because the calendar is tied to the seasonal cycle, and so planting on a particular date may be climatologically linked to the average date of the last freeze in spring in that location. Others, like planting at a time related to Easter, for example, seem less useful because Easter is not on the same date each year and using something that can occur on any date between March 22 and April 25 to determine your planting date seems a lot riskier, especially in years when Easter is early. In this week’s blog we will look at the importance of monitoring your soil for temperature and moisture conditions to ensure that your seeds and young plants have the best chance of survival.

Soil characteristics

Every soil has a set of intrinsic characteristics that describes its texture, its acidity, and its organic and mineral composition. It can be further described by environmental qualities including water and air content that give more information about what is in the soil on a daily basis. All of these are important in determining the success of a plant that is placed into that soil. Two of these characteristics that are very important for good plant establishment are the soil temperature and moisture present in that soil sample. Think of it like the “soil weather”, if you will. Soil temperature and moisture can vary a lot over just a few days if a front comes through and drops rain over your garden. In other periods it may not change much from one day to the next especially when temperatures are cool and the sky is cloudy which minimizes the heating of direct sunlight. Sandy soils tend to dry out much quicker than soil with a lot of clay content. The temperature and moisture content turn out to be very important for germinating many seeds and allowing the new plants to grow strong.

How does temperature affect germination and plant growth?

Most seeds germinate at when the soil temperature reaches an optimum value for that particular seed type. Our own John Porter discussed how to use this property to start seedlings indoors in spring before the outdoor soil reaches the most favorable temperature a few years ago. But many people make the mistake of using air temperature to decide when to plant rather than soil temperature, because when it feels like spring, gardeners’ thought turn to getting seed into the ground as soon as they can. However, soil temperature often lags the air temperature early in the season and that can lead to seeds germinating very slowly or sometimes just rotting away before they can sprout. Often, it just pays to wait until the soil has warmed up to the most suitable temperature for the seeds you are planting to germinate because your seedlings will be more robust and will grow more quickly.

How to obtain the soil temperature

The best way to get a soil temperature measurement is to do it yourself. There are a number of different inexpensive soil thermometers available to measure the temperature in your own garden plot. You might even find that it varies quite a bit around your property depending on the shading and type of soil you have, just like the local microclimate does. But even if you don’t have a suitable thermometer, there are online sources of soil temperature that can give you a general sense of what the soil temperature is in your area. Even if it is not exactly the same as your own back yard, it is probably close enough for you to judge when it is time to plant.

The National Weather Service does not measure soil temperature and moisture at their local airport stations because it is not useful for transportation, but they do provide some soil temperature and moisture information from satellites and other networks’ measurements because it is useful for hydrology, including watching for floods and droughts. Other agencies also collect this type of information. Many states also have statewide mesonets, including the University of Georgia network that I direct, that measure both soil temperature and moisture as well as weather parameters because they are so important for agriculture. Other countries may also measure this information. Keep in mind that the soil temperatures at those individual stations may not reflect exactly what is going on in your backyard because of differences in soil, sunshine, and exposure, but it will give you a general sense of what the pattern of soil temperatures is.

Once you know what the soil temperature is, wait until you know it is going to reach that temperature reliably, not just plant the first day it reaches that temperature. Generally you need about five days of consistent soil temperatures at the target temperature with no forecast for colder weather returning before it is safe to plant. That will help assure you that the seeds are in the best growing environment they can be.

How is soil moisture important?

According to the American Meteorological Society’s Glossary of Meteorology, soil moisture is “the total amount of water, including the water vapor, in an unsaturated soil.” The amount of moisture in the soil is determined by both weather conditions but also the soil type and vegetation in the area. It can be different at the surface than deeper down in the soil where the root zone of the plants is since it takes time for precipitation to percolate down to the areas where the most roots grow.

Like soil temperature, many state mesonets measure soil moisture in some way, although not all states do. The map in the link shows the distribution of soil moisture measurements from direct measurements across the Lower 48 states. There are also a number of satellite-detected soil moisture images available. For most gardeners, a general sense of how wet or dry the soil is can be enough to plant successfully. If the soil is too dry, the seeds may not germinate successfully but just sit in the ground until conditions improve and then sprout if they are still viable and have not been eaten. If the soil is too wet, the oxygen content of the soil will be too low because the soil water has filled all the pores in the soil structure, suffocating the new roots of any seedlings that develop. The sweet spot is somewhere in between, with enough moisture to swell the seed and nurture the new seedling without clogging the soil pores with too much water.

It’s almost time!

Now that you know how to watch for the perfect conditions (or at least close enough!) for your seeds to grow, I hope you will try your hand at doing your own observations of soil temperature and moisture and see what difference it makes in giving your garden the best start. Happy planting!