Backyard biocontrol – using natural enemies to wipe out invasive weeds

The agricultural-residential interface

Four years ago we moved to the family farm (where I grew up) and we’ve enjoyed restoring the 1 acre landscape around the farmhouse. Given that the residential part of this farm is surrounded by pastureland, there is a continual influx of weed seeds into our managed beds. While our thick applications of arborist wood chips have kept out many weeds, they still pop up where mulch hasn’t been applied yet or is too thin.

Photosensitized livestock will suffer severe sunburning after consuming Hypericum perforatum

One of these weeds is Hypericum perforatum (also known as Klamath weed or St. John’s wort), a species native to Eurasia. The latter common name can confuse gardeners, as there are several ornamental species of Hypericum also called St. John’s wort, but H. perforatum is easily identified by the perforations in the leaf. This invasive species is a problem for our cattle, as Klamath weed causes photosensitivity when it’s consumed and can be toxic in large amounts.

The weeds to the right of my raised beds include St. John’s wort, or Hypericum perforatum.

In the last few years H. perforatum colonized our stockpile of native soil waiting to be used in our raised beds. It was a small enough infestation that we could pull it all up, but a closer look revealed that some shiny metallic beetles were already busy feasting on the leaves. Putting on my IPM hat, I first needed to identify these interesting beetles. It didn’t take long to find out they were a Chrysolina species.

Chrysolina hyperici and C. quadrigemina (or St. John’s wort beetles) are also native to Eurasia and are specialist feeders – they only feed on Hypericum species. They were imported as biological control agents several decades ago and have been effective in controlling dense populations of St. John’s wort. C. quadrigemina in particular has been reported to feed on both ornamental and native species of Hypericum but not to the extent of causing significant damage.

Both species of the St. John’s wort beetle feed on the leaves, where they also lay thousands of eggs. The larvae that emerge from the eggs are voracious feeders and can defoliate dense stands of St. John’s wort. Like other animals that eat Hypericum perforatum, the larvae become photosensitive and generally feed before sunrise to avoid damage.

Hypericum perforatum infestation

Since biological control agents depend on the presence of their host, it’s important to retain a small population of the host. And because this particular beetle is a leaf feeder, one can remove the flowers of the plants to reduce reproduction, but maintain the plants to support the beetle.

Cinnabar moth (Tyria jacobaeae) was introduced to the US to help control tansy ragwort (Jacobaea vulgaris), another invasive, noxious weed

Many other introduced, invasive weeds can be controlled using carefully researched microbes and insects. Some of these biocontrol agents may already be found in your area – so it’s important to avoid using insecticides and fungicides, in particular, to conserve these garden assets.

Water: Garden Friend….and Foe? – Water, Relative Humidity, and Plant Diseases

We all know that water is essential for life and that we have to ensure our landscapes, gardens, and houseplants all have a sufficient supply of the stuff.  Forget to water your garden during a hot, dry spell and it could mean disaster for your plants.  But water can also create issues for plants, usually when it is in an overabundance – water helps spread and develop diseases on foliage and excess soil moisture can damage roots, creating opportunities for root rots and other diseases.  How do you meet the water needs of the plant while also avoiding issues associated water?  Understanding how water affects disease organisms will help, along with some tried and true Integrated Pest Management Strategies.

Water and Pathogenic Microbes

Both bacteria and fungi require water to grow and reproduce.  Most do not have a mechanism to actively take up and manage water, so they uptake water mainly through osmosis.  This means there must be some form of water present for those microbes that are actively growing and especially for processes like reproduction which use not only a lot of energy but might also be required to carry spores in order to spread.

File:Septoria lycopersici malagutii leaf spot on tomato leaf.jpg -  Wikimedia Commons
Septoria leaf spot, a common fungal disease of tomato that requires water for initiation and development.

Both pathogenic microbes and beneficial (or neutral) microbes require water to thrive.  It is one side of what we refer to as the disease triangle.  Water (along with temperature) are major components of the “favorable environment” side of the triangle, with the other sides being a plant capable of being infected and a population of pathogens capable of infecting.  Those last two sides meaning you have to have a population of the pathogen big enough to initiate or sustain an infection and a plant that can actually be infected by that pathogen.  For example – one disease spore may or may not be enough to start an infection (depending on the pathogen), but several hundreds or thousands definitely can.  And the pathogen has to be one that can actually infect the plant – it doesn’t matter if you have a million spores of Alternaria solani (one of two closely related fungi that cause early blight in tomatoes) on your cucumber plants, they likely won’t get a disease.  But if there are spores of A. cucumerina, a different species, you’ll likely get leaf spot on those cucumbers.  But it doesn’t matter if you have both a susceptible plant and a pathogen, there has to be a favorable environment (water and temperature) for there to be a disease infection. 

As this paper points out, water in the form of liquid (rain, ground water, dew, etc) and vapor (air humidity, fog) can provide the environment for microbe development in the soil and on foliage.  Microbes in the soil are ubiquitous as water is typically available in most soils (except in droughty or arid areas) , but excess soil moisture can create booms in populations for both the “good” microbes and the “bad” ones.  Microbes that live on foliage (sometimes referred to as epiphytic since they rely on moisture from the atmosphere) are much more likely to be water stressed since they are exposed to the atmosphere.  When there isn’t water available on the surface of leaves (from rain, fog, etc.) microbes tend to colonize around areas where water leaves the plant – stomata and to a lesser extent around tricomes and hairs. 

The paper also points out high atmospheric humidity is positively correlated with the number of fungi on a leaf surface. It’s also a requirement for diseases microbe spores to germinate, for filamentous fungi to break dormancy, for pathogen survival, for microbe movement on the leaf surface, and for disease infections to be sustained.  It is also shown that heavy precipitation increases water availability to these microbes thus hastening their growth.  Precipitation also dislodges and disperses pathogen spores and cells to adjacent plant tissues, and to leaves of nearby plants.  High humidity also makes leaf cuticles more permeable and promotes opening of the stomata, which can serve as an entry point for pathogenic infection.

Once inside the plant, microbes such as fungi and bacteria can thrive on the aqueous environment inside a plant, moving easily between cells or into the vascular tissue (depending on disease).  Pathogens that thrive in wet conditions, however, may initiate water soaked lesions on the plant to develop conditions favorable to their growth. 

Water, water everywhere – so is there anything you can do?

Of course, water is naturally occurring and in most places falls from the sky in some form or another.  In some places very little precipitation falls, in others there’s a lot. And don’t forget about the humidity, dew, and fog (which are often more common in places that get more rain, but provide moisture even in dry climates).  There are a few places where the atmospheric moisture levels are in that “just right” zone to support plant growth but not pathogen growth, which makes agricultural production of certain crops easier.  You could consider these areas the “Goldilocks” zone for crop production.  For example, a lot of seed crops are produced in the Midwest and arid north West, potatoes in Idaho, apples in Washington, etc.  The conditions there mean that, at least when those crops were getting established (before the advent of modern pesticides) in those regions, disease pressure was low. 

You can’t stop the rain, of course, if you’re in a place both blessed and cursed with abundant rainfall or atmospheric humidity.  But there are some things that you can do reduce the likelihood of diseases spread or supported by that water and humidity.

  • Evidence shows that there is a positive correlation between the density of planting and disease incidence.  Therefore, proper plant spacing and pruning can do at least three major things.  First, having space between plants, especially in the vegetable garden, can reduce the splashing of pathogens from one plant to the next during a precipitation event.  Second, it increases air flow through the plant, which can reduce the likelihood of pathogen spores that might float in and land on foliage.  Third, it reduces humidity in the immediate microclimate around the plant. The increased air flow in addition to the reduced amount of foliage that is releasing water through transpiration can have a significant effect on the humidity, which can have a big effect on the germination, establishment, and survival.  
  • Utilize diverse planting plans in the vegetable garden and the landscape.  Research shows that while having a variety of plants increases the diversity of disease organisms, it actually reduces the infection rate possibly because pathogens splashing from plant to plant are less likely to find a host plant if they are surrounded by non-host plants.  This practice is promoted in intensive vegetable plantings such as square foot gardening. 
  • As stated earlier, precipitation can drastically increase the population of microbes on foliage.  This also includes water from overhead irrigation.  For example, this study found that overhead watering of cabbage led to significantly higher and faster rates of spread of the black rot fungus as compared to drip irrigation.  Therefore, reducing or avoiding overhead watering can reduce the likelihood of disease incidence. 
  • Timing of watering may also contribute to disease development.  The dew point, which usually happens during the night time hours, is when the air is totally saturated at 100% relative humidity and therefore cannot hold any more water.  This is the point where excess moisture is deposited as dew on surfaces (another source of water on the foliage) and little to no evaporation of water already on surfaces happens (learn more at weather.gov).  As shared in this book chapter review, lower temperatures resulting in reaching the dew point can extend the time leaves are exposed to high moisture and result in higher disease incidence. 
  • As our own GP Linda Chalker-Scott points out in this review, mulching not only retains soil moisture, reduces erosion and more but also reduces the incidence of disease in plants by reducing the splashing of soil or spores from rain or irrigation onto the plant.  This drastically reduces disease spread from pathogens found in the soil or on plant debris.  The organic matter from organic mulches also has the benefit of increasing the population of beneficial microbes, which out-compete the pathogenic microbes. 
Mulching and drip irrigation can both significantly reduce disease incidence in gardens.
  • Crop rotation, where crops are not grown in the same soil or plot for a number of years, also reduces disease incidence by reducing pathogen loads in the soil or from crop residues left in the garden.  This study shows significantly reduced disease incidence on potato and onion when a crop rotation plan of four years is utilized (meaning that either onions or potatoes are not planted in the plot for a minimum of four years, with other crops planted between those years). 
  • If root rots and pathogens are a problem, try improving drainage around the garden. Adding organic matter can help with water permeability of the soil over time. Raised beds can also drain faster than in-ground gardens.
  • Of course, if you’re having lots of problems with certain diseases on your plants, these cultural controls may not be enough.  Finding resistant varieties may be a necessary step in breaking the disease cycle in your garden.

Overview

While water is required for plant growth, it can cause issues with plant diseases if there is too much or if it lingers on the wrong parts of the plant for too long.  Water from rainfall, irrigation, high humidity, fog, and dew can all lead to the initiation, development, and longevity of plant fungal or bacterial diseases.  Reducing the amount, persistence of water or humidity on or around foliage can significantly reduce the likelihood of plant disease incidence.  Methods such as reducing overhead irrigation, timing of irrigation, mulching, and crop rotation are key cultural methods in reducing diseases spread by water. 

Sources:

Aung, K., Jiang, Y., & He, S. Y. (2018). The role of water in plant–microbe interactions. The Plant Journal, 93(4), 771-780.

Burdon, J., & Chilvers, G. A. (1982). Host density as a factor in plant disease ecology. Annual review of phytopathology, 20(1), 143-166.

Café-Filho, A. C., Lopes, C. A., & Rossato, M. (2019). Management of plant disease epidemics with irrigation practices. Irrigation in Agroecosystems, 123.

Chalker-Scott, L. (2007). Impact of mulches on landscape plants and the environment—a review. Journal of Environmental Horticulture25(4), 239-249.

Krauthausen, H. J., Laun, N., & Wohanka, W. (2011). Methods to reduce the spread of the black rot pathogen, Xanthomonas campestris pv. campestris, in brassica transplants. Journal of Plant Diseases and Protection, 118(1), 7-16.

Rottstock, T., Joshi, J., Kummer, V., & Fischer, M. (2014). Higher plant diversity promotes higher diversity of fungal pathogens, while it decreases pathogen infection per plant. Ecology95(7), 1907-1917.

Wright, P. J., Falloon, R. E., & Hedderley, D. (2017). A long-term vegetable crop rotation study to determine effects on soil microbial communities and soilborne diseases of potato and onion. New Zealand Journal of Crop and Horticultural Science, 45(1), 29-54.

Diagnosing Abiotic Disorders I

Abiotic factors cause harm to plants resulting in symptoms. Abiotic disorders can look like damage caused by pests but do not spread in the same ways since the disease agent is not alive.

Insects and pathogens cause damage and disease in garden plants, but damage can also occur in absence of pests. We refer to these diseases as abiotic disorders. Plant pathologists consider abiotic disorders diseases because plants develop symptoms that reflect the changes in their physiology over time. Unlike outbreaks caused by insects or pathogens, abiotic disorders do not cause epidemics or as plant pathologists say “epiphytotics” because abiotic disorders do not spread the way insects and pathogens can. Like all diseases, abiotic disorders are a perturbation of plant physiology that show up as different or “not normal” appearance. Symptoms typically define most abiotic disorders since signs (of the actual thing causing the disorder) are not usually visible.

Since abiotic disorders do not require an organism to begin or complete a life history, they can occur at any time and are often of sudden onset. The reverse can also be true, depending on the agent causing disease symptoms which may not show for years in some disorders. Abiotic disorders are often associated with the degree to which a plant is adapted to its environment. Adaptation and establishment in an environment are different. New plantings  (those not yet established) do not tolerate abiotic extremes as well as established plants. Plants poorly adapted to the climate, soils or water of a region may be prone to abiotic conditions while plants adapted to their planting site thrive among the same abiotic factors.

Nutrient Disorders

Interveinal chlorosis is a symptom of nutrient deficiency. When on new leaves it usual is a micronutrient deficiency on older leaves a number of mineral deficiencies can result in chlorosis

Plants require mineral nutrients (which arrive in the sap flow after intake by roots) from soil solutions. While carbon, hydrogen and oxygen come from air and water, virtually all the other elements plants need for their growth and physiology come through the root system. Minerals are dissolved in water as ions and are available at various pH levels depending on their solubility characteristics. In general, under alkaline conditions many minerals are held in soil as insoluble precipitates and are unavailable; under acid soil conditions some elements again become insoluble and many leach away from the root zone causing soils to become depleted, especially of metals. Since roots take up minerals as ions (charged molecules) roots must be alive to regulate osmotic potential and the charge balance of ions entering and leaving roots. Anything that compromises root function can lead to inability to take up nutrients and eventually symptoms of nutrient deficiency. Compaction, flooding, root injury, poor soil food web conditions, and pathogens can all impair root function. Plants can show nutrient deficiencies for the following reasons:
• The minerals are missing from the soil
• The pH is not favorable for absorption of the nutrient which is insoluble
• The roots are not able to function and absorb nutrients
• Lack of mycorrhizae in soil or poor conditions for microbial growth

A soils diagnostic lab can help identify soil conditions and nutrient content of your samples, and suggest methods to provide optimum plant nutrition. Several soil samples should be taken all through the areas where affected plants grow, combined and sent to the lab soon after collection. Soil pH is like blood pressure – you can’t tell when it is too high or too low – so you need to have it tested. Knowing the soil reaction (pH) is the first step in investigating nutrient issues. Mulched plantings (with coarse tree trimmings chips) usually have few deficiencies in a wide range of soil conditions because nutrients are slowly but constantly provided and beneficial microbes can assist roots in nutrient uptake.

Temperature Extremes

High light intensity can damage any part of a plant if it is not acclimated to the radiation or if it is undergoing water stress. Here leaves of Privet were damaged by high heat levels
Temperatures that exceed native plant adaptations happened in 2012 and 2020 in California causing extensive damage to native oaks.

Temperature extremes cause injury and may cause abiotic disease to landscape trees. As the climate continues to warm, extreme hot weather is increasingly likely. In California, we had record all time high temperatures in the last three consecutive years and this year in the Pacific Northwest.   In some cases these temperatures were damaging to native tree species, suggesting that they are no longer adapted to the new normal temperature extremes. Years of record breaking freezing temperatures have declined, although cold temperatures can harm sensitive species if freezing occurs suddenly or for prolonged periods. Sunburn comes when temperatures exceed the ability of bark and leaves to adequately cool the tissue. Burned leaves fall from the tree and bark often splits, cracks and dies. This damage can become an important entry point for fungal pathogens such as Botryosphaeria spp. that cause canker diseases in most landscape trees. Planning for a warmer climate means selecting trees that can tolerate higher peak temperatures in summer while surviving the low temperatures of winter.

Air pollutants

Ozone causes “flecking” on pine needles. Image from Petr Kapitola

Another air pollutant is sulfur dioxide, which reacts in the atmosphere to form sulfuric acid,  and may reduce the pH of surface waters.  Acid deposition due to SO2 (the precursor) is an eastern US problem and often tied to coal use for electrical generation.  Air pollution damage to plants depends on the specific air pollutant, its concentration, and the sensitivity of the plant species, with ozone the air pollutant in California having the greatest effect on plants.


Air pollutants originate from a variety of both natural and anthropogenic sources.  Some, called primary pollutants, are released directly into the atmosphere.  Others, called secondary pollutants, are formed via atmospheric reactions of precursors.  Some air pollutants are both primary and secondary.  Ozone is a principal air pollutant in California which also affects plants. It is formed in the lower atmosphere when volatile organic compounds (VOC), i.e., short-chain carbon-containing compounds, which are released from a variety of anthropogenic as well as natural sources, react in the presence of sunlight with oxides of nitrogen (NOx) which come from internal combustion engines.

Ozone is toxic to plant cells because it is very reactive and quickly binds to plant tissues causing damage. Note that ozone in the stratosphere is necessary and protective of life.  It is the same molecule but has a different chemistry of formation. In urban areas, such as the Los Angeles Basin, pollutants may be held in the lower atmosphere by topography and meteorology, and ozone levels  may exceed federal standards for air quality, although much progress has been made since the 1960s . Conifers are particularly sensitive to ozone. Needle retention is reduced and the trees thin and appear yellowed.

While all plant tissues are susceptible to abiotic disorders, stems are most resistant, while leaves, shoots and young roots are perhaps most at risk of environmental factors that cause these disorders. Like biotic diseases, plants with abiotic disorders may require time to develop symptoms. There is a progression from slight to severe symptoms depending on the intensity and duration of the environmental factor causing the disorder. Below are some of the most common causes of abiotic disorders.

References

Costello, L., Perry, EJ, Matheny, NP, Henry, MJ, and PM Geisel. 2003. Abiotic Disorders of landscape plants a diagnostic guide. ANR publication 3420 University of California, Communication Services, Oakland CA.

Manion, P. 1981. Tree Disease Concepts. Prentice-Hall Inc., 399pp.

Schumann, G.L. and C.J. D’Arcy. 2010. Essential Plant pathology. 2nd ed. APS Press The American Phytopathological Society. St. Paul, MN. 369pp

“PoP” Goes the Weasel

How do you plan your work in your garden? One of the things that is most likely to affect what you do is rainfall. But how do you know when and how much rain is likely to fall? One way to get an idea of the possibility of rain is to look at something called “Probability of Precipitation”, or as we call it, “PoP”. How often have you heard someone say that the weatherman (or woman) was wrong because they predicted 30 percent chance of rain and they did not get anything? Or someone else says there was only a 10 percent chance of rain and they got flooded? If you understand how these forecasts are made, it might help you plan your outdoor activities, including your garden work and when you water.

Source: John Robert McPherson, Creative Commons

How is “PoP” defined?

According to the National Weather Service (NWS):

PoP = C x A where “C” = the confidence that precipitation will occur somewhere in the forecast area, and where “A” = the percent of the area that will receive measurable precipitation, if it occurs at all. The forecast is what we call a “conditional” forecast—that means it depends on two different things, one of which requires the other to occur. It’s important to keep in mind that these forecasts are made for a particular period of time (often 12 hours) and for a particular area (the forecast zone). The first part of the calculation is whether or not it will rain at all anywhere in the forecast zone during the time that the forecast covers. The second is how much of the forecast zone will be hit by precipitation sometime during the forecast period.

How likely is it that precipitation will occur?

The first part of the equation above, “C”, is whether rain will occur or not in the forecast zone during the forecast period. Sometimes that is easy to determine if a big high-pressure center is over the area and no rain is expected anywhere in the region. That means that the first part of the equation is zero, and so the PoP forecast would be zero. If a strong front is moving through your area or a tropical storm is headed your way, the probability of rain somewhere in the area is probably close to 100%. But often, the likelihood of rain is not so clear. What if you are not sure about the timing of the front or the tropical storm? If it moves slower than expected, it might not make it to the forecast area before the clock ends for that forecast period. Or if you are not sure the conditions are going to be right for a rain shower to occur, then you might or not get precipitation, depending on the actual conditions. Then C becomes something between 0 percent and 100 percent, depending on how much you trust the computer models that produce the forecasts.


NWS radar image for June 26, 2021, at 4:28 pm ET. The arrow shows the direction of motion of the storms.

How much area will be covered by the storms if it does rain?

As I am writing this, it is raining at my house just southeast of Athens, GA (you can see the tiny yellow splotch on the radar map above). Sometimes it is clear that rain will cover the entire forecast area during the forecast period. But most of the time, we think it will rain in parts of the forecast area, but it will be “hit or miss” rainfall from discrete storms, not widespread coverage. The second part of the equation, A, is the forecaster’s estimate of how much of the area will be hit by rain sometime during the forecast period. It’s not as easy as you think, because those storms are moving, and they cover more of the region than you might expect. In my research I have found that often the PoP forecast is too low, and that rain as estimated by radar covers a wider area than you might expect based on the forecast. Fortunately, the NWS does provide radar estimates of precipitation that are calibrated by actual ground-truth rainfall data. The map below shows the 24-hour rainfall ending on the morning of June 27, 2021, including the rain you see in the maps above. For my CoCoRaHS rain gauge, the 0.05 inches I got correspond quite well to the light blue on the map.

24-hour rainfall estimated by radar June 27, 2021 at 8 am ET

Where do you get PoP forecasts?

So where do you get PoP forecasts and how do you use them? Most meteorologists provide PoP forecasts on their broadcasts or written forecasts. I tend to use the similar Precipitation Potential from the NWS hourly forecasts because they go six days out, which allows a longer planning horizon. You can see a simplified example of a forecast graph below. The “Rain” categories of Slight Chance, Chance, Likely, and Occasional correspond to Precipitation Potentials of roughly 5-25%, 25-50%, 50-75%, and 75-100%. The amount of rain in inches is shown superimposed on the bars, so you get an estimate of how wet it will be in that time period. You can find instructions for how to get your own hourly forecasts at my blog.

The hourly graph allows me to plan well ahead of whatever outdoor activity I am doing. The forecasts show hour by hour how likely precipitation is, including the chance of thunderstorms, and an estimate of how much rain will occur. Keep in mind that the forecasts six days out are not likely to be as accurate as the ones for tomorrow, but they are still useful for planning purposes. Using this information tells me when and how much rain to expect.

Example of an hourly weather forecast for Tifton, GA, on July 5 and 6, 2021. Other variables like wind and humidity are available but are removed here for simplicity.

How do you use PoP forecasts in your planning?

These forecasts are especially useful for determining when to irrigate or apply insecticides or fertilizers that require specific wet or dry conditions to work properly. If you can see rain will be starting soon, you might choose not to water your garden unless the rain amounts are likely to be small or the chance low. Or maybe you want to mow your lawn now before it gets wet. If you are planning to fertilize and if it needs to be watered in, now might be a great time! If you need to apply a treatment that requires wet leaves to be effective, then you might wait until after the rain is over rather than applying now and seeing the chemical wash away in the storm. These precipitation forecasts can help you make the best use of your time by providing targeted, timely information on when rain will occur and how much is likely to fall when it does.

Figure courtesy of WCNC and Meteorologist Brad Panovich (@WxBrad on Twitter)

For more information on PoP forecasts, check out ” Do You (Or Your Meteorologist) Understand What 40% Chance of Rain Means?” by Dr. Marshall Shepherd, my colleague at the University of Georgia.

Mycorrhizae: “If you build it, they will come”

“Field of Dreams”

The movie “Field of Dreams” is a family favorite – we love how baseball and the supernatural are interwoven to create a great story. If you haven’t seen the movie, you should – and for those of you that have, you know why it was important for Ray to build the baseball field. Like the magic that unfolded once that physical space was provided, botanical magic emerges from garden soils that support mycorrhizal life. Garden product peddlers have taken advantage of the scientifically-established relationship between plants and mycorrhizal fungi by selling inoculants. And gardeners tend to focus on which of the many brands of inoculants to buy, rather on questioning their efficacy.

Choices, choices, choices

I’ve attached a link to my peer-reviewed fact sheet on mycorrhizae for a more in-depth discussion about this symbiotic relationship, but the bottom line is this: inoculants don’t work. To understand why, we need to consider a modified version of the disease triangle. Many gardeners are familiar with this concept, which depicts the three criteria needed for plant disease to manifest: the presence of the pathogen, the presence of a host plant, and environmental conditions conducive to pathogen growth. Pathogen spores are EVERYWHERE in landscape and garden soils – they just aren’t activated unless their host is present and environmental conditions allow their germination. Likewise, mycorrhizal spores are EVERYWHERE in landscape and garden soils. We can make a mycorrhizal triangle to visualize the three criteria for needed for mycorrhizae to develop.

While our understanding of mycorrhizal relationships continues to expand, we do know some of the environmental factors needed for successful inoculation:

  1. Soil oxygen. Mycorrhizal fungi are aerobes, meaning they are active when sufficient oxygen is present.
  2. Woody debris on the soil surface. Mycorrhizal species are also decomposers of woody material. There is increasing evidence that a natural woody mulch (not sawdust, not bark) is required for mycorrhizal establishment. Fungal hyphae colonize the debris, extract nutrients, and transport them to their host’s roots. Arborist wood chips are an ideal mulch in this regard as they absorb water and provide an ideal substrate for hyphal development.

There is a robust body of peer-reviewed research conclusively demonstrating that commercial inoculants applied to plants in landscaped soils have no substantial effect on the development of mycorrhizae. This lack of efficacy has induced some inoculant manufacturers to add fertilizer, especially nitrogen, to increase plant growth and fool consumers into thinking the inoculant was responsible.

The image on the left is the label from a mycorrhizal inoculant. Close inspection (middle image) reveals addition of a fertilizer, which is identical in NPK content to a fish fertilizer (right image).

And here is the lesson “Field of Dreams” provides: if you build it, they will come. Build a healthy soil by mulching with a thick layer of arborist wood chips. Not only do they provide nutrients and absorb water, but their presence reduces soil compaction and increases aeration. You can be assured your plants will be successfully inoculated with your soil’s native mycorrhizal species.

This Quercus garryana seedling is already inoculated with native mycorrhizal fungi

Catch my Drift? Herbicide Drift, Curling Tomato Leaves, and Food Safety

There’s all kinds of maladies that can strike your garden plants throughout the season- diseases, insects, negligence, and more.  But one common issue we are seeing more and more here in the corn belt and other places with lots of crop production is herbicide drift.  Of course, you don’t have to have a corn or soy field nearby to have issues with drift – it can happen anywhere and anytime an herbicide is applied and proper precautions aren’t taken, even when you or a neighbor are just treating a small area in the yard.  There are other avenues of herbicide damage on plants as well, such as using herbicide-treated grass clippings as mulch in the garden.

 A wide variety of plants can be damaged by herbicide drift from a variety of different products – trees, shrubs, roses, vegetables, and more.  The damage can be slight to severe, and unless the dose is large most plants will grow out of the damage.  Vegetables and fruits, though, are of particular concern due to the potential food safety risk from residues of unknown herbicides on the plants.  Therefore, it is especially important to be able to identify signs of herbicide drift and take the appropriate course of action which is usually and unfortunately removal of the plant from the garden. 

I have to remove the plants!?!?

Yes, you read correctly, I said removal of the plant!  I, along with many of my extension colleagues, encourage gardeners who have drift or herbicide damage on their plants to remove them from their gardens. Why take such a drastic measure, especially if the plant may actually recover and “grow out” of the damage?  The answer is mainly one of safety.  Since it is likely impossible to know exactly which chemical or product formulation was used there’s no way of knowing if the product is safe to use on consumable crops, whether its residue is safe, or whether it is systemic and has a residual effect.  A gardener cannot know if there is a pre-harvest interval where the crop will be safe after a certain passage of time or if it will never be safe.  And even if you do know the product (let’s say you were the one that used it or you know what is being used by the neighbors) it is likely that there won’t be safety information for use on fruit and vegetable crops, since we don’t typically apply herbicides to plants we want to keep growing.  You should also remember that application of such herbicides to fruit and vegetable crops, even if accidental, technically constitutes an off-label (and illegal) application of an herbicide to a non-target crop or pest. 

What are the most likely fruit and vegetable plants to be damaged from herbicide drift?

While just about any plant can be damaged by herbicide drift if enough herbicide gets on the plant, there are a few plants that seem to be more susceptible to herbicide drift.  This means that these plants exhibit damage with smaller doses of herbicides than others and will show damage while other plants nearby remain unfazed.  The plant that we get the most calls about are tomatoes.  This is the vegetable garden crop that is the most susceptible to herbicide drift and just so happens to be the most widely planted crop in the garden.  The other edible crop that seems to be highly susceptible to herbicide drift is grape.  While grapes aren’t nearly as common as tomatoes in home gardens, wineries in regions with high herbicide use rates are struggling to keep their vineyards going due to the damage.

I live nowhere near a big farm, how do I keep getting drift damage?

Of course, drift can come from anywhere, even a small application of herbicide on a neighborhood lawn or garden.  But under the right weather conditions (high temps and wind) some herbicides like dicamba can volatilize and drift for 2-3 miles or more.  Even if you think you live nowhere near a farm or other area where herbicides might be used you can get drift from miles away.  This makes it hard to pinpoint where the damage is coming from in order to sleuth out what exactly was used.  This is especially tricky here in our area where the city of Omaha is surrounded on all sides by farmland, and even has pockets of productions fields sandwiched between residential areas.  Unfortunately, one of the prime herbicide application times in our region is shortly after most gardeners plant their tomatoes so we get lots of calls and questions that end up being drift.  Thankfully there’s usually still time to replant tomatoes, but it isn’t fun telling people that started plants of their favorite or special varieties that they’ll have to rip them out and go buy new plants. 

The kicker is that drift can be random.  It can be one or two plants out of a bed of twenty, or one plant on one side of the garden and another somewhere else, or an entire field full of plants.  It just really depends on the wind patterns and concentration of herbicide. 

Is it drift?  Or is it something else? 

At first glance it can be hard to tell if an issue is drift or something else since the signs can look like some other problem until you get up close.  There are a wide variety of herbicides on the market and therefore there can be lots of different signs.  The most common types of damage you’ll see are light/white colored and necrotic spots from exposure to broad-spectrum herbicides like glyphosate, and curling, twisting, stunting, yellowing, and epinasty from broadleaf herbicides like 2,4-D and dicamba.  Epinasty is an unusual, twisting growth pattern on the leaves that result when one layer of the leaf (usually the upper layer) grows faster than the other.  You can get weird strappy looking leaves, weird margins, and other irregular growth patterns.  The damage from broadleaf herbicides can sometimes be mistaken for heat or drought damage, viral diseases, or even excess watering, all of which cause leaf curling of some sort.  I’ll share a few tomato pictures below to demonstrate herbicide damage vs other types of leaf curling.  For a great pictorial guide to herbicide damage symptoms, check out this resource form the University of Tennessee

Symptoms of broadleaf herbicide (such as dicamba or 2,4-D) drift damage on tomato. Notice the irregular margins, strappy appearance, and curling of the leaves. The damage is usually limited to a small area on the plant. Photo: Patty Leslie

Note the irregular growth patterns of the leaves in this sample. Herbicide damaged leaves cannot be flattened out to look normal. Photo: John Porter
Widespread damage, likely from application of herbicide-treated grass clippings as mulch. Photo: John Porter
Leaf curling likely from excessive heat, NOT herbicide damage. Note that the leaves could be flattened to look normal. Photo: Scott Evans

Can you avoid drift?

Unfortunately, you can only control drift from the herbicides you apply yourself.  Pesticides such as herbicides can be used safely and effectively if used appropriately.  Reading and following the label instruction is important and is the law, paying special attention to wind speed, temperature, and application equipment, e.g., how fine of a mist does the nozzle create.  Drift from the neighbors’ lawn treatment or a nearby farm is really outside of your control, so being watchful for signs of drift is important.  Sheltering susceptible crops, like tomatoes, using something as a windbreak might be helpful.  As this journal article points out, a windbreak or vegetative buffer around wetlands offers some protection and I noticed a similar effect recently in one of our Master Gardener project gardens.  Our Master Gardeners grow thousands of pounds of produce a year for local food banks, and on a recent visit I noticed that about 25 percent of their tomato plants were showing signs of drift (and they were removed and replaced).  The pattern was interesting – the only plants damaged were the ones on the outside edge of the garden and the ones along wide walkways in the garden.  But plants in the interior were spared.  So perhaps planting less susceptible crops on the exterior of the garden and along walkways to act as buffers might work. 

And while it isn’t useful for home gardeners, specialty crop producers (like those all-important wineries) and beekeepers can register for a program called DriftWatch where they can be informed when spraying will take place on local farms. 

Is your landscape “Sustainable”?

The word “sustainable” gained new life over the last few decades as the concepts of sustainable agriculture and now sustainable landscapes were developed. But what actually are “sustainable” landscapes? This is not something that is easily defined, so I offer my own ideas on the subject here. We can think about this and be thoughtful about landscapes and garden choices as we grow, plant, and maintain landscapes at home and in public spaces.

While this landscape has some of the elements of a sustainable landscape, it is very ugly, with tired artificial turfgrass. The first element of a sustainable landscape is that has an appropriate level of quality.

A sustainable landscape provides benefits

If we start with soil, and nothing growing in it, we can move forward adding landscape elements and benefits begin to emerge. Plants provide habitat for animals including arthropods. As the diversity of plants in a landscape increases, so does the diversity of visitors that use that vegetation. The sculpting of the land may create water catchment areas that help sustain soil moisture. Hardscape (walls, patios, water features and rocks) may create visual focus points. Plants provide many benefits such as sound absorption, dust collection, shade, food, and of course can also be aesthetic. The most sustainable landscape provides its benefits with a minimum input of water, fertilizer and labor to maintain.

While this landscape is visually appealing with specimen trees and broad swards of turfgrass it is not sustainable. The amount of water required to grow poorly adapted trees (some of which are now diseased) in this California climate and the energy required to maintain (mow turf) will require significant on going investments of time or money and hydrocarbons to fertilize and maintain it. Typical of many older landscapes there are no mulch zones.

A sustainable landscape is appealing

Why expend energy or spend money maintaining an ugly landscape? Landscapes in order to be sustained, must appeal in some way to those that use them. In some cases plants in landscapes are adapted to their environment and require little applied water, pruning or other maintenance in order to survive and provide benefits.

Sometimes addition of color to a landscape will help its visual appeal. Surveys of gardeners suggest that colorful landscape are more appealing than those that are only green in color.

Points of interest within a landscape make it appealing. Also, hiding the landscape with gates, shrubs or walls provides intrigue and beckons you forward to explore the unseen parts. While mass plantings of the same plant material can be stunning so can specimen trees or other plants that are strategically placed for high impact. Landscape art either man made or nature made (rocks and logs) can be become the focus of a landscape making it appealing.

In surveys of Master Gardeners this landscape is consistently rated higher than others because of its use of: color, specimen plants, attractive hardscape, presence of trees, and walls that provide some intrigue. The landscape is also easy to maintain and has a low hydrocarbon footprint

A sustainable landscape often contains trees

Trees are the workhorse of landscapes. They provide shade and thus reduce energy costs in landscapes and they are extremely visually aesthetic. Trees are very important for birds, insects, squirrels, and other animals. Trees remove carbon from the atmosphere and feed the soil food web with the captured carbon. Trees help increase the capture of rainfall and the water infiltration rate of soils. While trees do require maintenance (which can be expensive), maintenance costs can be reduced by proper selection, pruning and placement in the landscape. Trees also have proven health both (physical and psychological) benefits for people who live or reside near them.

Keukenhoff gardens in the Netherlands is world famous and has millions of visitors while it is open each year. Keukenhoff is sustainable because of the millions of visitors and sponsors that pay for its maintenance, the plentiful rainfall in the Netherlands, and the Benefits that it provides millions of people
If we remove the trees from Keukenhoff we still have the tulips, but the landscape loses much of its interest and charm.

A sustainable landscape should not consume excessive amounts of energy

The traditional landscapes I grew up with included lawns in the front and rear of residences. This of course required frequent mowing, often with gasoline powered equipment. Shrubs were planted that required shearing with electric or gas powered hedge clippers. Since mulches were never much used, fertilizers (derived from petroleum) were used to push growth which was clipped and hauled (using petroleum to power the trucks) to a landfill. As you can imagine a lot of energy is utilized to maintain such landscaping. Much of the petroleum-based energy expenditure can be mitigated by using more mulch especially if it is produced on site, limiting the expanse of turfgrass to needed areas, and planting or utilizing adapted plant materials to the site and climate. Surround trees with tree chip based mulches, not turfgrass.

This traditional landscape requires excessive pruning of the tree and shrubs and mowing of the turfgrass. Some labor is mitigated by using stone mulch on the side of the yard.
This landscape may be over-planted but use of mulch cuts down the necessity of mowing, prevents weeds, and provides a place to recycle yardwaste in the landscape

A sustainable landscape should be water efficient

For those of us in the west we continue to suffer multidecade droughts. Water use efficiency is necessary for our landscapes to be sustainable because water is expensive and limited. For those that have excess water landscapes need to manage the excess water well without suffering erosion or soil nutrient losses that compromise the landscaping.

Sustainable landscapes provide room for waste recycling

One problem with landscapes that don’t use mulch is that there is no place to recycle used plant clippings. If landscapes are fertilized and irrigated to produce lush growth that is then disposed of with a waste hauler, this is not sustainable. It is best if clippings can be resused as mulch under shrubs or in other out of sight mulched places.

Sustainable landscapes use adapted plants

Adapted plants are not necessarily native plants but plants that can live in the soils at the site with the amount of water that is available to them with a minimum of extra care, fertilizer, requirement of pruning or other inputs (pest management) to keep them looking good.

There are likely many other tenets of sustainable landscapes, but these are some of the key factors. The landscape should be adapted to the climate, provide huge benefits and require less maintenance and then it is, by all means and metrics, sustainable.

This landscape uses garden art, fences and a specimen plant (Dasylerion longissimum) to achieve impact. In the springtime the Wisteria next to the residence adds color. The landscape makes efficient use of water and is adapted to survive with rainfall. Stone mulches help cover the soil.

“Water, water, everywhere…

Did it rain enough last night to water your garden? Have you started running the sprinklers and aren’t sure if they’re running enough? Perhaps you’re not sure that new drip system you installed is doing its job. Or maybe you just want to be more efficient and careful with your water use. How can you know moisture is getting deep enough into the soil to benefit your plants. Is there an easy way to find out?

Yes there is – a simple soil probe will do the trick.

A soil probe can be anything long and sturdy enough to penetrate the soil at least 12 inches (~30 cm.). Handmade soil probes, long screwdrivers, skewers, even the spit from an old rotisserie grill will all work.

A probe made of metal will work best and for safety it should have a handle of some sort. If there’s no handle you should wear sturdy gloves when using it. This set of  22″ screwdrivers was purchased at the local outlet of a national low cost tool franchise. It meets all the requirements and is inexpensive. Plus it’s a set so there’s one for you and one to share!

While you only need the probe to go 12″ into the soil it’s helpful if the probe itself is longer, if only for convenince. The probes are shown here with a yardstick for scale. (Yardstick = 36″=~91.5 cm.)

So you now have a soil probe, how do you use it to measure soil moisture depth? Easy-peasy.
Insert the probe straight into the soil at the spot you want to test. You’ll need to use firm pressure but don’t force it into the soil. The probe will pass through moist soil but stop when it hits dry. And this is true for any soil type, sand, loam or clay. When the probe stops, grasp the probe right at the soil surface and pull it out. The part beyond your hand towards the probe tip shows you how deep the moisture is.

Note: if you have rocky or stony soil the probe may hit a rock and stop. Usually you can hear or feel that it hit a hard object. Just adjust the probe’s postion and try again.

For trees, large shrubs and deep rooted grasses the probe showing a 12″ moisture depth is adequate. For shallower rooted plants or annuals 4-6″ is enough. Monitoring soil moisture depth is an easy way to know if your landscape or garden is adequately watered. Water is a precious resource, let’s not waste it.

To estimate how much rain has fallen on your property, check out this site:
https://water.usgs.gov/edu/activity-howmuchrain.html?fbclid=IwAR3SFjeaflrsXyCtZ_qdUUeltuK9qzYolmybq0wz5KNH2xP9KdJf1g_uckk

When normal isn’t normal

You may have read in the news earlier in May that NOAA has updated their “normals” for temperature and precipitation at stations around the country. In climatology, normals are the calculated averages over a specified time period. Usually, we use a 30-year period to capture what the average weather is like in a time period that is about the length of a generation, but now NOAA is also calculating normals based on other time periods like 15 years. Utility companies often use 10-year normals because electricity-generating technology and energy demand is changing so quickly that 30 years is considered too long.

Source: Marc Schloesser, Creative Commons

Why do they update the normals every 10 years?

Normals are updated every ten years, so the new period of 1991-2020 is replacing the older normal period of 1981-2010. They only do it every ten years because a lot of work goes into quality control of the data as well as adjusting for station moves, missing data, and changes in observation time. All of those events can introduce artificial “climate change” into the record, leading to averages that don’t really represent the current climate at the location of the station being described. Climatologists follow rigorous methods of making these corrections, and even scientists who are skeptical about their techniques by and large end up with nearly the same corrections if they follow scientifically and statistically accurate methods. NOAA has provided some FAQs that explain more about the process of creating the new normals if you are interested.

How are the normals changing?

Determining what a “normal” temperature is when the temperatures are relatively stable is easy, because you can use any long-term average to describe the expected temperature. But when the climate is not stable but is changing over time, what you think of as “normal” weather changes as cooler decades get replaced by warmer decades. For example, here is a graph of the annual average temperature for the Midwest with 30-year normals plotted on it for 1961-1990 (green), 1971-2000 (blue), 1981-2010 (violet), and 1991-2020 (yellow). Early in the record, the 30-year averages (not shown for the early time periods) did not change all that much from one decade to the next because there was no trend towards warmer conditions. But now, every new set of normals gets warmer. We are not living in the climate that our parents or grandparents grew up in! This Washington Post article by Bob Henson and Jason Samenow provide an excellent overview of all the changes that we are seeing and why those changes are occurring. We can expect the next set of normals to be even higher as the temperature continues to rise.

Data from the Midwestern Regional Climate Center.

How are the normals changing across the country?

The annual average temperature is not changing by the same amount everywhere. The map below shows that even though most of the lower 48 states are getting warmer, the upper Great Plains got cooler when the latest normals were calculated. Western Texas and parts of New Mexico had the largest increases in temperature. NOAA also has these maps for select months.

Of course, it is not just the annual average temperature that is changing. The minimum temperatures are increasing at almost twice the rate that the maximum temperatures are rising. Most but not all monthly temperatures are rising at many stations. The precipitation is changing in northern and western high-elevation areas from snow to more rain. Most parts of the US are getting wetter, but the Southwest is getting drier. And the rain is coming in higher intensity bursts, with longer dry spells between precipitation events in many areas.

As temperature and precipitation change, other variables that are related to heat and moisture are also changing. The length of the growing season is increasing in most of the country, allowing gardeners to plant new varieties of heat-loving plants but stressing plants that prefer colder temperatures. This is a concern for peach farmers in Georgia, for example, since peach trees need a certain number of hours below 45 F to set a good crop of fruit. As the temperature rises, it becomes harder for the trees to get the cold weather they need to produce enough blooms. Other plants like lilac, which I enjoyed every spring when I was growing up in Michigan, do not grow in Georgia because of the heat and may someday be scarce even in the Midwest. Growing degree days (a measure of the amount of time above a base temperature, commonly 50 F, used to track plant development) are increasing, affecting the growth patterns of commercial crops as well as garden plants. Humidity is also rising, leading to more fungal diseases and more oppressive working conditions for gardeners and farm workers who are affected by both the higher moisture levels and more frequent extremely hot days. At the same time, higher evapotranspiration from plants accelerates the water cycle, making droughts (and floods) more likely.

Where do you find your local normal weather?

If you are interested in finding your new “normal” temperature and precipitation and comparing it to the old values at your location, you can find instructions at my daily blog. Of course, there are many other places to find it as well—just do a search online and several sites should pop up. If you want to do an average over a different set of years, you can use the Custom Climatology Tool from the University of Nebraska-Lincoln to do those calculations.

Ultimately, the changes in the climate reflected in the new normals will show up in other garden-related values such as the USDA Plant Hardiness Zone, although it’s hard to know exactly when those values will be updated. Even without knowing exactly what zone you are likely to be in over the next decade, with the continuation of rising temperatures that we expect, you can try out plants that are just on the warm side of your current zone to see how they do. Of course, your local microclimate will also affect their ability to thrive, so don’t forget to consider that too.

Should we just get rid of “normals” since they keep changing? I don’t think so, since they do provide useful information about what we expect over a number of years. You can use normals to determine what clothes to have in your closets, how much heating and cooling you need for your homes, and what to plant in your garden. Just be aware– “normal” is no longer normal in a changing climate.

Rooting around – the differences between taproots and mature roots

A seedling with green cotyledons and emerging radical

Most of us have witnessed dicot seed germination at some point in our lives – watching the coytledons transform from seed halves to green, photosynthetic structures, while the radicle developed into the seedling root system. This seedling root – or taproot – is important to seedling survival as it buries itself in the soil to provide structural support and to give rise to fine roots for water and nutrient absorption. But that’s where much of our visual experience ends – because we don’t see what’s happening underground. Without additional visual information we imagine the taproot to continue growing deep into the soil. And while this perception is borne out when we pull up carrots, dandelions, and other plants without woody root systems, the fact is that woody plants do not have persistent taproots – they are strictly juvenile structures. Understanding the reality of woody root systems is critical in learning how to protect and encourage their growth and establishment.

Mature carrots have taproots. Mature trees do not. Photo courtesy of Pixnio.

Trees, shrubs, and other woody perennials all have juvenile taproots just like their herbaceous counterparts. But these long-lived plants develop different morphologies over time, which are primarily determined by their soil environment. Water, nutrients, and oxygen are all requirements for sustained root growth. Gardeners always remember the first two of these needs, but often forget the third. And it’s oxygen availability that often has the biggest effect on how deeply root systems can grow.

Roots grow where they can. Sometimes that zone can be very shallow, as this coastal forest photo shows.

Whole-plant physiologists have known for a long time that “roots grow where they can” (Plant Physiology, Salisbury and Ross, 1992). But this knowledge has become less shared over time, as whole-plant physiologists at universities have been largely replaced by those who focus on cellular, molecular, and genetic influences (and can bring in large grants to support their institution). Sadly, many of these researchers seem to have little understanding about how whole plants function. Simply looking at the current standard plant physiology textbook (Plant Physiology and Development, Taiz et al., 2014) reveals as much. (To be fair, there is now a stripped-down version of this text called Fundamentals of Plant Physiology, [Taiz et al., 2018] but even this text has little to do with whole plants in their natural environment.) If academics don’t understand how plants function in their environment, their students won’t learn either.

The Table of Contents for Plant Physiology and Development. You won’t find a discussion of woody root ecophysiology in here.

Well. Time to move on from my soapbox moment on the state of higher education.

Roots grow where oxygen is plentiful. It becomes a limiting factor as soil depth increases. Photo courtesy of Wikimedia.

Let’s look at what happens with a young tree as it develops. The taproot grows as deeply as it can, but eventually runs out of oxygen so vertical growth stops. At the same time, lateral root growth increases, because the levels of oxygen closer to the soil surface are higher. These lateral roots, and their associated fine roots, develop into the adult root system, continuing to grow outwards like spokes on a wheel. When pockets of oxygen are found, roots dive down to exploit resources. These are called sinker roots and they can help stabilize trees as well as contribute to water and nutrient uptake.

Gardeners and others who work with trees and other woody species would do well to remember that woody root systems, by and large, resemble pancakes rather than carrots. These pancakes can extend far beyond the diameter of the crown – so this means protecting the soils outside as well as inside the dripline.

Typical root structure of a mature tree in its natural environment. No taproot here!