The Garden Professors™

We at the Garden Professors stress the importance of accurate diagnoses of plant “problems.” Often, problems aren’t due to pests or disease, and sometimes they aren’t problems at all. That got me thinking about a tree oddity I saw earlier this year when I was visiting my daughter in Walla Walla. Near her office at Whitman College stands a mature box elder (Acer negundo), whose lower crown has large swaths of pale yellow leaves (Figure 1). From a distance, one might think of several reasons these chlorotic leaves have developed: lower crown leaves and branches are routinely less productive than upper canopy leaves and as a result receive fewer resources from the tree. Lack of water in particular can cause early fall color change.

But that’s not what’s going on here. A closer examination of the leaves (Figure 2) also reveals a lumpy trunk (Figure 3) – and venturing around the trunk we find multiple trunks that have fused together (Figure 4). What become obvious through careful observation is that those pale yellow leaves are associated with the smaller diameter stems wrapped around the main trunk.

• 

• 

• 

Unfortunately, Whitman College does not have an online tree database for me to access, so I don’t know when this tree was planted or what variety it is. But I am fairly confident that it is Acer negundo f. auratum Schwer. (That “f.” stands for forma, which is synonymous with subspecies.) Anyway, this natural variation was recorded in 1893 in Gartenflora 42:202. More current references to this variant erroneously call it a cultivar (‘Auratum’). It’s identified as having golden-yellow leaves with smooth undersides.

Box elder is widely regarded as a fast-growing, weedy tree with challenged aesthetics. The auratum form, with its chlorotic leaves, is less vigorous and would be better suited for a college campus landscape. It’s reported to only reach 25’ at maturity, with its yellow leaves becoming nearly white in the summer.

And that life history characteristic is what spelled the near-demise of the originally planted tree. Varieties, subspecies, and cultivars are often grafted to species rootstock, which is a faster method of propagation than by seed. Careful management of this young tree would have included removal of suckers as they appeared at the base. Left unchecked, a vigorous sucker rapidly outgrew the scion, which then became embedded in the new, dominant trunk.

The take-home message here reflects the importance of onsite field diagnostics. In this case, a photo of a chlorotic leaf does not tell the whole story and would likely result in a misdiagnosis.

Happy holidays to you and your trees!

I am writing this post on Thanksgiving Day, and I can hear football in another part of the house while I sit here with cat in lap. Here in Georgia, we are still officially in the growing season, although that will end in the next few days since a cold front has pushed through today, ushering in much colder conditions that will result in temperatures in the mid-20s next week. It will be a while before we see snow, though, and in fact we only get it every couple of years in Georgia, so we may not get any snow at all this La Nina year, since La Nina winters are usually warmer and drier than usual in the Southeast. Many of my friends from more northern areas have already experienced killing freezes and even snow so today I want to talk about how snow on the ground affects your gardens.

What does the phrase “blanket of snow” mean?

When you hear the term “blanket of snow”, what does it mean to you? To me it means enough snow to completely cover the ground. Quora says it is a metaphor: “Everything is or was covered with snow thick enough to hide the actual objects and their shapes like a blanket would hide the objects it was covering.” Certainly, a thick enough covering of snow will mask the shapes of objects underneath it, just like the blanket on my bed hides the outlines of my legs. But I also think of a blanket as an insulator that keeps heat trapped underneath it and a blanket of snow can also do that for the ground beneath it. Snow also has weight, an early snowfall landing on autumn leaves can quickly strip a tree of its leaf cover if the snow is heavy enough to release the leaves from their branches. Linda has discussed damage to garden trees and shrubs from snow in a previous post.

When does snow form and fall?

Snow can come from several weather sources. In a previous blog post I discussed lake effect snow, and this week is likely to have some very big lake effect snows downwind of the Great Lakes (up to 5 feet!) because of the combination of very cold air with the record-setting high temperatures the Great Lakes are experiencing this year. Snow can also be caused by upward motion of moist air over elevated land, which becomes snow when it rises above the freezing level and drops precipitation on the mountains. For most people, snow comes when a large area of low pressure brings cold air into contact with warmer moist air at the surface. The dense cold air causes the lighter warmer air to rise over it. The air temperature drops as it goes up leading to bands of snow where the cold air and the moisture meet. If the cold air lags behind the surface low pressure, then it is unlikely to cool off the moist air enough to get snow and you may get a cold rain instead. This is the most likely way most of us will get snow if we don’t live near a large lake or mountain range.

How does snow insulate the ground?

Snow insulates the ground from the cold air above the snow by trapping air within the snow cover. This is not unlike how a down comforter works. The snowflakes fall against each other in random orientations that leave a lot of air between the individual flakes. The trapped air serves as a barrier between the really frigid air that is over the surface of the snow and the soil beneath it. In some cases, the snow is so effective at insulating the soil that the soil temperatures can be above freezing while the air above the snow cover is much colder. This is especially true when the skies overhead are clear because the top of the snow radiates thermal energy up to space very effectively when there is no cloud cover. If the soil stays above freezing, then pests and weeds will continue to live in that soil until a longer freezing spell comes along later in the winter when the ground is less protected by the snow. However, the insulation can protect from the desiccating effects of very cold, dry air on the plants that are waiting under the surface for spring temperatures to bring them back to life. A winter drought in the wheat fields of the Great Plains can lead to severe damage to the winter wheat crop since the lack of insulating snow cover leads to soil temperatures too low to sustain the wheat plants into the next growing season.

The amount of insulation snow cover provides depends on the density of the water in the snow cover. You can think of this as the ratio of snow depth to snow water equivalent, the depth of water that the snow would have if you melted it and measured the water volume that was left. Typically, the ratio of snow depth to water equivalent is roughly a 10 to 1 ratio—in other words, 10 inches of snow is equal to 1 inch of liquid water. But that varies widely depending on the temperature and weather conditions at which the snow occurred. A heavy lake effect snow near 32 F can be a ratio more like 6 to 1 while a really cold Arctic or high-altitude snow can be more like 15 or even 30 to one, resulting in a very fluffy snow that is dry and powdery with the snow crystals barely sticking to each other.

Different shapes of snowflakes affect snow cover density

The difference in the ratio of snow depth to water equivalent is due in part to the different shapes that snow crystals form depending on what the temperature and relative humidity are where they are forming in the atmosphere. The snowflakes that grow in the highest humidity levels and form near freezing are called dendrites. These are the typical snowflakes depicted on Christmas cards and in children’s books, with six-armed patterns that can be very ornate and beautiful. The dendrites usually cause the fluffiest snow covers because the edges of the flakes are rough and catch against each other, resulting in a lot of space between the flakes. Other shapes of snowflakes are denser and are also smoother, resulting in a closer packing of snow crystals that lead to a tighter snow cover. The longer the snow cover sits on the ground, the more dense it becomes as the rough edges of the snowflakes melt and smooth out, resulting in a tighter packing of the snow cover that becomes heavier and harder to shovel.

For gardeners, a blanket of snow can be not only a thing of beauty but a way of protecting your garden plants from the most extreme cold air. It can also be a way of providing moisture that will be needed by the growing plants in spring once the next growing season begins. So if you get snow, enjoy it as it covers your winter garden but also walk and drive safely if you have to go out in it. If you live in an area that does not get snow, enjoy the pictures that others post on social media and think about how a blanket of sparkling ice crystals might look in your garden.

Coming up in December: end of year summary

I plan to post a summary of the 2024 season in my December blog post. There is a lot to talk about this year, not just in the Southeast, and I hope to cover a good bit of it in that article. In the meantime, happy Thanksgiving to those of you in the United States and a happy Christmas and holiday season to all who celebrate it.

Ants are a very familiar and recognizable group of insects in our homes and gardens. Due to their common presence on or around our plants and garden pests, some people consider that they may be the cause of some of the issues that we see. For the most part, ants play important and diverse roles in their ecosystems and are usually beneficial to us in our garden settings. As with every situation, however, there are always exceptions.

Ants are in the family Formicidae, within the order Hymenoptera (making them relatives to bees and wasps). They are also eusocial, meaning that they share characteristic traits including a queen (though some species have multiple queens that peacefully share a nest) taking care of brood cooperatively (usually through workers), and reproductive division of labor (meaning that certain groups within a species play a role in reproduction while others do not). Eusocial insects are able to collect a large array of resources, store/share them within their colonies, and can have interesting and complex methods of communication through pheromones. Other eusocial insect groups include bees, wasps, and termites- though there are more eusocial ant species than all of these other groups combined with an estimated 12-20,000 species (and likely even more that are undescribed).

Ants also have very interesting and elaborate communication, movement, and mating behaviors. They send individuals to scout out ideal sources of food and nesting areas, and then use trail pheromones to navigate their way to these locations. When the mating season arrives, winged male and virgin female ants take nuptial flights and then go on to start new colonies. These newly mated queens store the sperm from these nuptial flights and will use these stores to selectively fertilize her eggs for the duration of her life.

Ants are omnivores, and feed on a variety of organic materials including fungi, nectar, seeds, plants, arthropods and other small animals (acting as predators or scavengers). Even though they do sometimes feed on plants, they rarely do enough physical damage to be very noticeable in most situations. With the exception being species of leafcutter ants (primarily found in tropical climates such as central and South America). These ants form complex societies and even farm their food. Like their name suggests, they cut leaves off plants and take them back to their large underground nests in which they cultivate and feed on the fungus that grows on these chewed leaves. They can take a significant amount of vegetation to accomplish this task, though plants often easily recover by producing new leafy vegetation. In temperate climates, we don’t need to worry about these ant species ruining our favorite ornamental tree or shrub.

There are also several ant species found in lots of different climates, which can be nuisance pests due to their nesting habits, behaviors and/or close proximity to humans – but I will not be discussing these ants in this post. The goal of this blog post is to discuss examples of common ants that can be found in our yards and gardens and what they might be up to. The Ants in our Plants, so to speak.  

Ants as Beneficial Garden Guests

Many ants are great generalist predators, especially when they work together! In fact, one of my first memorable entomological observations was a large white grub (the larva of a Scarab beetle) in my family’s lawn in Pakistan being swarmed by 40-50 ants that were working together to take down this sizeable opponent. I was 7 years old at the time, but I recall being endlessly fascinated by what I saw, and continued to observe the epic battle for nearly an hour.

If a large grub (or even slightly larger animals) didn’t stand much of a chance against a determined colony of ants, smaller soft-bodied arthropods would likely be no match! In fact, ants have been recognized as great biological control agents in agriculture, especially in tropical climates. That being said, they can also have a few behaviors that can make them detrimental to our agricultural (and garden) productivity, as I will explain in the next section.

Ants as companions to other insects

In some situations ants can act in a way that is contrary to our gardening goals by supporting, protecting, and partnering with other common pest insects. These are often referred to as ‘symbiotic relationships’ (where these organisms have a close association with one another, that may benefit one or both of these groups). You may be familiar with some of these associations, which often involve a honeydew producing insect (such as an aphid or a scale insect) being closely guarded and “farmed” by a group of ants.

The reason that several ant species associate with these honeydew producing insects is due to the fact that their waste (a sugary substance that is excreted from a diet rich in plant sugars referred to as honeydew) is an excellent nutrient rich food source. These ants will often “milk” these sap-sucking insects by manipulating their abdomens with their antennae in order to coax out more honeydew. Having such a great source of food in such close proximity also gives the ants an incentive to protect it – which means that these honeydew producing insects basically have ant bodyguards that can defend them from natural enemies (such as generalist predators and parasitoids). In some cases, ants will herd or move these sap-sucking insects to juicier plant tissues, and to safer locations. When these ants move to a new nesting site, they will bring aphid eggs with them in order to establish a new “herd”.

Seeing this in action can be quite a sight to behold. In fact, if you are seeing ants grouping around some of your garden plants in larger quantities, take a look to see if you can spot some of these honeydew producing pests as well.

This symbiotic relationship between ants and honeydew producing pests can also have significant economic and ecosystem impacts! As a meta-analysis by Anjos et al. (2022) demonstrated that although ants in various cropping systems can reduce the abundance of non-honeydew producing pests, their impact on honeydew-producing pests is reversed! This analysis showed a variety of instances in which ants decreased the number of natural enemies, and increase the abundance of these honeydew-producing pests (Anjos et al., 2022).

Ants in turfgrass

Since many ant species nest in complex underground colonies, they can move large portions of soil in order to create these dynamic living spaces. Ant nest mounds in locations where we don’t want to see them can often be a nuisance to us, and sometimes even detrimental to our plants. I receive calls about this in a turfgrass setting (especially if the turf isn’t very dense or competitive), where your plants are mowed low enough to make even smaller nests noticeable. Even though these ants feed on a variety of common turf pests like white grubs and cutworms, their nests can sometimes be unsightly. Although this usually isn’t a problem in home gardens (and raking small mounds, using a hose, and increasing your mowing height can be simple fixes which could cause the ants to relocate), in situations like golf courses where low mowing heights are an important component of play mechanics, this can be more of a problem.

As you would expect, larger ant nests that happen to be built in your home gardens can be an even larger problem, because piling large quantities of soil over turfgrass is not great for the health of the turf. Some of the ants that produce larger nests include Allegheny Mound Ants (found in the Atlantic Coast of the U.S.) which can build some large and very conspicuous nests that can be over a foot tall and multiple feet wide. Additionally, since ants like to locate their nest entrances in sunny locations, they can damage vegetation in close proximity that may be shading the nest entrance. They accomplish this by biting the plants and depositing formic acid into them and, when persistent enough, can even take down larger vegetation (such as trees) through a painstaking process of hundreds of these formic acid deposits (although this isn’t very common since they prefer to nest in more open spaces).

Ants on Peonies

If you grow peonies, you may have been waiting for this section of the Ants in Our Plants blog post, as you’ve likely seen ants on or around your peonies, especially around the flowers. This is another commonly observed mutualism that exists between some flowering plants and ants (where the flowers lure ants as a source of protection from other pests), the origins of which can be traced back to the Cretaceous Period. Peonies have extrafloral nectaries located on the base of their flower buds. These produce a honeydew-like material which is a rich source of sugars, lipids, and amino acids utilized by ants as a source of food. As ants track their way to these sugary food sources, they also protect the peonies from other flower-feeding insects such as thrips. These ants don’t harm your peonies at all so there is no need to worry about them: just marvel at this cool association between two different organisms next time you are enjoying your prized peonies.

Ants in Trees

Sometimes we can find ants in old tree cavities, and around logs and stumps. Although there are a variety of ant species that nest in these habitats, the group of ants most commonly seen in proximity to our home gardens are usually carpenter ants (Camponotus species). These ants primarily chew through dead wood, and create nesting sites in imperfections and cavities of older, often damaged and deteriorating hardwood trees. They excavate smooth nests within these cavities, and you can sometimes see a characteristic pile of sawdust around the entrance. In some situations these ants can be a structural pest in homes, especially if you have water damaged wooden structures (since damp wood is easier to chew).

They play an important role in nature by helping to break down dead and decomposing wood and cycling nutrients. In our ornamental trees, significant damage is rare and often indicates that a tree has other significant damage that is more of a concern than the ants themselves. Therefore, treatment is not usually recommended for the trees themselves, though some may choose to treat if the trees are located in close proximity to wooden structures that can potentially be damaged by these ants.

I’m hoping that this post illuminated some of the interesting and diverse roles that ants play in and around our gardens. More than anything, though, I hope that this inspires you to be observant and to go out and explore some of these interesting and complex associations between insects and plants that we can often see in our very own backyards!

Resources

Ants in Agricultural Systems
https://www.sciencedirect.com/science/article/abs/pii/S1049964420306058
https://royalsocietypublishing.org/doi/10.1098/rspb.2022.1316

Farmer Ants and their Aphid Herds
https://www.mcgill.ca/oss/article/did-you-know/farmer-ants-and-their-aphid-herds

Mound Ants
https://www.canr.msu.edu/news/mound_ants

Ants on Peonies
https://ipm.missouri.edu/MEG/2018/5/antsOnPeonies/

Carpenter Ants
https://www.extension.iastate.edu/news/2009/jun/060503.htm

I thought this would be a fairly straightforward individual to research and write about. But no, talk about a rabbit hole.
Have you ever known someone that makes you ask, “They did what?” or “They’re where?” or “What are they up to now?”
If so, then you’ll recognize the type in this installment of People and Plants.

Friedrich/Frederick Adolph Wislizenus  was a German-born American medical doctor, explorer and botanist. The name Wislizenus traces its etymological roots back to Poland, specifically the town of Wislicza.
Friedrich A. Wislizenus is known for his printed recollections of travels to New Mexico territory and northern Mexico. He was born in Königsee, Germany on May 21, 1810, the youngest of three. The children were orphaned very young and a maternal uncle took them in. After completing “high school” at the Rudolstadt Gymnasium, Friedrich continued his studies at the Universities of Jena, Goettingen and Würzburg. In April 1833 he took a break from his schoolwork by actively participating in a revolutionary uprising against the existing German government. (Another participant of this event was Ferdinand Lindheimer.) The expected groundswell of support from the citizenry didn’t happen while a strong police presence did. In response Friedrich suddenly decided he should leave town.

He wound up in Zürich, Switzerland where he matriculated at the University of Zürich. His studies again suffered a brief interruption when he joined a movement to free Italy from monarchical rule. The expedition was disarmed by Swiss troops before reaching the border. There was nothing for our Friedrich to do but go back to the college from which he graduated in 1834 with high honors in Medicine.
Thereafter he decided to move to Paris to work but found no opportunities, so in the fall of 1834 he sailed for New York to hopefully establish himself in his profession.
By 1837 he had journeyed to Mascoutah, Illinois on the outskirts of St. Louis and was practicing his trade. Finding the practise of a country doctor’s life dull, monotonous, and lacking in remuneration, he relocated to St. Louis in 1839. 

Before settling down to resume his medical career Friedrich decided to make an extended journey into the farthest West. Scraping together his meager savings, he left the banks of the Mississippi in the spring of 1839 as a member of one of the annual expeditions of the Rocky Mountain Fur Company. His travels lead him into the far Northwest to the Wind River Mountains and then over the Rockies. Friedrich eventually reached Fort Hall on the Snake River, near the present site of Pocatello, Idaho, which at the time was the Southernmost trading post of the English who still held Oregon Territory in I839.
That’s a lot of ground to cover in six months.

From Ft. Hall he intended to cross the Sierra Nevadas and wander into California. But this dream was shattered for the lack of a competent guide. So he ventured back along the banks of the Arkansas River to the Missouri border. This voyage proved to be quite an adventure due to the inability of finding facilities, supplies, and the lack of equipment. He finally reached St. Louis in 1840. His record of this journey of almost six months duration was first printed in German in 1840. His son later translated it into English and published it along with a biographical sketch in I9I2. Its observations on the flora and fauna are quite detailed and the Linneaus binomial scientific designations are often included. Topography and geologic findings are also noted along with meteorological reports.

Friedrich resumed his medical practice and soon became involved in various civic activities. He was a regular attendant at the Western Academy of Natural Sciences where he found a kindred spirit in Dr. George Engelmann. The two were to become life long friends to the extent of taking care of the other’s patients when one of them was out of town.
In 1846, finding he could no longer ignore his “itchy feet,” he joined a merchant expedition to Santa Fe in New Mexico Territory. With a goal in mind and the necessary instruments in a horse-drawn wagon, he and an assistant intended to gather information concerning a part of the continent about which little was known.

Upon reaching Santa Fe the intrepid band found the headlines and town chatter were full of news of the Mexican-American War. Despite the situation the group determined to travel south, cross the border, and make it to the state of Chihuahua by September, which they did. Immediately upon reaching their destination the entire company was imprisoned. Not being a man to waste an opportunity, Friedrich put the several months in a secluded mountain village to good use. The enforced stay resulted in collection of notes, observations and sketches concerning the flora, fauna, topography, and weather of northern Mexico. Finally in the spring of 1847 the prisoners were freed by Colonel Alexander Doniphan. Friedrich accepted Colonel Doniphan’s offer of a temporary appointment as surgeon in the U. S. Army and continued with the soldiery to the mouth of the Rio Grande. He then returned to St. Louis via New Orleans before 1847 had ended.

Due to the efforts of Senator Thomas H. Benton, whom he’d become acquainted with, Wislizenus was summoned to Washington, D.C., and requested to publish his recollections, Memoir of a Tour to Northern Mexico in 1846 and 1847 by A. Wislizenus, M.D. This volume corrected many erroneous views of the western country and provided detailed descriptions (with maps and sketches) of the lands near the Rio Grande. The Senate ordered printing of 5,000 copies, which was a lot for the time, for distribution.

Among the trophies brought from his travels were many new plants, both as samples and sketches. These were later studied by his good friend and colleague Dr. George Engelmann who named many of the specimens after Wislizenus.
Dr. Wislizenus also has a lizard named after him. The animal was first documented by Friedrich in Santa Fe, New Mexico Territory.

The next several years saw Friedrich getting married, becoming a father and traveling the world, including Turkey, a visit to his hometown, Panama and finally getting to visit the West Coast of America. He returned to St. Louis in 1852 and spent the rest of his life there. He pursued scientific and civic interests, being one of the founders of the St. Louis Academy of Science and the Missouri Historical Society. He continued to indulge his love of meteorology and botany as long as his failing eyesight would allow. He died in September 1889 and he and his wife are buried on their estate near Kimmswick, Missouri on a high bluff overlooking the Mississippi River.

Dr. Wislizenus’ writings are available here:
https://www.biodiversitylibrary.org/creator/253456#/titles

To see list of plants attributed to Dr. Wislizenus go here:
https://tropicos.org/specimen/Search and type Wislizenus in the Senior Collector box and click on Search.

If you find yourself singing “How Dry I Am” these days, you might be under the influence of a large, stationary area of high atmospheric pressure. This past month, most of the eastern United States has been trapped in this pattern, with warm temperatures, clear skies, and no rain at all for most of the past month. In fact, many cities are on the verge of setting new records for low or no precipitation for the month of October, and some may even break records for the driest month ever. In this post, I will discuss what high pressure is, how it affects precipitation and health, and what impacts these may have on your gardens.

What is atmospheric pressure?

Pressure is a measure of the force of the atmosphere pushing down on the surface of the earth. This is caused by the weight of the air above that surface. As you go up in the atmosphere in an airplane or as you climb a mountain, the amount of air that is above you decreases and the pressure goes down. Pressure also changes horizontally as differences in the temperature around you cause variations in the density of the air, leading to areas of high pressure where the air is more dense and low pressure where it is less dense. Wind is just the movement of air in response to differences in pressure as the atmosphere tries to equalize pressure everywhere horizontally. In the vertical, the pressure gradient pulling air up is balanced by gravity pulling the air down. The wind at the surface is controlled not only by the variations in pressure at the surface but also by friction and the revolution of the earth around its axis, which diverts moving air to the right of the original movement in the Northern Hemisphere and to the left in the Southern Hemisphere (we call that the Coriolis force). Because of the combination of these forces, air tends to spiral into the center of the lowest pressure and spiral out of the center of the highest pressure.

How does atmospheric pressure relate to precipitation?

As the air moves into the center of low pressure, it meets in the middle and rises up, since it cannot go down into the earth. Rising air cools off as the pressure decreases and eventually the water vapor in the air condenses and forms clouds and sometimes precipitation. In the middle of high pressure areas, the air sinks, leading to air that is heating up and evaporating any clouds that might have formed elsewhere. Skies may be bright blue due to the lack of water vapor in the descending air. A complete circulation is formed when air at the surface of the high moves out away from the center towards areas of low pressure, then rises up and spreads out away from the surface low, moving into the top of the high pressure column and sinking towards the surface. You can see this schematically in the diagram below.

When an area of high pressure persists over an area for a long time, the sinking air leads to persistent sunny skies and low humidity, making it very difficult for clouds and rain to form. In summer, these long-lasting high pressure centers can lead to heat domes and oppressive and dangerous conditions such as occurred in the Pacific Northwest a few years ago. In the winter, high pressure is often associated with cold outbreaks of frigid, dry and dense air moving south from the Arctic (in the Northern Hemisphere). Those conditions lead to freezes caused by temperatures that fall to 32 F (0 C) or lower due to low winds and no clouds to trap the heat near the surface.

This fall has been a textbook case of a strong high pressure that has parked over the eastern half of the United States, causing day after day of warm and sunny weather and almost no rain at all. I often think that a drought is just too many days in a row of nice weather (assuming you prefer sunshine, which not everyone does). Fall is the time of year when we tend to have the longest periods of dry weather in the Southeast, but the dry spell does not usually last as long as it has done this year. If you enjoy warm and quiet weather (or sometimes cooler and less humid weather if some dry air has moved in from the north), fall is the time for you to really enjoy your garden before it shuts down for the winter. The dry conditions are good for many farmers too, because the harvest of commodity crops like cotton are easier when the plants dry up. Grape growers also appreciate the dry conditions because it concentrates the sugar in the grapes, leading to more tasty wines. Large-scale areas of high pressure around 30 degrees North and South latitudes are related to the formation of deserts like the Sahara and the Desert Southwest in the United States because the sinking air prevents the occurrence of precipitation, although it can rain there too when the monsoon blows moist air into the region and heat makes it rise to form clouds and rain.

How does high pressure relate to health?

Air pressure variations can lead to health issues in some people. Most people seem to be affected by low pressure, which can cause sinuses and ears to hurt and bother some people’s joints. Changes from high to low pressure associated with the movement of cold fronts also bother many people with arthritis and can cause headaches in some sensitive people. In high pressure areas, most people feel more alive and active. It is hard to know how much of that is due to the pressure and how much is due to the persistent sunshine and lack of rain, though. Interestingly, there is some evidence that more women give birth when the air pressure is low, especially when there is a big change due to an incoming tropical cyclone.

Sinuses, inversions and trapping of aerosols

Air quality also suffers under high pressure because the sinking air traps pollutants near the surface of the earth. This can include soot, smog, and even pollen. The temperature of the air above the surface often rises with height before cooling off as you go higher in the atmosphere. We call this an inversion because we normally expect temperature to decrease as you rise away from the earth, and instead it rises over a layer near the ground before cooling off above that layer. Farmers spraying some herbicides are not allowed to spray when there is an inversion because it can lead to concentration of the chemical near the ground and drift into neighbors’ fields, causing damage to the plants there. This can also happen in home gardens if you have a neighbor that uses a lot of chemical sprays in his or her yard. In the worst cases, a strong and persistent inversion can lead to dangerous levels of pollutants that can cause harm to people with lung conditions like asthma. Smoke from wildfires can also get trapped under the inversion, adding to the pollutant load.

How you can plan for high pressure impacts

If you know that high pressure is forecast for your area, you can use that information to plan for the kind of weather you are likely to experience while it is in place. If it persists over the area, it will be dry and you may have to increase your watering. Plants may get dustier and could be affected by trapped pollution and chemical treatments, leading to spotting or discoloration of the plants and eventually death as the pollutants affect the soil. If you need to work outdoors during persistent high pressure, make sure you monitor the air quality levels so you do not irritate your lungs or dry them out in the low humidity of the sinking air.

Enjoy the days of high pressure as you work in your gardens in the sunny conditions. Low pressure will soon come and bring rain and clouds. We need both to keep our gardens and our gardeners happy!

While the onset of autumnal leaf color change reminds us that winter is coming, there are many other reasons why leaves turn red. Knowing why and how leaves turn red is key in accurate diagnosis.

These are examples of leaf reddening misdiagnosed as phosphorus deficiency:

• 

• 

• 

These leaves are not phosphorus deficient, either.

Congratulations to accurate diagnosticians at UCANR! This is indeed phophate deficiency. Damage is not localized as in the previous images.

Here are some other underlying causes behind leaf reddening.

If you’re interested in learning more about how and why leaves turn read, be sure to download my most recent factsheet, appropriately titled “Why do leaves turn red?” ] It’s peer-reviewed and relevant to any part of the world.

Enjoy this Halloween treat!

For those of us in the Southeastern United States, this past week has been a whirlwind of preparation for Hurricane Helene, followed by the terrifying storm itself and now, for some people, months of clean-up and houses, yards, and gardens that may never be the same. My post this week (I did not get to it last week because of the impending storm) will be about how to prepare for an extreme weather event, including where to find accurate and timely information on weather forecasts and how to prepare your house and garden for the extreme weather you may suffer. While we just went through a hurricane, this could apply equally to an ice storm or blizzard, derecho, tornado, or any rapidly occurring weather event. No matter where you live, you will experience extreme weather at some point and need to be ready for it. My post from last month, https://gardenprofessors.com/how-would-your-garden-handle-a-12-inch-rainfall/, was surprisingly prescient, since Helene also brought extreme rainfall to the Southeast, particularly to western North Carolina and Virginia and to the area around Atlanta, GA. Our hearts go out to all of the people affected by Helene and other serious weather disasters.

Preparing your property and garden for damaging wind and rain conditions

The best way to deal with an extreme weather event is to prepare for it well ahead of time. I used to play a game with my son when he was young: “Name all the ways that something could go wrong (in his case, what could make a train or airplane crash).” It is a surprisingly effective way to make you think about possible dangers that might lurk around your home if a severe weather event occurs and what you might have to do to minimize them. If you can identify the risks of an event, then you can address them before the roof starts to blow off and the trees start falling.

The first step is to look around the outside of your house for any potential threats to your property that could become safety issues in strong winds or heavy rains. If you have trees, are they healthy? Are there any low-lying or diseased branches that could come loose in a strong wind and hit your house or car? Could a 50-mph wind from the southwest (or whatever direction your storms usually come from) blow them into your windows or onto your roof? Are there any areas on the outside of your house that are in poor repair and in danger of failing in a storm such as a loose railing or fence? If you like decorative items in your garden like garden gnomes or shiny balls, can you remove them before a storm approaches so that they don’t become wind-borne missiles? You should also consider potential areas where erosion might be enhanced by the ground cover or slope and determine if there are ways to slow the flow of water to preserve your soil and garden design.

Once you identify potential problems in your yard, you can repair them before a storm comes. But you should also make sure that you have adequate insurance coverage and that you document what you have before a storm hits with an inventory or video recording. You might also need special insurance to cover extreme events; I read today that less than 1% of all residents of western North Carolina had flood insurance, even though many of them live in flood-prone areas. Floods are not covered by most homeowners insurance policies. Of course, there could be many reasons for not buying it, including high costs, but if an extreme event occurs, you are going to wish you had it.

Identifying location concerns that could affect your safety

No matter where you live, there are bound to be risks due to the location of your property. If you live on top of a hill, you are prone to experience more lightning and higher winds and potentially more snow since the temperatures at higher elevations are generally lower. If you live in a river valley, your property and gardens would be more vulnerable to floods but also to freezes since cold air drains downslope to low-lying areas. When you decide where to live, consider the land the house and garden are on and also how you get there, whether you have to cross rivers or go through areas that are heavily wooded, which could be a problem in high winds but also potentially in a wildfire. Where you get your water could also be a concern if your well has an electric pump and power is likely to go out.

Get ready to go or stay

Preparing for an extreme event means planning for providing food, shelter, and water for your family and pets if you plan to stay and planning for an evacuation if it is not safe to stay. There is a lot of information out there about this planning process and I am not going to list everything here but you will find some links at the end of this blog that you may find helpful. As I write this almost a week after Helene came through the Southeast, there are many people here whose power is still out and who may not be able to travel because of all the trees and power lines that are down in their neighborhood and who have no access to clean water because the water plants were destroyed or the electric pumps on their wells no longer work. It usually takes about 72 hours for outside help to start getting supplies into affected areas, so you need to plan for several days and perhaps much longer to provide everything you need. Eventually, help will come from a variety of federal, state, and local agencies and non-profits but often the first help comes from a neighbor with a chain saw or extra water. If you evacuate, you will need a plan to get out safely, including more than one possible evacuation route, plenty of gas for your car, copies of important documents, and a place to go, which may include housing for pets or livestock.

Getting the weather information you need to make good decisions

Once you have your plan for severe weather, you need to monitor the situation for potentially threatening situations. A smartphone weather app is an easy way to get your daily weather for regular planning purposes but is not adequate for situations when weather might be changing rapidly since the apps are often updated only once or twice a day. In severe weather, an old forecast is a bad forecast because things may be quickly changing and you need the latest information.

There are a number of apps and websites that you can use to monitor extreme weather events, including the National Weather Service local forecasts for frequently updated information, watches for potential severe weather, and warnings for when it is spotted. The National Hurricane Center is the official source of hurricane and tropical storm warnings and their maps are updated every three hours when a storm is present. Beware apocalyptic posts on social media that often show a single worst-case model run that has practically no chance of occurrence just to gain attention and clicks. Stick to expert guidance from trusted sources. You can also get information on what local conditions you might experience and the timing of wind and rain using the hourly weather forecast from the National Weather Service, although in a rapidly changing extreme weather event even these may not change quick enough to capture the evolution of the storm. Weather radios and some smartphone apps that are designed can provide information about watches and warnings based on your location. If you live in an area that is prone to flooding or severe weather or is in a floodplain or near the coast of the ocean, then you should pay careful attention to rain in the area upstream of you are or the likely storm surge from a tropical cyclone. Be prepared to evacuate if emergency managers ask you to.

Planning ahead can save lives

Extreme weather is something that we all need to plan for but that planning often falls by the wayside because of other pressing tasks. But if you can take the time to do some simple advance planning and be prepared to act when the event starts, you can save yourself a lot of work later on when the clean-up begins and may save your houses and gardens. You may even save your family from serious consequences! I hope that it never becomes an issue for you, but I also know that many communities, businesses, farms, and families are dealing with the consequences of extreme weather now. Thanks to help from many people, they are starting to rebuild their lives, but it will be a long time before things get back to normal and for some, it may never be the same.

Some useful links:

Georgia Department of Public Health: Severe Weather Preparedness

Resident’s Handbook to Prepare for Natural Hazards in Georgia (good for other locations too)

Bankrate: How to Create a Home Inventory

FEMA: Hurricane Preparedness and Evacuation Planning

I often feel slightly nauseous after a day of debunking misinformation online, in emails, and in person. Others who selflessly give their time and energy to the same efforts probably feel the same. An antidote counteracts poison; in a very real sense, those of us who guide gardeners through the six circles of horticultural hell are routinely exposed to the mind-numbing dregs of lazy thinking.

What are the six circles of horticultural hell, you may ask? After playing with word lists and acronyms I have come up with the ABSURD approach, as in “don’t be ABSURD with your gardening information.”

This is a first draft of ABSURDity and I imagine it might get tweaked and shaped a bit. But it’s a good mnemonic device for educators to consider using, right along with the CRAP test.

A = anecdotal. Anecdotal evidence is simply one person’s observations that are not supported with scientific evidence. Reporting that your roses grew better when you used compost tea is an anecdote. Anecdotes are often collected by advertisers and called “testimonials” which sounds vaguely legal and therefore more reliable.

B = bogus. Bogus information is verifiably false; factual evidence exists to disprove it. Claiming that water droplets will scorch leaves on hot days is bogus.

S = scam. Scammy sources of information exist to sell stuff. Websites selling seeds for nonexistent flowers whose pictures are generated by AI are scammy.

U = useless. Useless information promotes something that has no effect. Adding eggshells to gardens for any purpose is useless.

R = ridiculous. Ridiculous recommendations defy even common sense. Placing plastic forks into the soil to discourage animals from digging is ridiculous.

D = dangerous. Dangerous products and practices can injure people, pets, and the environment. Putting mothballs in your landscape to discourage nuisance wildlife is dangerous.

You can find many more examples of ABSURDities in our 15 years of blog archives. Simply type in the word you are looking for and have fun diving down the (mothball-free) rabbit holes!

Early in August, Hurricane Debby made landfall in the Big Bend of Florida’s Gulf Coast, crossed into Georgia east of Valdosta, and moved leisurely northeast. The remains of Debby crossed the coast, briefly moved over the Atlantic Ocean before turning northwest and making another landfall northeast of Charleston then headed north into New England. While the winds decreased quickly once Debby made landfall, it dropped huge amounts of rain along its path. The highest official rainfall amounts topped 12 inches, but I heard of some unofficial measurements of 20 inches in a few isolated locations. Pictures of erosion in peanut fields and standing water in cotton stands filled my inbox, and it made me wonder how a garden would cope with so much precipitation in such a short period of time. I want to take a few minutes today to discuss it.

What happens as the rain is falling?

Tropical rainfall (or rain from any heavy thunderstorm) contains a large volume of water in droplets falling from a height of several miles. Generally, the raindrops are large and fall in a shape that looks more like a hamburger than the typical teardrop we often see in pictures. As the drop falls, the bottom flattens out under the influence of the air that it is falling through and surface tension holds the drop in a rounded shape. Raindrops fall at different speeds depending on how big they are. Large raindrops associated with warm weather and strong updrafts like the ones we see in thunderstorms and hurricanes can have a falling speed of up to 20 miles per hour, while smaller raindrops fall at lower speeds of 2 miles per hour and drizzle at much slower speeds of a few feet per second. The speed at which the drop falls is known as its terminal velocity and is reached when the force of gravity pulling the drop downward matches the friction of air slowing the drop down as it falls.

What damage does the rain do when it hits the ground?

When the falling raindrops hit leaves, they can cause damage to the plants. The biggest raindrops are heavy and falling fast so they can break off or damage the foliage, although not as much as hail does. We see this especially in fall when the leaves are loosely attached to the branches after they start to change color and die. A heavy rain (or snow if you live in a colder area) can remove a lot of leaves in a short time, making the fall colors muted or non-existent as the leaves are washed to the ground.

When the rain hits bare ground it can cause soil particles and microbes to splash upward. Farmers know that rainy weather can lead to more diseases because of the enhanced transmission of fungal spores and other pathogens up onto the crops. Of course, the wet soils can also provide a lot of moisture in the air around the plants that can fuel the development of fungal diseases like powdery mildew, especially when plants are close together with little ventilation by the wind. Mulch may be able to help reduce the transmission of spores but creates a moist environment around the roots of the plants that can cause problems if it keeps too much water in the ground. One advantage of arborist wood chips is that the wood absorbs moisture and releases it later and the pores between wood chips are large and can store a lot of water before it runs off. It can also preserve soil moisture between rain events and it can reduce the impact of raindrops on the soil surface, limiting nutrient loss and splashing.

Where does the rain go when it hits the ground?

Once rain hits the ground, some of it sinks into the ground (infiltration) while the rest might either run off if there is a slope or stand in a puddle if there is a low-lying or flat area. The rate at which water can enter the soil depends on the characteristics of the soil. If it is gravel or coarse sand, it can take in as much as 0.8 inches of water per hour. Sandy loams can take in 0.4 to 0.8 inches per hour, loams 0.2 to 0.4 inches per hour, and clay soils less than 0.2 inches per hour. The rain can also compact the soil, making the movement of oxygen around the roots more restricted. The water that does not sink into the soils will either sit at the place the rain has fallen or will move downhill under the force of gravity. Flowing water will often carry a lot of soil and debris with it, so the movement of topsoil and nutrients like nitrogen from field to stream can cause problems for both environments. Silt from the floodwaters can cause loss of oxygen in streams and murky conditions affecting the stream health or the silt can be deposited in low-lying areas. In the biggest floods, large deposits of sand can ruin fertile bottomlands by covering healthy soil with thick deposits of unproductive silt and sand. The erosion can also dig deep trenches through gardens and farm fields, leading to loss of plants and making the movement of farm equipment across that rutted land difficult especially since it can also get bogged down in the mud.

If the water pools in a low-lying area the soil can become saturated for a long time. This causes damage to the plants standing in the water because without oxygen, the roots die. In the short term the loss of oxygen can cause the plants to wilt. If it lasts for a long time it is likely to lead to the death of the plants in low-lying areas. In Hurricane Debby, some farmers are starting to see flooded cotton plants become reddish and stop growing, reducing the eventual yield of the crop. The dying roots can also give off ethylene gas, hastening the ripening of crops like tobacco and shortening the time that farmers have to harvest it before it rots. Farmers are often told to harvest crops that grew in flooded areas separately from upland crops because the negative impacts of oxygen deprivation can stunt plants, ruining their yields and increasing the likelihood of aflatoxin in peanuts or other toxins which can decimate the value of the crop.

What can gardeners do to prepare their gardens for heavy rains?

In the future, the frequency of heavy rains is expected to increase in the United States as well as other parts of the world because of more water vapor in the atmosphere as the planet warms. What can gardeners do to make sure their gardens can withstand heavy rain events? The first thing to do is to understand what type of soil you have and how much water it can absorb. If you have sandy soil, heavy rain will have less of an effect than clay soil but it might mean you have to water a lot more often. In Debby, some areas that received 12 inches of rainfall needed irrigation less than a week after the storm moved through because the sandy soil of the coastal plain just does not hold much water. The next thing you should consider is the local topography of your garden. Are there areas that are natural channels for water? Make sure those areas are lined with material and plants that can withstand water and reduce erosion. If you have low-lying areas, use those to plant rain gardens with plants that are more adapted to wet conditions.

Rain is a necessary part of gardening unless you live in a desert and provide your own water to your gardens, but it can cause a lot of damage if the water is not managed carefully. Take the time to study your garden and understand where the water will flow and your garden will be much more likely to take a 12-inch rainfall in stride with minimal damage. It might even thrive!

As gardeners, we often assign human characteristics to our plants as a way of feeling more connected to them. We talk about their preferences and dislikes for certain environmental conditions and even for each other. The idea that plants have feelings has caused many to believe that plants are sentient and capable of making deliberate choices. (We’ve discussed plant sentience in previous posts that you can see here, here, here, and here.)

• 

• 

I could spend my time debunking all the books, websites, and social media accounts that promote the pseudoscientific side of companion planting. But this popularized version is a horticultural zombie: it never dies. Instead, I’d rather discuss the ways that plants can change their environment physically, chemically, and biologically – which can influence the survival of other plants. The table below summarizes these methods.

I encourage you to download and read my recently published Extension manual – it’s free and peer-reviewed. In addition to providing solid scientific advice, it will help you understand why the classic example of companion planting – The Three Sisters – may be of historic and cultural interest but is unlikely to benefit plant productivity or soil quality.

Below are some evidence-based companion planting strategies for your gardens and landscapes. More are also available in the Extension manual linked above.

• Perennial companion plants will take a year or two to establish. Annual companion plants should be used if immediate benefits are desired.
• If you are growing perennial crops, avoid using annual companion plants that require yearly soil disruption. Crop growth and yield can be negatively affected.
• Use living mulches on pathways, between rows in vegetable gardens and orchards, and other locations that are not densely planted to reduce competition. Living mulches play a crucial role in protecting soil from erosion as well as biological and chemical degradation, and this improvement may outweigh any drawbacks from competition.
• To reduce competition among desirable plants, choose species whose roots are less likely to interfere with one another. Intersperse large taproot vegetables like carrots and radishes with those whose root systems are shallow and widespread, like corn, onions, and lettuces.
• Avoid invasive species and aggressive native plants. They will be overly competitive for resources like sunlight, resulting in reduced growth and vigor of other species.
• A well-chosen organic mulch will improve plant growth and productivity. A woody organic mulch, such as arborist wood chips, will enhance mycorrhizal populations, improve overall soil health, and control weeds. Arborist wood chip mulches also house predatory spiders and insects, such as ground beetles.
• In vegetable gardens, try to intercrop different species so that individuals of the same species are as far apart as possible from each other. This will reduce the ability of pest insects to infest an entire crop.