The Garden Professors™

Last April 30, 2022, I wrote a post about what a third year of La Niña meant for gardens. La Niña, you might remember, is an atmosphere/ocean phenomenon driven by unusually cold water in the Eastern Pacific Ocean (EPO) that shifts the jet streams that steer storm systems around the world. In La Niña winters in North America, it is often linked statistically to colder and wetter than normal conditions in northern parts of the United States and north into Canada and warmer and drier conditions in the southern tier of states in the US. Just a couple of weeks ago, NOAA announced that the long La Niña has finally ended and that we are now in a neutral period. In last year’s post, I described the difference between La Niña and the opposite climate phase, El Niño, and how they affect climate conditions around the world. So now you might be wondering how this switch to neutral conditions and then potentially to an El Niño later in the year will affect your gardens in the coming growing season.

Elfen-Krokus (Crocus tommasinianus), AnRo0002, Commons Wikimedia

How good was last year’s forecast?

A discussion of how accurate the forecast for last winter was can be found in NOAA’s ENSO blog. As you can read, the temperature forecast was much better than the precipitation forecast (as it usually is). That is not surprising because many causes of rain and snow occur on very small spatial scales from a variety of physical processes that are not always well-captured in our current climate models. You can hear a more detailed discussion of historical ENSO patterns by David Zierden, the Florida State Climatologist, in this 15-minute video for the March 2023 Southeast Climate Monthly Webinar starting at minute 28:35).

As a result of the cold and snowy conditions in northern parts of the country, spring there has been delayed, and my friends in the Upper Midwest have only started to see daffodils and other early spring flowers, while in the Southeast, our azaleas and dogwoods are already in decline about a month earlier than usual (you can track this at the National Phenology Network website we’ve discussed before).

What happens next?

Now that La Niña has ended, we are in neutral conditions. That means it is difficult to predict the climate during the next few months because without La Niña (or El Niño) to give us statistical guidance on what climate to expect, almost anything can happen. The variations in climate in neutral seasons are caused by local variations, other interactions in climate on regional or global scales, and other factors that are not always well understood. In addition, we are in spring, which historically gives us the least trustworthy predictions for the coming seasons due to what is called the “spring predictability barrier.” That means that while we think we are likely to swing into an El Niño in the next few months, the atmosphere might have other ideas and could keep us in neutral conditions for quite a while before we switch to an El Niño.

This year, we are already getting signs in the Eastern Pacific Ocean that a switch to El Niño will come quickly. The water near the coast is already quite warm and the warm pool is starting to stretch out to the west. In other words, El Niño-like conditions are already present in the EPO, but have not lasted long enough yet for an official El Niño to be declared. That usually takes several months of monitoring to make sure this is not just a short-term change.

What do neutral and El Niño conditions mean for the Northern Hemisphere growing season?

Usually, ENSO conditions do not strongly affect the NH summer climate. That is because both La Niña and El Niño are strongest in the winter months and tend to weaken or disappear in the summer. Local conditions including soil moisture variations such as drought, ocean temperature variations, and local weather systems have a much bigger impact on growing season weather than ENSO does in most areas.

However, the ENSO state does have one strong influence. That is in the tropical activity in the Atlantic and Eastern Pacific Oceans. When neutral conditions or La Niña conditions occur, the jet stream aloft is weak and it is easier for tropical waves to develop into tropical storms and hurricanes, so neutral and La Niña seasons tend to be more active and have more named storms. When an El Niño occurs, the jet stream is unusually strong and that keeps tropical waves growing vertically into strong storms, so the number of tropical storms and hurricanes in the Atlantic is usually lower in years when an El Niño is occurring. Of course, it only takes one storm (Hurricane Andrew in 1982 was in an El Niño year) to cause tremendous damage if it hits somewhere vulnerable. In contrast, the storm activity in the EPO increases in El Niño years due to the pool of warm water there.

This year, the likely storm activity may be more tied than usual to the ENSO state. If we see a quick switch to El Niño after just a few months of neutral conditions, the Atlantic may be most active early in  the season, while storms later in the season will be suppressed. By comparison, in the western US where some moisture enters the country through EPO storm activity, you may see an increase in thunderstorms that are fed by the water vapor entering the country from the storms in the EPO.

What does this mean for your gardens?

If you live in an area that normally gets rain from Atlantic tropical activity, even if it is not from actual tropical cyclones or hurricanes, you can probably expect drier conditions this year, or at least more variable rainfall from less organized systems as more precipitation will be produced from local influences. El Niño does tend to delay the onset of the Southwest Monsoon, so that could be something to watch next year, although it may not have much impact this year. In Texas, the switch to neutral conditions means rain in April through June is more likely, which would be great for avoiding drought. If you live in an area that does not receive much moisture from the tropics, it will be hard to make a good forecast because the statistics just don’t give much guidance. We do know that globally, El Niño years tend to be very warm, so it is likely that 2023 may be one of the warmest, if not THE warmest, since global records began in 1880 due to the long-term rise in temperature from greenhouse warming. Warmer weather will mean a longer growing season, more hot days and nights, more humid conditions (unless a drought occurs), and more diseases and pests that thrive on the warmer conditions.

What do we expect next winter?

If you like to plan far ahead, you can see seasonal forecasts from NOAA’s Climate Prediction Center at Climate Prediction Center – Seasonal Outlook (noaa.gov). If you are not from the United States, your own country’s weather service may provide similar outlooks for your region. Here in the US, we are likely to see wetter and cooler conditions in the southern US as the jet stream steers storm systems over us (days will be cooler because of the clouds, but nights won’t necessarily be colder than usual since clouds trap nighttime heat). In northern states, warmer and drier conditions will be more likely, and that could mean an earlier start to the growing season next year. Now that something to look forward to if you are a gardener!

Tulipa “El Niño” 2015, Retired electrician, Commons Wikimedia.

“Dirty Dozen” is one of those short, alliterative phrases that’s easy to remember and fun to use. In today’s post, I’m applying it to garden products whose production or use can be damaging to the health of ecosystems, environments, and even humans. How many of these products are in your garden shed, or appear in ingredient lists of other products? Each short description below has one or more links to additional information. After you count them up, see how you rank on the Charlotte Scott Meme-O-Meter^TM

Product manufacture damages ecosystems

Peat moss. Peat moss bogs are slow-growing ecosystems that store vast amounts of carbon. Removing peat moss destroys these ecosystems (which can take centuries to regrow) and releases C0₂ into the atmosphere. Do a little experimentation with sustainably sourced or locally available crop residues and see if you can find a more environmentally friendly substitute.

Kelp. Kelp, or macroalgae, are the basis of intertidal and subtidal food webs. Removal of these plants creates underwater deserts where little life can be found. Restoration can be done, but it’s a slow and expensive process. Ask yourself why you think you need kelp and compare this to the facts.

Bat guano. Seems like a reasonable use of a waste product, right? Nope – and Dr. Jeff Gillman explained why in a post more than a decade ago. If you want a sustainably produced, manure-based fertilizer, you can find many options at garden centers.

Products whose use damages garden and landscape soils

Rubber mulch.  Recycling used tires is a good idea; grinding them up and putting them on top of your living soil, not so much. There are much better mulch choices out there.

Landscape fabric. Landscape fabric does not control weeds, nor is it permeable once installed. It’s a sheet mulch that restricts water and air movement between the soil and the atmosphere. Weeds will quite happily colonize the surface while the roots of desirable plant struggle for water and oxygen below the barrier.

Plastic mulch. There is nothing worse (see chart in this link) you can put on a living soil unless your intention is to kill everything under it. You may see it used in agricultural production, but that doesn’t make it a good choice for your gardens and landscapes. Again, there it a much better alternative to this and all other sheet mulches.

Epsom salt. Would you be so excited about buying this stuff if it was correctly labeled as magnesium sulfate? That’s all it is – an inorganic chemical. Despite its soothing name, Epsom salt is not a cure all for anything except a magnesium deficiency in the soil. Overuse can create nutrient imbalances in soils.

Products whose use damages plants and plant-associated microbes

Phosphate fertilizer. The most overused nutrient in home gardens and landscapes, and one that can cause iron deficiency in plants. The only time you should add anything containing phosphate – including compost or other rich organic material – is if you have a soil test indicating a deficiency.

• 

• 

DIY garden remedies. Self-proclaimed gardening experts come up with all kinds of home-made potions as safer alternatives to conventional fertilizers or pesticides. There are good reasons that both fertilizers and pesticides are regulated at state and/or national levels: it’s the only way you can know exactly what the active ingredients and when, where, and how to apply these chemicals. To follow some “chemical-free” recipe from the internet is playing Russian roulette in terms of collateral damage to soils and non-target organisms.

Wound dressings. Unfortunately, many gardeners don’t understand that plants and people respond differently to injury. While antibiotic dressing and bandages are good for healing our nicks and cuts, trees have a completely different response. Slathering black goo or paint over tree wounds is the last thing trees need to seal damage naturally.

Product whose use can be harmful to human health

PAM hydrogels. PAM (polyacrylamide) hydrogels have limited usefulness in home gardens and landscape – worse, they have to potential to injure people and pets. (There is also a list of references used in the linked article.) After these same materials are used in labs for gel electrophoresis (used for DNA analysis, for example), their disposal is generally regulated. No such regulations exist for using them in the landscape. Hydrogels based on starch or other natural polymers are fine – but avoid anything that contains “acrylamide” or “acrylate” on the label. Better yet, use a well-chosen mulch to absorb and release water to the soil.

Time to take the quiz!

All jokes aside, sustainability and gardening require constant adjustment and learning. You came here, you read through this list, and you are thinking critically about your practices.

Continue reading It’s all Greek to me…

If you’ve been following the national weather this week, you might be wondering if the groundhog has developed a split personality this year. Is winter over or are we in for six more weeks of cold? While the eastern half of the United States is feeling the effects of record-setting high temperatures and one of the earliest springs on record, the western U. S. is observing cold and snowy conditions all the way down into Southern California. I’ve heard reports of snow reaching all the way down to Tucson in southern Arizona.

What is an atmospheric wave?

Some of you may be wondering how we can have some tremendous differences between the eastern and western U. S. at the same time. The reason we see such differences is linked to the wavy pattern of the upper atmosphere, which helps direct warm or cold air into different regions. Today I will discuss atmospheric waves and how they affect the surface weather.

Since air is a fluid, it should not be surprising that it has waves in it. In fact, there are many different types of waves that can occur in the atmosphere, ranging from small-scale pressure waves downwind of mountain ranges to the largest planetary-scale oscillations in pressure. Sometimes you can see some of those smaller-scale waves in clouds occurring above you.

Where do atmospheric waves come from?

What ultimately drives the atmospheric circulation is the temperature difference between the equator which is warm due to direct sunlight and the poles which are cold because whatever little sun they get, especially in winter, is of low angle and there is not much solar energy reaching the surface to heat things up. This temperature difference causes a difference in atmospheric pressure that makes winds blow to try and equalize those differences.

Variations in land versus sea, the rotation of the planet, and differential heating from other causes like drought all contribute to the atmospheric developing waves that look like large swings in the pressure patterns at mid-levels in the atmosphere. Areas where the pressure is low are called troughs and areas where it is high are called ridges, like the patterns we see in topographic maps. Surface fronts are located near the boundary between troughs and ridges where the contrast in temperature and humidity is often the greatest.

How is the current wave pattern driving these big temperature swings?

This week there is a tremendous ridge of high pressure located in the eastern US while a very deep trough is located over the western part of the country. Warm air blowing up from the south under the ridge has brought record-setting high temperatures to many parts of the East, while cold air blowing in from the north behind a strong surface low pressure center has caused winter storm and blizzard warnings all the way south to the mountains near San Diego. Record-setting low temperature and wintry conditions are occurring in those areas.

The warm temperatures in the East are of special concern for gardeners since they have caused the first leaves and blossoms to occur as much as a month early in some locations, according to the National Phenology Network. Since the average frost date for those areas is a month or two later, the likelihood of damage to fruit crops is high since freezing temperatures could significantly reduce the production of fruit in gardens and orchards if they occur in the next month or two. It could also cause damage to many other flowering plants that are fooled into blooming early due to the unusual warmth.

Will atmospheric wave patterns change in a warmer world?

Is the unusual warmth due to global warming? This is a hotly debated topic (if you will excuse the pun) with both proponents and skeptics weighing in. Many scientists believe that as the arctic regions warm up faster than the equator (a process known as “Arctic amplification”), the atmospheric wave patterns will become more amplified, with both deeper troughs and stronger ridges occurring. That could mean more extremes in both hot and cold weather as these waves occur. Other scientists caution, however, that the computer models used to study the large-scale wave patterns don’t always agree with global warming as a root cause. If we do have generally warmer conditions that are punctuated by significant cold outflows our gardens will have to be able to survive the increasing variability of the weather, especially in winter and spring when there is still a significant pool of cold, dry air near the poles available to flow south into mid-latitudes.

Please feel free to share how this crazy weather has impacted your own garden. Are your trees blooming early, or has snow covered everything? If you are not in the United States, what do you see going on in your own neighborhood? We are interested to know!

It’s still too cold here in the Pacific Northwest to see much happening outside, so it seems a perfect time to write about something you can’t see anyway. That “something” is the movement of water and dissolved substances through two pathways: the xylem and the phloem. And before you roll your eyes and go watch TikTok videos, keep in mind that learning about these transport systems is critical to understanding how plants work and caring for them appropriately. To prevent brain overload, we’ll focus on how xylem works this month and tackle phloem next month.

• 

• 

Functional xylem is composed of dead, lignified cells connected into a series of tubes that move water one way – from the roots to the leaves. You can think about xylem like a giant straw sucking water out of the soil and moving it into the atmosphere. You’ll find dissolved substances in xylem water, such as soil minerals and root-stored compounds including growth regulators and sugars. Since this is a one-way highway, everything in the xylem ends up at the end of the straw, which is primarily the leaves. Most of the water dissipates into the atmosphere through the stomata (a process called evapotranspiration) and the dissolved substances are left behind.

I mentioned that sugars can be found in the xylem, which will confuse gardeners who correctly associate sugars moving through the phloem. That’s generally true except during late winter when some trees, most famously maples, will produce a sugary sap in the xylem. While the exact mechanism of sap production remains unclear, we know that the sugars are coming from storage sites in the trunk and require a freeze-thaw cycle to enter the one-way xylem highway.

While many people see this process as the plant “wasting” water, it is the only way that soil minerals can reach the leaves. In the summer, evapotranspiration lowers leaf temperature through evaporative cooling. Thus, doing anything to interfere with xylem function (like using antitranspirants) will have a long-term, negative effect on plant health. Likewise, anything in the soil that’s taken up by roots may end up in the leaves – for better or worse.

• 

• 

Gardeners need to think about this last caveat carefully. Plant species are highly variable in their abilities to regulate what goes into the xylem and what is left behind in the root tissue. Regulation is controlled by a barrier called the Casparian strip, which is a ring of living cells that require water (and its contents) to pass through their membranes to enter the xylem. You can think of the Casparian strip as a customs office at a country’s borders: some things are allowed in, and others are forbidden. Depending on how selective this border crossing is, soil contaminants can be left behind in the roots or carried through the plant. This is why it is so very, very, important to have your vegetable garden soils tested for heavy metals and other contaminants, and to take precautions if contaminants are found.

If you have fruit trees in your garden, then you may already be aware of the importance of winter cold for the development of blooms and fruit. Some fruit varieties like apples, cherries, peaches, and blueberries all need a certain number of hours of temperatures below 45 F to prepare the trees to bloom in the spring. Surprisingly, other trees you might not think of as fruit trees also need a period of cold conditions to produce a good crop, including pecans and olives. Many other garden plants also require cold periods to provide the best blooms, such as lilac and hydrangea. I miss seeing lilacs here in the Southeast because we just don’t get cold enough to meet their chill requirements, although new varieties that can flourish in warmer conditions are being developed.

Why do some plants need winter cold?

The cold conditions over winter cause the plants to go into dormancy. That helps protect the plants from harsh conditions over the winter. Once the plants experience the number of chill hours required for that plant, they are ready to begin the blooming process once the temperature warms up. If the fruit trees do not get enough hours of cold over the winter, they do not bloom well in the spring and blossoms may be delayed or bloom at irregular times or not bloom at all. Leaf emergence may also be affected. The potential yield of fruit is reduced due to the lack of a strong uniform bloom.

Scientists count the number of hours a tree or orchard is below 45 F and measure the accumulation of those hours as “chill hours.” Some publications call them “chilling hours” instead. They are usually accumulated starting around October 1 and go through the winter into the spring bloom period when bud break occurs. An alternate method for calculation only counts the hours between 45 and 32 F. Chill hours are different from cold hardiness, which is the lowest temperature that a tree or plant can tolerate without dying. Recent research has allowed scientists to develop more sophisticated methods that dynamically calculate “chill portions”—these calculators take into account variations in temperature over time and can reset the calculation if cold conditions return after a warm spell.

Picking the right fruit variety for your garden

Most types of fruit other than citrus and each variety has a preferred minimum number of chill hours for that variety to set a good fruit crop. If you live in the Southeast, you may be able to plant varieties of blueberries that require only a few hundred chill hours, where in the north you may be able to plant a more cold-hardy variety that requires over 1,000 chill hours for the best bloom. Because of the cold outbreak we had this December, some of our blueberry varieties are already blooming this year in the warmer weather we have had in January.

If you are planning to plant a fruit tree in your garden, you will want to pick a variety that matches the expected number of chill hours for your area. If you plant a variety that expects a lot of chill hours but your location gets only half that amount, you will not get very good yields from your trees since the plants won’t break dormancy correctly. But if you pick a variety that requires a low number of chill hours for your area, then the tree will be ready to bloom as soon as the chill hour requirement is met, which could put them at risk for a frost if they bloom too soon. So picking the right variety for your area is crucial! Most nurseries can provide the recommended number of chill hours for the fruit variety you pick.

In the future, as temperatures get warmer under the influence of greenhouse warming, most areas will see a decrease in the average accumulation of chill hours over time. We are seeing this at my weather network stations in Georgia and in many other states as well. If you plant something that has a long lifetime, you may want to plant varieties that require fewer chill hours than your current climatological average to make sure they will thrive in a warmer climate in the future.

Where can you find information about chill hours for your location?

For average numbers of chill hours for the United States, check out the map below. Keep in mind that your local microclimate will affect the number of chill hours your garden will receive.

Here are some sources of current chill hours for this year:

The Georgia Weather Network has a tabulation of stations with current accumulations since October 1 at https://weather.uga.edu/aemn/cgi-bin/AEMN.pl?site=AAAA&report=ci. Many other state agricultural weather networks provide similar information. You can find a list of many of them listed under the Partners tab at the National Mesonet Program website.

The Midwestern Regional Climate Center has a current map for the contiguous U.S. similar to the at https://mrcc.purdue.edu/VIP/indexChillHours.html. An interactive version based on GIS maps is at https://mrcc.purdue.edu/gismaps/vipstndata.htm.

AgroClimate has a chill hour calculator for Florida and Georgia at https://agroclimate.org/tools/Chill-Hours-Calculator/ which allows you to choose the chill hour model and the time period of interest.

Don’t forget to check out our archives!

If you are interested in this topic or have other questions about gardens and gardening, we encourage you to explore our archive of blog posts to see if you can find the answer to your questions. Here are a few of the previous stories about winter chill you might find interesting:

Chill hours and bud break in Christmas trees:

https://gardenprofessors.com/and-now-for-something-completely-different/

https://gardenprofessors.com/the-walking-dead-christmas-tree-edition/

Chill hours and lack of flowering in landscape plants like lilac:

https://gardenprofessors.com/why-doesnt-my-plant-flower-part-1/