The Garden Professors™

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Soil characteristics

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

How does temperature affect germination and plant growth?

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

How to obtain the soil temperature

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

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

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

How is soil moisture important?

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

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

It’s almost time!

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

Like many home gardeners, we maintain a compost bin (a lovely 3-bin system built by my husband). I don’t need very much compost as the only organic matter I add to our landscapes is an arborist wood chip mulch. We do have a couple of raised beds for veggies and I do need organic matter for potted plants, so our compost goes there.

A few weeks ago I was preparing our raised beds for spring planting. (Actually, I should have done this in the fall but better late than never.) In any case, my raised bed preparation consists of a few very easy steps:

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1. Clear out any weeds or veggie residue.
2. Pull all wood chips to one side, leaving soaker hoses exposed.
3. Lay down a thin layer of compost
4. Replace wood chips and add more as needed to raise mulch level to at least 4”.

This is a great way to preserve and enhance the soil environment, while inhibiting weed growth. In the spring, I only have to pull the chips back and plant seeds or starts.

But back to the compost. We only add kitchen scraps and yard debris to our bins. Nearly everything is unrecognizable after it’s been composting, save a few eggshell pieces. So imagine my surprise and unhappiness when I found partially decomposed and even intact tea bags in my finished compost.

Now, I drink a lot of tea. I go through 4-8 teabags a day. Most of those teabags are of the simple paper variety, but I do get fancy pyramidal tea bags on occasion. Many of the sellers of these teabags claim their products are biodegradable, and some are made of silk or some other degradable fiber. But most are made of nylon. And they are full of microplastics.

This problem was reported years ago by The Guardian, which I managed to miss until recently. This article is well worth reading for those of you who drink tea and compost the teabags. Here are a few salient quotes:
“A single silky plastic tea bag at brewing temperature (95C) releases… microplastics,…nanoplastics… and polyethylene terephthalate (PET) into a single cup of tea.”
“To put it unscientifically, the amount of plastic found in these tea bags is more than we ingest from just about anything else.”

I’ve written before about the dangers of unwanted chemicals in corrugated cardboard and advised against its use on soil or in compost. Now we need to add nylon teabags to the list. The research reported in The Guardian is alarming enough that I will no longer use pyramidal teabags in brewing my tea. I won’t even compost the tea leaves in these bags, as they are contaminated by the brewing process.

Have you found uncomposted items in your finished compost that surprised you? Make a comment below!

I’ve been thinking about snow and winter precipitation a lot this week as Georgia digs out from a record snowfall that hit the southern part of the state with amounts of up to 9 inches. This is the second snow event this winter, something that does not happen very often here, although I know it is much more common in many other parts of the country and the world. The heavy snow band stretched all the way from eastern Texas to eastern Virginia up the East Coast, bringing record snow amounts to many areas along the path, including a very heavy snowfall in New Orleans. In the past week I have seen references to snow as “poor man’s fertilizer” and I wondered where that came from and whether there was any truth to it. I discovered that it is a saying that is often used by farmers and even turned up in “Farmer Boy” by Laura Ingalls Wilder. The book tells the story of Laura’s husband, Almanzo, growing up on a farm in upstate New York in 1866. His father told him snow was a good thing and called it “poor man’s fertilizer.” We will explore what that means in this week’s Garden Professors blog.

Why is snow called “poor man’s fertilizer?”

Snow is sometimes called “poor man’s fertilizer” because, like rain, it can add nutrients to the soil when it falls and sinks into the ground. As snowflakes and rain droplets fall through the atmosphere they pick up trace minerals from dust particles and other nutrients like nitrogen compounds that are needed by plants as they grow. Lightning can also fix nitrogen in rain or snow, although the impacts are limited to areas right near the lightning strikes. The nitrogen compounds include nitrate, ammonium, and dissolved organic nitrogen. It is estimated that between 2 to 12 pounds of nitrogen are deposited per acre in the U.S. per year from rain and snow (at best about ½ pound/acre per inch of snow and usually much less than that). The highest amount of nutrients will occur in the first flush of rain or snow, since once they are washed out of the air, it takes a while for them to build back up.

Lightning contributes 5-10% of the nitrous oxide and the remainder comes from human activity such as emission from automobiles and industrial plants. Ammonium can make up 25-75% of the total N in precipitation and comes from soil microbial activity and manure and urea fertilizer outgassing. Since some of the nitrogen comes from human activity, it is probably no surprise that nitrogen levels in rain are higher in the eastern United States than they are out west due to the prevailing west-to-east winds over the continent. One of the results of this is too much nitrogen falling in some locations, enhancing crop yields but harming forests, waterways, and other natural landscapes.

The main benefit of the snowfall is the length of time it takes to melt, since that provides nitrogen to the soil over a longer period and reduces runoff. This is in contrast to rain, which often falls so fast that only a limited amount of the water can percolate into the soil. The rest runs off into streams and lakes resulting in more nitrogen there and less deposited into the ground. And of course snow is free so everyone that it reaches receives benefits at no cost compared to having to purchase and apply commercial fertilizers that add nitrogen to the soil, hence the reference to “poor man”.

What other benefits of snow cover are there?

In addition to providing moisture and nitrogen to the ground, the snow cover can provide other benefits to your gardens. The blanket of snow can provide an insulating layer between the very cold Arctic air that sometimes blows in from the north and the ground where plant roots and dormant plants are waiting out the winter months, as I discussed in November 2024. If the snow is high enough it can also provide some protection for tree trunks from the nibbles of hungry deer and rabbits looking for winter dining as well as protection for small rodents and other critters that are trying to avoid the eyes of hawks, owls, and foxes, who also need to eat.

Snowfall also serves as an important water source for many communities around the world, storing water in the snowpack in winter and slowly releasing it during the summer for use by communities and agricultural producers downstream. This is becoming a problem as more precipitation falls as rain instead of snow as the climate gets warmer because it changes the seasonal availability of the water. Meltwater from snow can also recharge the groundwater that can provide water through wells. The water from snow can also provide water for animals in winter, who may have limited sources of moisture to survive.

What limits are there to fertilization by snow?

Snow does not always provide much benefits to gardeners and farmers. If the ground is already frozen the meltwater may not be able to penetrate the surface and will run off before it can enhance soil nitrogen. If the soil is too permeable, the meltwater may sink down so deep that plants cannot reach the nutrients until later in the season when their roots stretch down to deeper levels, if at all. Even with a consistent snowpack over time, the amount of nitrogen that is able to infiltrate the soil is highly variable.

Enjoying the snow

While snow does provide some fertilizing effect due to the presence of nitrogen in the meltwater, the amount is not large. It’s highly variable due to the amount of dust picked up by the snowflakes as they fall through the atmosphere as well as how much and how fast the snow melts when it hits the ground. If the ground is frozen, very little may make it to the root zone. Testing your garden soil for nitrogen content each spring will allow you to determine how much nitrogen is already there and how much should be added using commercial fertilizers to provide the optimum levels for plant growth. But whether or not snow really is “poor man’s fertilizer”, it provides nourishment for the soul as you see this enjoyable and beautiful contrast to the greenness of your summer garden.

Even though some may not be fond of them, we understand that most spiders are beneficial, excellent predators of arthropod pests and are extremely interesting organisms. Gardeners are often really enthusiastic about this group of arthropods and enjoy observing them in outdoor landscapes, usually at a comfortable distance. There are some of us who enjoy the company of these creatures in closer proximity, even indoors and as pets (myself included).

Writing a post about spiders has long been on the back of my mind, and a topic that has been requested multiple times. So as I sit in front of my fireplace on this snowy Montana evening, thinking back to the several spiderwebs speckled in between my wood pile, I thought that this month would be the perfect time to do some spider research. For this post I will be focusing on the outdoor spiders that we can commonly find in our gardens: who they are, what they do, and how we can make more of a hospitable environment for them.  As usual I will caveat this by saying spiders are a broad group, and since I can’t exhaustively cover them in the scope of this post I will share resources for you to learn more at the end.

Types of Spiders in the Garden

Spiders are arachnids (class: Arachnida) and share this category with scorpions, ticks, and mites. They fall within the order Araneae, containing more than 50,000 species across 134 different families, making them the largest group of arachnids in terms of species diversity. They have 2 body segments, 8 legs, chelicerae with fangs, and spinnerets that can produce silk. Although all spiders can produce silk, not all of them make webs. Nearly all spiders are venomous (except for 2 families which lack venom glands) however, most spiders either do not bite humans, do not have venom potent enough to negatively impact us, or fangs capable of penetrating our skin. Spider venom is primarily used to immobilize and subdue their prey which are often smaller arthropods like insects. There are a few spiders of medical importance (as we know) who can be found in close proximity to humans, and some people can have an allergic reaction to spider venom (like with many insect venoms). Most spiders are carnivorous, feeding on small arthropod prey, and some of them supplement their diet with plant products like nectar and pollen. One species (Bagheera kiplingi) is known to be primarily herbivorous. For the most part though, spiders are amazing generalist predators most of which are not dangerous to humans and all of which will not seek you out and hunt you down (despite what some exaggerated spider-related media and tropes may claim).

Jumping Spiders

Arguably the cutest group of spiders around, jumping spiders (family: Salticidae) can sometimes even convince the most spider averse people to take a second look in appreciation. Jumping spiders comprise the largest family of spiders, containing more than 6000 species worldwide. These often brightly colored critters with distinctly large eyes do not spin webs, but actively hunt prey, often during the daytime with their excellent vision. Aptly named for the large leaps they can make while hunting for prey or escaping threats, they can be extremely entertaining to watch around the garden. They can use their silk to make small insulated shelters under leaves, bark, or between rock cracks. They also produce compounds like glycol and other proteins which act like an antifreeze, allowing some of them to remain active in colder and even freezing temperatures.

Orb-Weavers

The characteristic mascot of spiders in the garden, orb-weavers (family: Araneidae) build the very familiar large, circular spider webs that we all can easily picture. These magnificent builders lay in wait of prey that flies or crawls onto the sticky parts of their webs. After biting their prey to immobilize it, they proceed to wrap them in silk. Most of them are active at dusk and will rebuild their webs each day.

Wolf Spiders

Wolf spiders (family: Lycosidae) are another group of non-web spinning spiders known for their active hunting abilities. Some will wander around the ground, searching for prey, while others wait in burrows for an unsuspecting victim to walk by. These hairy grey, black, and brown spiders have excellent eyesight and many of them are primarily nocturnal hunters. Females lay their eggs in a silk sac and actively protect them by carrying them around. Once they hatch, the mother will carry these spiderlings (in some cases, over 100 of them) on her back, which is quite a spectacular sight to behold.

Ground Spiders

Ground spiders (family: Gnaphosidae), similar to wolf spiders, are nocturnal hunters who use their quick speed to hunt down and chase after prey. They use their sticky silk to entangle their prey, immobilizing them. This hunting behavior allows this group of spiders to target prey larger in size than themselves. During the day they can be found in silk shelters. A really cool group in this family includes the ant-mimicking ground spiders (genus: Micaria). The first time I spotted this spider under a rock in Kentucky, all of my entomological instincts were telling me that it doesn’t quite look like an ant. Upon closer observation, the 8-legs gave it away (but not before our whole group was thoroughly impressed by its ant-like appearance).

Crab Spiders

These sometimes brightly colored and distinctly shaped spiders (family: Thomisidae) can also be found worldwide. They are called crab spiders because of the way that their two pairs of front legs (which are longer than the rest of their legs) are positioned, in addition to their sideways and backwards movement which can be crab-like. These are another group of non-web making spiders which act as ambush predators. Sometimes referred to as “flower spiders”, they can be found perched on a flower, waiting for a visitor to stop by for some nectar before they pounce.

Lynx Spiders

Lynx spiders (family: Oxyopidae) are another group of ambush hunters that target prey species found on plants. Similar to crab spiders, some species can also capture pollinators while they patiently await them on flowers. Due to their often green and brown coloration, they can camouflage themselves in plants, making hunting for prey easier. Species in this group are also known to be important for biocontrol in agricultural systems.

Funnel-Weavers & Grass Spiders

Funnel-weavers (family: Agelenidae) are another cosmopolitan group of spiders known for their very quick speeds and their unique webs. Named for the structure of their webs, which looks like a flat sheet that tapers into a funnel-shape, these spiders lay in wait for prey that walk across these sheets, triggering vibrations that cause the spider to ambush them. Although these webs are not sticky (unlike sections of orb-weaver webs), they do contain a lot of silk fibers that can entangle their prey. Once the prey have been subdued by a quick bite, the funnel-weaver grabs their tasty meal and retreats back into the safety of its funnel. Not to be confused with funnel-web tarantulas, these spiders can create their funnel-shaped webs in leaflitter, on soil, or in grass. This family includes grass spiders and also the common house spiders. Some species may seek refuge indoors during the winter time (an example being Hobo spiders), which can sometimes be a nuisance for people.

Sheet Weavers

Sheet weavers or money spiders (family: Linyphiidae) are a group of very tiny spiders, containing over 4700 species worldwide (making them the second largest family after jumping spiders). Although they are very widespread, their small size makes them easy to miss. Many species of Lyniphiids are considered excellent biocontrols of small soft-bodied arthropod pests such as aphids.

Harvestmen

Although Harvestmen (Order: Opiliones) may greatly resemble them, they actually aren’t spiders, even though they are arachnids. They do have 8 legs, but only have one body segment, no fangs or venom glands, and do not produce any silk. They are sometimes referred to as daddy-longlegs, not to be confused with daddy-longlegs spiders, which are in fact spiders in the family Pholcidae. There are many tall tales associated with this group of spiders so to learn more about them check out the resources. Harvestmen prefer moist environments such as caves, leaflitter, and under logs. They are omnivores and opportunistically feed on decaying vegetation, carrion, animal waste, and small arthropods. They can also aggregate together to retain moisture (which can be quite a sight to behold).

Benefits of Spiders

Due to their carnivorous diets, diverse hunting behaviors, and widespread distributions, spiders are excellent beneficial organisms which can reduce pest populations in a wide variety of landscapes. They are also well-known as naturally-occurring biological controls in many agricultural and horticultural systems. Several studies have been conducted which demonstrate the benefit of a variety of groups of spiders in these systems. Spiders can reduce populations of common groups of pests including caterpillars, aphids, leafhoppers, planthoppers, and beetles.

Many groups of spiders can be found in agricultural systems. An analysis by Young and Edwards (1990) demonstrated the presence of over 600 spider species spanning 26 families found in 9 specific field crops in the United States. According to their analysis: 5 spider families comprised the majority found in field crops including Salticidae [jumping siders], Linyphiidae [sheet weavers], Araneidae [orb-weavers], Theridiidae [tangle-web spiders], and Lycosidae [wolf spiders] (Young and Edwards, 1990). A study conducted by Akhtar et al. (2024) showed 45 spider species spanning 13 families in maize crops in the Punjab region of Pakistan. The families that made up the majority of these species included Araneidae [orb-weavers], Lycosidae [wolf spiders], and Salticidae [jumping spiders] (Akhtar et al., 2024). These are just a couple of examples of studies that have been conducted, though there are many more you can find!

Several studies have also demonstrated that presence and density of spider populations resulted in an increase or improvement in crop production. A meta-analysis conducted by Michalko et al. (2019) evaluated 58 studies conducted on the impact of spider density on crop performance and found an overall positive result. Agricultural pest insects were suppressed in situations of higher spider density in 79% of cases. Their efficacy in biocontrol varied depending on the type of crop, but was highest in rice, grape, cabbage, and wheat systems (Michalko et al., 2019).

As more research continues to be conducted, I am sure that we will find many more instances in which spiders improve crop productivity through the suppression of common pests.

Protecting and Conserving Them

All of these studies show that the presence of beneficial organisms like natural enemies can be important natural biological controls which can assist us in having a more productive garden (whether the scale of production is large or small). As such, thinking about protecting and conserving these awesome generalist predators is in our best interest.

Implementing practices that can reduce negative impacts on spiders, and creating a landscape that favors them can have wonderful benefits for our gardens. Much of this can also be intuitively considered when you think about the biology and hunting behavior of these groups of spiders. A study conducted by Mashavakure et al. (2019) on the impact of farming practices on spiders in southern Africa showed a variety of common trends which can be adapted for gardens of different scales. In this study, they showed that the two factors that had the largest impact on spider populations were tillage and mulching (Mashavakure et al., 2019). Practices with minimum tillage had highest populations of Lycosidae [wolf spiders], Gnaphosidae [ground spiders], and Salticidae [jumping spiders] (Mashavakure et al., 2019). Plots that had the lowest mulching levels also had the highest populations of Gnaphosidae [ground spiders] and Thomisidae [crab spiders] (Mashavakure et al., 2019).

Structural complexity and diversity of vegetation is another way that you can conserve and increase beneficials in the landscape (including spiders). Having a variety of plants of different sizes and maintaining this habitat year-round can provide shelter, hunting spaces, and overwintering sites for spiders in the home landscape. In addition, reducing practices that can harm beneficials including practicing IPM and reducing the use of broad spectrum insecticides, also goes hand in hand with creating more habitat for spiders.

I hope this post illuminated some of the diversity and beauty of our favorite 8-legged garden companions. Even if some may not want to snuggle up to them, we as gardeners can always appreciate the importance of these amazing creatures in our landscapes.

Resources:

Daddy-longlegs myths: https://spiders.ucr.edu/daddy-long-legs

Overview of Spiders in Agriculture: https://vegcropshotline.org/article/insect-spotlight-spiders-an-overview-of-their-role-in-agricultural-systems/

Young, O. P., & Edwards, G. B. (1990). Spiders in United States field crops and their potential effect on crop pests. Journal of Arachnology, 1-27.
https://www.researchgate.net/profile/G-Edwards/publication/256082787_Spiders_in_United_States_field_crops_and_their_potential_effect_on_crop_pests/links/00463539065408dc43000000/Spiders-in-United-States-field-crops-and-their-potential-effect-on-crop-pests.pdf

Akhtar, N., Tahir, H. M., Ali, A., Ahsan, M. M., & Abdin, Z. U. (2024). Assessment of Biodiversity and Seasonal Dynamics of Spiders in Maize Crops of Punjab, Pakistan. Journal of Asia-Pacific Biodiversity.
https://www.sciencedirect.com/science/article/pii/S2287884X2400061X

Mashavakure, N., Mashingaidze, A. B., Musundire, R., Nhamo, N., Gandiwa, E., Thierfelder, C., & Muposhi, V. K. (2019). Spider community shift in response to farming practices in a sub-humid agroecosystem of southern Africa. Agriculture, Ecosystems & Environment, 272, 237-245.
https://www.sciencedirect.com/science/article/pii/S0167880918304821

Once again we wander down the path of botanical history.

George Julius Engelmann was a botanist, physician, and meteorologist, but is remembered primarily for his botanical monographs. George, also known as Georg, was born Feb. 2, 1809 in Frankfurt am Main, Germany, the oldest of thirteen children, nine of whom reached maturity.  Unusual for the time, his parents established and ran a successful school for young women there in Frankfurt.

Like most privileged young men of the time, George attended gymnasium. He started to take an interest in plants when he was 15 but was also keen on history, languages, and drawing. With the help of a scholarship, 1827 found him studying sciences at the University of Heidelberg. In 1828 young Engelmann, being embarrassed by his participation in a recent student political demonstration, decided to transfer to the University of Berlin for a couple of years. He then moved on to the University of Würzburg where he graduated in 1831 as a Doctor of Medicine. With shades of things to come, his dissertation for the medical degree was more related to botany than to medicine. It was devoted to morphology, the structure and forms of plants, and was illustrated with five plates of figures drawn and transferred to the lithographic stone by Engelmann himself. It was published in Frankfurt in 1832 under the title of De Antholysi Prodromus.

Spring and summer of 1832 found him in Paris where he was leading, ” a glorious life…in spite of the cholera,” but changes were on the horizon. His uncles wanted to make land investments along the Mississippi River and enlisted him to be their agent. In September of that year George sailed from Bremen to Baltimore and made his way to family already living in Illinois near St. Louis. For the next three years, to get a better lay of the land which he’d been hired to sell, he made many long horseback journeys alone through southern Illinois, Missouri, and Arkansas. While he did use his recently acquired regional knowledge in his new job, his botanical notes waited to be used in the future.

Tiring of the land agent role by late 1835, he moved to St. Louis to start a medical practise. Apparently needing more to do, in 1836 he founded a German newspaper called Das Westland. It contained articles on life and manners in the United States and was widely read and appreciated in the United States and Europe. Four years later his medical practise was well established and he’d earned enough money to make a trip back to his hometown. There he fell in love, got married, and the newlyweds then returned to America. When they landed in New York City Engelmann met Asa Gray, already a well known American botanist. A close friendship developed between the two which was ended only by death. 

Eventually Engelmann’s medical practise in St. Louis became so successful he no longer needed to keep office hours: he simply saw patients in his study. This allowed him to take long vacations and devote more time to his preferred botany and biology studies. An 1842 monograph on dodders, A Monograph of North American Cuscutinae, had established his reputation as a botanist. He was one of the earliest to study Vitis (the grape species) of North America; nearly all that is known scientifically of these plants is due to his investigations. One of his most economically important discoveries was of the immunity of the North American grape to the plant pest Phylloxera, which became very significant later in the century during The Great French Wine Blight.

In the 1870s French vineyards came under attack by Phylloxera vastatrix which feeds on grape vines roots. Growers observed that certain imported American vines were resistant to the insect’s feeding habits. The French government dispatched a scientist to St. Louis to consult with the Missouri state entomologist and Engelmann, who had studied American grapes since the 1850s. Engelmann verified that certain living American species had resisted Phylloxera for nearly 40 years. Additionally it was found that Vitis riparia, a wild grape of the Mississippi Valley, did not cross pollinate with less resistant species which was the cause of previous growing failures. Engelmann arranged to have millions of shoots and seeds of V. riparia sent to France which eventually provided Phylloxera resistant rootstock and saved the French wine industry.

Other difficult plant groups Engelmann studied include cacti, conifers, mistletoes, rushes, and yuccas. In 1859, he published Cactaceae of the Boundary which studied cacti on the United States/Mexico border. A unique aspect of Engelmann’s cacti studies is he established, for the first time, the classification of these plants based on floral, fruit, and seed characteristics.  The source he referenced for this was Dr. Wislizenus’ Expedition from Missouri to North Mexico. Engelmann eventually published two books on cacti, both of which are still valued references. Other monographs he published are Notes on the Genus Yucca (1873) and Notes on Agaves (1875). The latter was illustrated with photographs, which is something we tend to expect now but was quite forward thinking at the time.

In addition to his writing, both alone and in collaboration with others, Dr. Engelmann was also a founding member of the St. Louis Academy of Sciences and the National Academy of Sciences. He was instrumental in the establishment of the Missouri Botanical Garden by encouraging Henry Shaw, a wealthy St. Louis businessman, to develop his already extensive gardens to be of scientific as well as public use. What was then called “Shaw’s Gardens” eventually became the Missouri Botanical Garden. Engelmann’s botanical collection, which contains the original specimens from which many western plants have been named and described, was given to the Missouri Botanical Garden. This gift of almost 100,000 specimens led to the founding of the Henry Shaw School of Botany at Washington University in St. Louis, where an Engelmann Professorship of Botany was established by Shaw in his honor. His legacy also lives through the many plant species named in his honor, including Engelmann oak (Quercus engelmannii), Engelmann spruce (Picea engelmannii), Apache pine (Pinus engelmannii), and Engelmann’s quillwort (Isoetes engelmannii).

Engelmann died in 1884. He was interred next to his wife, who passed away in 1879, in the Bellefontiane Cemetery in St. Louis.

The botanical works of the late George Engelmann, collected for Henry Shaw, esq. /Ed. by William Trelease and Asa Gray.
https://archive.org/details/mobot31753000060878

PPT on digitizing Engelmann’s collection
https://www.slideshare.net/slideshow/digitizing-engelmanns-legacy-4745573/4745573

How did your garden grow in 2024? Was it a lush playground full of beautiful flowers and plentiful produce? Or was it a sere landscape of brown, wilted foliage? How your own garden fared in 2024 was certainly dependent on where you live, what you had planted, and how you took care of it, but it was also subject to the variations in weather and climate in your area. This week we will take a look back at the climate conditions in 2024 and look forward to what it might mean for 2025. Now is the time to look at your seed catalogs and dream!

What controlled the climate this year around the world?

With just a few days to go in 2024, it is quite clear that this will be the warmest year we have ever measured since official global records began in 1880. There are three main factors that controlled the climate in 2024, although of course there are also local variations due to smaller-scale weather events. The contributing factors are the warming trend across the world caused by greenhouse warming of the planet, the El Niño that dominated the Eastern Pacific Ocean in the first half of the year, and the unusual warming of the Atlantic Ocean this year which provided fuel for the growth of Atlantic tropical cyclones this year as well as raising the global temperature.

Impacts of the rising temperature trend

Rising temperatures for the world are well-documented by scientists across the globe and are generally linked to increases in the amount of greenhouse gases like carbon dioxide and methane from sources like the burning of fossil fuel. This is not a new concept and can be found in scientific literature going back to at least 1856 when Eunice Foote discovered that carbon dioxide trapped heat in her home laboratory. Many scientists since then have corroborated that effect and others have shown that the primary source of greenhouse gases like carbon dioxide is the burning of fossil fuel, although there are other sources as well.

A timeline of global temperature for January through November (we don’t have the values for December 2024 yet since it is not quite over) shows that this is almost certain to be the warmest year on record so far. But since temperatures are still rising, we can expect to see more record-setting warm years in the future. The rise in global and regional temperature that is occurring now may not be apparent on a day-to-day basis due to short-term variations caused by passing weather systems, but the changes are reflected in increases in normal temperatures when they are updated every ten years. Longer-term changes like the drift in Plant Hardiness Zones also reflect this upward trend. Heat waves across Northern Europe and in South America in 2024 have also been attributed in part to the warming trend. Of course, winter will still occur and we will continue to get cold periods, just fewer of them than in the past.

El Niño to neutral conditions

The second major impact on the climate in 2024 was the lingering El Niño that was occurring as 2024 began and lasted until early June. The warm water in the Eastern Pacific Ocean associated with El Niño helped raise the global temperature during the first half of the year and affected the climate around the world. In North America, an El Niño is associated with warm dry conditions at high latitudes and wet cool conditions in southern latitudes as the jet stream is shifted to the south, bringing storms, clouds, and rain along with it. Once the El Niño ended in June, neutral conditions prevailed until the end of 2024, although the last few months we almost reached the threshold for the opposite phase, La Niña. Climate patterns associated with La Niña were starting to appear late in 2024, leading to dry conditions across southern parts of the United States and wet conditions farther to the north. In fact in the Southeastern US most areas were in drought for a good part of the summer except areas that were hit by tropical cyclones like Beryl, Debby, Francine, Helene, and the remains of Rafael. By the end of 2024, over 87% of the lower 48 states were covered by drought or abnormally dry conditions, a big change from early in the year.

Notable droughts also occurred in Brazil and other parts of South America and in northern Europe. These droughts were also associated with record-setting warm temperatures as high pressure over those areas tamped down any development of rainstorms and caused clear skies which increased temperatures. You can look at maps and timelines of specific areas of the world or country at https://www.ncei.noaa.gov/access/monitoring/climate-at-a-glance/. The map above shows how the drought status in the US changed over the year with some areas getting much wetter and others drying out. Note that areas with tropical storms and atmospheric rivers this year experienced a lot of changes from month to month but it did not affect the total change over the year by much.

Record-setting warm temperatures in the Atlantic

The third impact on this year’s climate was the abnormally warm temperatures in the Atlantic and Gulf of Mexico. These record-setting temperatures have been linked to decreases in aerosols from burning of fossil fuels by ships crossing the ocean and in more recent literature, to decreases in low clouds over the ocean. Both of these can lead to more sunlight reaching the surface of the ocean and increasing its temperature. Those warm sea surface temperatures led to a larger number of tropical systems than usual in the Atlantic Ocean by providing a source of fuel that helped them develop into full-fledged tropical storms and hurricanes. There were 11 hurricanes and 18 named storms in the Atlantic Basin this year, the 5^th largest number in the satellite era. The number and intensity of tropical cyclones in other parts of the world like the western Pacific are also attributed in part to warmer ocean water. The Philippines experienced five different typhoons in just a few weeks, causing tremendous damage there, and other areas of Southeast Asia also felt the impacts of tropical systems.

What do we expect in 2025?

By January I expect that the La Niña will be officially declared. Whether or not it is, though, we can predict that the early part of the year will show the characteristic pattern of a weak La Niña, including a shift to the north for the jet stream over the United States. That will bring cloudy and colder weather to the northern states and warmer and drier conditions to the southern states since the jet stream is what is pushing our precipitation-producing systems around. These conditions will likely be reflected in the soil moisture present during the spring planting season, so I expect dry conditions in the Southeast that could affect germination of seeds. Wetter conditions in the North should not have this problem but farmers could have trouble getting into the fields to plant if it is really wet and cool, leading to delays in establishing crops. This La Niña is likely to be fairly weak, so it may only last for a few months before we return to neutral conditions. NOAA’s predictions are that the neutral conditions are likely to last for most of the summer. That means we are likely to get another active Atlantic tropical season. The South could be fairly dry except where the tropics bring storms to the area again in 2025. If you are in other countries, you can find more information here or check with your local authorities for how your region usually responds to a La Niña in winter and later in the year.

How did your garden do in 2024? What are you looking forward to in 2025? Let us know in the comments. We are happy to get your questions, too, as we plan for our blog posts in 2025. Many thanks for the comments we have received in this past year and for your support of our blog! We appreciate it!

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.

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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