The Garden Professors™

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/

Companion plants! Great, what a good idea. When you first hear the term and think about the concept it sounds great but there is a lot to not like about it. The term “companion plants”  implies that these plants are partners and they “enjoy” each other’s company.  The term is an anthropomorphism or overlaying of human qualities on non-human organisms.  A more appropriate term may be plant associates or plant associations, a term taken from plant ecology, which has more basis for use.

Plants naturally grow together in groups which are called plant communities. These plants evolved under certain climate, soil, and environmental conditions that allow them to live together in the same place. Coastal sage scrub, oak woodland, and juniper pinyon woodland are some common plant communities where I reside in Ventura County. All of the plants growing in these communities receive winter rainfall and summer drought (Mediterranean climate) to which they are adapted to grow in. Plants growing here either resist drought through specific plant adaptations such as reflective leaf surfaces (white sage), abundant trichomes (sycamore), microphylly (buckwheat), succulent water storage (agaves, yucca and other lily family bulb forming plants), and C-4 metabolism (grasses). Some plants avoid drought by growing in the rainy season, setting seed and then remaining dormant during hot dry weather. Plants can grow in this climate because they have the adaptations to do so.

Plants compete for resources and while doing so may provide a place for other plants to grow. Trees have an advantage over grasses because they can grow above, catching the sun and shade the grasses out. But shade may also provide a place for shade adapted plants to grow. Plants surviving challenges in a specific environment may end up growing together. Woody plants also provide perching birds a place to defecate and spread seeds. This is why unexpected things may grow under other plants. Shade may even be necessary for development of proper form. We have noticed in studying western hackberry (Celtis reticulata) that the tree has no apical control and will not develop into a tree shape when grown in full sun. When grown in shade apical control is present and the plant grows a straight trunk. Birds commonly eat hackberry fruit and likely disseminate it under the canopies of other plants. I don’t think the hackberry minds growing as a blob but its “companion” plants cause it to change form due to changes in light intensity.

Some plants live very closely with others. Mistletoe is a great example. Leafy mistletoe is a hemiparasite deriving its energy from sunlight of its own leaves but utilizing water and photosynthate from its host. Similarly there are free living plants such as Indian paint brush (a member of the Orobanchaceae) that are also hemiparasitic using their roots to extract benefit from neighboring plants. Holoparasites are true parasites deriving all their nourishment from their hosts, e.g., Dodder (Cuscuta spp.). Dwarf mistletoe is also holoparasitic as it largely lacks chlorophyll. These plants are always found on or near their hosts but it is hard to call them true “companions.” The plants clearly associate with each other and in some cases are detrimental as one of the plants stands to gain nothing from the interaction.

One popular example of “companion planting” is The Three Sisters (TTS) polyculture of corn, squash and beans. This agricultural system is said to be synergistic. Corn provides support for beans and shades the squash, and beans provide nitrogen fixed from the air for the other two members of the system. The system was “practiced” by indigenous Americans all across the continent. Soils, rainfall and climate are quite diverse across the United States, and I am sure that TTS agriculture had mixed success. It is an interesting thought that the human diet can be satisfied by these crops and likely the combination was more about ensuring sustained calories and nutrients for those who grew them. In one published study there was no increase in production when comparing TTS to mono-cultures of the component parts, nor was N increased in soil. This makes sense since it’s not available until the plant dies giving up its nitrogen to the next crop which is the basis of legume cover cropping. Continued use of the TTS system is a zero sum game as corn and squash will rapidly use all the nitrogen from the previous year’s legume crop.

Mutualism is the concept that interactions between two organisms benefits both. There are many examples of plants that have a mutual relationship with insects, birds, fungi and bacteria. I found no examples of plants that have mutual relationships with other plants, e.g., “companion plants”, common to the scientific literature. I thought this was unusual so I called a friend who is a plant ecologist and asked her the question. At first she was enthusiastic and pointed to non-plant-plant relationships. As I redirected her to only plant-plant interactions we could not identify anything. My suspicion is I have missed something important or we will discover one day that there are plants evolved to help one another but for now, it evades me.

There is no doubt that one plant can help another but it’s incidental and not a sign of a mutual relationship. Most plants evolved to grow in communities because the growing conditions are suited to all. Knowledgeable gardeners and landscape architects will group plants that grow well together. This is only common sense.
Understanding how plants grow in nature informs gardeners about adaptations and this in turn elevates the practice of horticulture.

References

Martinez, R.T. 2008. An evaluations of the productivity of the native American ‘Three Sisters’ agriculture system in northern Wisconsin. M.S. Thesis. University of Wisconsin-Stevens Point, College of Natural Resources.

Marsh, E. 2023. The Three Sisters of Indigenous American Agriculture. National Agricultural Library (USDA). https://www.nal.usda.gov/collections/stories/three-sisters

As we close out 2022, I thought I would 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.

What were the average climate conditions in 2022?

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 28. 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 2022 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 north central part of the country, where temperatures were colder than normal. This pattern is consistent with the La Niña that we have been experiencing for most of the year, although individual months did vary.

Precipitation was more variable, as the map of precipitation percent of normal below shows. The driest areas in California and the Central Plains are consistent with the extensive droughts that covered those parts of the country throughout the year. The eastern Coastal Plain also shows overall drier than normal conditions for the year as a whole but the timing of wet summer and dry fall caused a lot of problems for the farmers there. The wettest areas were New Mexico and Arizona (due to a vigorous monsoon), the Southern Appalachians and Mississippi, and the Florida Peninsula due to the heavy rain associated with Hurricane Ian.

What extremes did we see in 2022?

The averages show the overall conditions that occurred this year, but don’t begin to capture the extremes in temperature and precipitation that occurred. These extremes get washed out in the averaging process but are far more likely to cause serious impacts than deviations from normal conditions over the whole year. These extremes caused 15 billion-dollar disasters in the U. S. alone as of October 11, and I certainly expect that extreme events since then, including last week’s cold outbreak and snowstorm, will add to that number.

In 2022, we experienced a number of heat waves with record-setting temperatures, including unusual warmth in South America, Europe, Asia, and Australia, where their highest temperature ever recorded (50.7 C or 123.3 F) was tied in January, their peak summer month. The United Kingdom experienced their hottest year ever, including temperatures in excess of 40 C for the first time. Much of Asia was also very hot in 2022. In the United States, the Pacific Northwest saw heat waves in both August and October, with the Southwest experiencing blistering heat in September and the Northeast in August. By comparison, cold outbreaks occurred both in January and early February and in December, with an Arctic outbreak spreading southeast from Alaska down to southern Florida, bringing extensive freezing conditions that caused significant damage to citrus in Florida and Georgia, bursting water pipes, and a lake effect blizzard in Buffalo NY that eclipsed their previous record-setting snow event set just a month earlier.

Precipitation was just as variable, with floods and droughts occurring around the world this year. Some of the more notable flood events include the rainfall in Pakistan in August that put a third of their country underwater, the floods that destroyed the northern entry to Yellowstone Park in June, and the local flash floods that occurred in eastern Kentucky in summer and fall, and the southwest monsoon that began in June with precipitation 200-800% of normal, easing drought in that area. At the very end of the year, as I am writing this, an atmospheric river event on the West Coast is bringing heavy rain to areas of California that have been plagued by drought all year. That may provide some relief from the dry conditions going into 2023.

Many other areas of the world experienced significant droughts in 2022. They include an extreme drought that occurred over most of Europe, causing damage to many crops and limiting navigation on local rivers. This was also true in the United States, where the long-lasting drought in the central United States led to record low levels on the Mississippi River, stopping barge traffic that usually transports grain from the Midwest down to the Gulf of Mexico. Drought covered over half of the United States for many months in 2022, although it waxed and waned in some areas with the movement of rain-producing systems.

How about the tropics?

While La Niña usually means that the Atlantic tropical season is active, this year was oddly quiet between early July and the end of August, with no storms observed during this period for only the third time since 1950. But once the season resumed, we saw Hurricane Fiona (affecting Puerto Rico and Nova Scotia, although it stayed offshore for the continental eastern U. S.) and Hurricane Ian in September. Ian caused tremendous death and destruction to southwestern Florida as it crossed over the peninsula, dropping feet of rain before it moved into the Atlantic Ocean and then recurved west into South Carolina as a weakened storm. In November, Hurricane Nicole brought devastating coastal flooding to areas that were previously affected by Ian, although it caused less damage than Ian did. Damage from tropical systems was not confined to the tropics, however, as the remains of Typhoon Merbok hit the west coast of Alaska in September, causing significant coastal damage with its incredibly strong winds.

What does the past year teach gardeners?

Most of the United States as well as the rest of the world experienced a warmer climate in 2022, so gardeners will continue to need to choose plants that are appropriate for their warming climate zones. But they will also need to prepare for extreme conditions; devastation by individual storms as well as 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.

Thank you for another great year!

Finally, 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 2022 garden challenges (weather or otherwise) in the comments along with your plans for how you plan to address them in 2023 and beyond. I look forward to reading them! We will see you again in the New Year.