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

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.

My Soil Is Crap, Part II

Last month in my blog My Soil Is Crap Part I, I tried to dispel the myth that you can diagnose soil problems by just looking at your soil. While the color of a soil does impart some diagnostic qualities, most soils are not easily analyzed without a soils test. A complete soils test will give a textural analysis including useful information about water holding capacity and a variety of chemical analyses. Soil reaction or pH is an essential component of any soil test (and is often unreliable in home soil test kits). Soil reaction affects the availability of plant required mineral salts. Most soil tests give a measure of the salinity sometimes call TDS, or total dissolved salts (solids). Finally specific mineral content of soil is usually analyzed – in particular macronutrients are usually quantified. With these data a great deal can be predicted about the “grow-ability” of your soil. Soil tests can also help guide attempts to modify soils. The biology of soils is not easily or routinely analyzed through soils tests.

Soil Harm

Soil can be “harmed” in several ways–making it less able to grow plants. Or another way to look at this is that soil can be enhanced in several ways to grow plants better. First let’s examine the harm. Soil can be physically harmed by tilling with a rototiller. Tillage destroys structure and the natural clods and peds that form over time because of a soil’s innate qualities. Structured soils support plants and help prevent disease. Tilled soils will in time resume their native structure, but the amount of time required is quite variable depending on soil type. Soil structure can also be squished– this is compaction. Compacted soils hold less water, take water in slowly (so more runoff) and have less air space for gas exchange. In severely compacted soils roots have difficulty penetrating so plants don’t grow well or at all in compacted soil zones. Compacted soils are common in parks, school yards and public areas. Finally soils can be damaged chemically and biologically. Excessive salts from fertilizers applied in excess can compromise roots causing fertilizer burn. Soil residual herbicides from overapplication can have toxic effects on plants growing there or nearby. Herbicides and salts often accumulate along roadsides where they are used to melt snow and ice or control weeds.

Compacted, saturated or layered soils can build toxic gases that reduce metals in the soil, creating hazardous conditions for plant growth

Climate affects on soil

Climate can modify soils making them less than optimal for growing plants. In areas of high rainfall, soils may become deficient in certain ions such as metals, which tend to leach from soil, leading to increased acidity because these ions help maintain pH neutrality. In areas where precipitation is less than evaporation, salts tend to accumulate in soil and soil reaction rises above neutral. The ideal soil pH for most plants is 6.8. At this pH, most plant-required minerals are available for absorption by roots. As pH moves above 8 or below 5, soils are said to be alkaline or acid and various minerals are less available to plants. Soil reactions between pH 6.8 and 7.2 usually pose few problems for most plants. Some plants that are “acid loving” like blueberries are adapted to grow in low pH soils where nutrients are supplied by decaying organic matter. For these kinds of plants, some soil modification may be necessary (unless you live in a climate where such plants are natives). Testing your soil pH is very important to understand nutrient availability in general.

Amending vs Mulching

Arid soils are usually low in organic matter. In climates with more rainfall where forests or grasslands naturally occur, soils have higher organic matter content. Typically organic matter ranges between 1 and 5% of total soil solids. Organic matter supplies carbon for soil microbes and is necessary to promote soil structure. Organic matter can hold and release positively charged (cations) soil mineral nutrients used by plants. Organic soils have the highest cation exchange capacity (CEC), a measure of soil fertility. Soil organic matter tends to bring soil pH back toward neutral. Very acid or alkaline soils can be modified by adding organic matter. Finally, organic matter may contain nutrients that help plants grow. Sometimes amending with a nutrient-rich compost will give annual plants quite a boost (see Calendula images below) While arborist chip mulches yield nutrients to soils slowly over years, composts provide nutrients immediately, and they can be easily over-applied depending on what is required for a given soil to grow the intended plants. If you are going to amend a soil, be sure that the amendment has enough nitrogen in it. Well-formed composts, high in plant required mineral nutrients but not overly salty, make excellent amendments.

Adding amendment to planting holes of perennials is not recommended because it has little long term effect

Perennials, including all woody plants, generally do not benefit from amending because they rapidly grow out of the amended zone in the planting hole. Unless you amend an entire site, not much will happen. Also, once perennials are set in the ground you can’t amend again. Mulches of arborist chips, fresh or aged, are best for perennial plantings. Mulches can be replenished as needed without disturbing root systems. Raised beds are often amended heavily, and rightly so, since these planting situations amount to large containers that need a more porous “soil”. Since raised bed plantings are usually annuals, amendment can be added again as needed between crops. Composts make suitable amendments. Compost qualities, especially salinity, should be carefully measured or monitored before using, or through a bio-assay as detailed in my last blog.

Adding minerals and fertilizers

Gardeners generally buy and add fertilizers without concern to harming their plants. This is a big NO. Excess levels of phosphate can interfere with uptake of other needed minerals. Applying fertilizer to landscapes above what is needed can pollute creeks and other bodies of water. It is important to let your soil test guide fertilizer applications. Usually there are enough fertilizer elements in most soils that landscapes can remain unfertilized, especially if leaf litter and mulches are utilized. If plants show deficiency symptoms be sure to check your soil reaction to make sure that the pH is in a growing range for the plants you are cultivating. If the pH is right but you still have symptoms, then consideration of fertilizers based on soils tests is appropriate.

There is some confusion about use of minerals as amendments. Lime is used to raise pH and often brings soils back into production in high rainfall areas where soils are too acid. Gypsum does not alter pH of soils but is often called things like “clay buster” or “compaction reliever” This is because salt affected clay soils have too much sodium which is replaced by calcium when gypsum is applied to a sodic soil relieving some of the particle dispersion. Most gardeners do not have sodic soils (which are greasy and poorly productive) but just plain old clay or clay loams. Gypsum supplies sulfate as an anion and calcium as a cation and if sulfur or calcium are deficient gypsum can be helpful. Gypsum is not needed in most gardens. Gypsum does have a fungicidal effect against root rot organisms (Phytophthora) and can be added to reduce root rot hazard. Epsom salts (magnesium sulfate) are often recommended for rose culture, but there is no research showing any benefit from their application to roses. In our trials in California, application of Epsom salts had no effect on rose bloom quality or quantity. Some soils low in magnesium could benefit from magnesium sulfate but these are fairly rare.

Some Soil changes are not long lasting

The textural nature of soil (i.e., relative amounts of sand, silt and clay) does not change over time. While we can add organic matter, it breaks down and disappears rapidly. Water quality, evaporation, and rainfall drive soil change. These factors tend to bring soil back to its “native” conditions. Irrigated soils may be affected by the quality of the irrigation water. So if you are trying to grow blueberries in Las Vegas, this will be a challenge that likely can’t be met by soil modifications. Growing plants adapted to the type of soil and climate you have is best. Growing exotics that require a different soil formation process will always be an uphill battle better suited to container culture.

References:

Blakey, D. 2021. Adjusting soil pH in California Gardens. UCANR publication 8710. https://doi.org/10.3733/ucanr.8710

Downer. A.J. and B.A. Faber. 2021. Organic Amendments for Landscape Soils. UCANR publication #8711.

Downer, A.J., and B.A. Faber. 2019. Mulches for Landscapes UCANR publication #8672.

Faber, B.A., A.J. Downer, D. Holstege, and M.J. Mochizuki. 2007. Accuracy varies for commercially-available soil test kits analyzing nitrate nitrogen, phosphorus, potassium and pH. HortTechnology: 17:358-362.

Messenger, B.J., Menge, J.A., and E. Pond,. 2007. Effects of gypsum on zoospores and sporangia of Phytophthora cinnamomi in field soil. Plant Disease 84(6): 617-621

My Soil is Crap

My Soil is Crap! Or is it?

Over several years of teaching basic soil science to arborists, master gardeners and students something started to coalesce into a trend. If I ask my students do they have “good” soil, many say no. I have heard Master Gardeners complain their soil is terrible or that a certain soil is bad in some way. People form opinions about soil based on its color, texture, odor, or even how plants grow in it (perhaps the most diagnostic quality). So how do you know if your soil is “crap”? Soil is a combination of physical, chemical and biological properties not all of which are obvious from a casual examination. Soil is infinitely variable depending on how it was formed and what has happened to it. Many soils are fragile and their growing properties can easily be harmed.

Soil forms from its parent material or rocks that weather over time to form smaller and smaller particles

Soil Formation
To understand soil you need to understand how it forms. Soils are often depositional, forming as particles are deposited in place from wind, or water or other weathering factors. Deep soils form from the alluvium  as water washes particles down from mountains. Terraces along streams also form soil deposits when they overflow the stream bed. Almost all soils form from rocks that are referred to as the parent material. The kind of rocks that form the parent material determine the minerals that will dominate that soil. Exotic soils like serpentine soil contain large amounts of magnesium but lack calcium. Soils can be young (not deep or fine textured) or very old (deep clays). One of first things gardeners should seek to find out is if they have “native” soil or are gardening on fill. Soils are also modified by climate especially rainfall. High rainfall areas have leached soils, are usually forested, and have acid soil reaction (pH). Arid soils usually have excess salts, and tend toward being alkaline. Understanding soil formation helps to understand what kind of soil you have and how to utilize it best for your garden.

Residential landscapes are often on fill soils with various textures and interfaces. Here decomposing granite surface soils cover the actual clay loam textures underneath. Soils can vary significantly on the same property requiring multiple tests and actions for their treatment.

Fill is not Soil
One of first things gardeners should discover is if they have “native” soil or are gardening on fill.  Fill around homes and cities is not soil in the natural sense. Fill soil is not formed in a natural process, it will not have the predictable qualities of soils and may be extremely variable even on a single property. Soil maps are available from your cooperative extension office and on line from the NRCS (https://websoilsurvey.sc.egov.usda.gov/App/HomePage.htm). The web soil survey is a map of naturally occurring soil types. Soils are described in detail and understanding your soil type will inform its ability to grow plants, hold water and minerals, etc.

Soil Physical Properties
No matter which soil you have, gardeners will want to know what to do to make it better for growing their plants. The physical characteristics of soil are important for gardeners to understand. Soil texture is described by analyzing the content of various particle sizes. Sands are composed of large particles silts have intermediate size particles and clays contain the finest particles. Soils texture is the relative content of sand, silt and clay particles and are described by their content of these particles such as a “clay loam” Pure loams are relatively rare because they have equal measures of sand silt and clay and are considered the most arable soil textures. A clay loam has more clay than the other particle sizes but enough to still be considered a loam. Textural classes are described by the soil triangle. You can diagnose your soil texture by using a ribbon test where you feel the soil and analyze its qualities. A laboratory can separate the particles and give an exact analysis. Soil texture affects horticulture directly as it determines drainage characteristics, moisture content and mineral holding capabilities.

Soil Chemical Qualities
One of the most defining chemical qualities of soil for gardeners is nutrient content. Minerals or elements in soils are highly variable based on soil age, their formation processes and the parent material from which they  developed. Fine textured soils have more mineral nutrients and storage capacity than coarse textured soils. Sands tend to be hungry for plant nutrients and clays are usually rich in nutrients. This is because as particle size decreases the electrical properties of soil become more negative in charge and tend to retain positively charged mineral nutrients. You can estimate nutrient content by seeing how plants grow in a given soil without fertilization. If weeds are abundant and happy, the soil may contain adequate amounts of the 18 different elements necessary for plant growth. The only way to accurately know the nutrient content of a soil is to have it analyzed in a soils lab. There are other blogs at this site that tell you how to take a soil sample. Never fertilize a soil that already grows plants well as you will be polluting surface waters and contaminating streams with excess fertilizer elements that can leach or run off.

A well structured soil has water-stable aggregates, pore spaces, roots, hyphae, organic matter etc. This kind of soil is the product of a robust soil food web.

Biological Qualities of Soil
The most elusive quality of soil is the biological quality. Soils are ecosystems of organisms. Much has been written about the soil food web and it is a critical part of how soils and plants interact. While we can see worms and small arthropods; bacteria, fungi and nematodes are not visible. It is difficult to visually assess soil biology. However there are some indicators. “Healthy” soils are often well structured. Soil structure is a physical description of the way soils form aggregates, clumps and clods. Well structured soils have abundant pore spaces, bits of organic matter, and have distinct clods or clumps. Often these clods are water-stable, that is, if you put a soil clod in a jar of water it will not dissolve. This is an easy test you can make of your soil. Place a clod in water and leave it there over night if it dissolves it is not a water-stable aggregate. Water stable aggregates from from the action of soil microorganisms that bind soil particles with polymers as well as the hyphae of fungi which connect particles together.

Soil Carbon Drives Soil Biology
Healthy soils have more carbon in them then soils that are not biologically active. Organic matter is an important part of soil and is added as litter or mulch breaks down and by plants themselves as they deposit carbon through exudates and associations with microorganisms. Plants can add as much as 20% of their carbon captured through photosynthesis into soil through root exudates and microbial association. Carbon is food for microbes and an essential component of a healthy soil. Soil with large amounts of organic matter are dark in color (but so are many low OM clays so don’t be fooled). Again the only way to know exactly how much organic matter is in soil is by a soil test. A detailed soil organism analysis may not help you that much because it is difficult to assign specific roles to groups of organisms living in soil. If we provide organic matter (fresh wood chip mulches in perennial plantings) the food web will grow to utilize it and we do not need to worry about who is using the carbon.

A bio-assay of three soils (2 cups each) planted with radish and carrot. From top left to bottom right: clay loam; silt loam and potting medium

Despite all these factors soils are still a bit magical. Even with soil surveys, and soil analyses you really can’t tell if a soil will grow well until you try to do so. In my University class I am having my students do a simple bio-assay (growing seeds in soils) The assignment was to grow radish and carrots in three different soils, hoping that some would show up signs of damping off disease. I did the experiment as well. My seedlings were grown in a silt loam, a clay loam and a potting medium. The soil-based differences are very visible. The clay loam grew the largest seedlings. Bio assays such as this are helpful to see what the growing qualities of soil are. They don’t tell the entire story but they are very interesting for comparative purposes. Bio assays are great to do before you purchase soil for raised beds or if you are gardening in a new soil that you don’t know much about. In the next blog I will touch on how, when, and why soils should be modified to enhance your garden.

The contrarian rosarian–debunking rose mythology

Roses are perhaps the most frequently cultivated landscape plant across America. Rose gardens are common to parks, landscapes, botanical gardens and for homeowners. Everyone seems to have an opinion about rose culture and there are numerous clubs and societies to support the hobby of rose growing. This week I am in the midst of pruning my rose fertilizer study here in Santa Paula California. I have 240 roses of eight varieties and my thoughts are on roses now, so I offer this blog to dispel some of the myths about rose horticulture.

Myth I–Roses are difficult and require a lot of pesticides

Roses grow well in California soils. A selection of varieties here in Santa Paula CA

Most roses grow easily in most soils in most places. Roses tolerate environmental extremes very well. They grow in many climates and tolerate below freezing temperatures during winter dormancy and high temperatures during summer. Current rose varieties have been developed through breeding of wild rose types. Floribundas, hybrid T roses, grandifloras, shrub or landscape roses, climbing roses and dwarf roses offer the enthusiast a variety of forms and functions in the Rosa genus. In the early 19th century Empress Josephine of France gave rose development a great boost in her own garden at Malmaison. Her patronage of rose research led to the development of thousands of varieties in Europe and later in the United States. The genetics of garden roses is now quite diverse. Because of the diversity of roses some grow better than others, some are highly disease resistant some are very susceptible. Like all plants, roses develop various kinds of diseases and attract pests. Because they are grown commonly in gardens there are many rose pesticides available for use. In my decade of rose research growing hundreds of roses, I have never used pesticides to maintain them. Susceptible varieties could be treated with pesticides or gardeners can chose to avoid varieties that host pests and focus on ones that are not so afflicted. With so many varieties available to gardeners there will be strong varieties and weak ones, pest prone and healthy. The variety you select will determine the necessity for pest control. Many many roses are relatively pest free and grow well without any treatments.

Myth II Roses Require lots of irrigation

The idea that roses need more water than other landscape plants is a horticultural misnomer. In the Central Valley of California roses are grown for production to consumer markets and they typically are furrow irrigated once every eight days in the growing season. Even during triple digit weather, they are held to this schedule without damage.

Can you tell which one got Epsom salts? No. there is no difference between roses grown with applied magnesium sulfate vs those not receiving the treatment.

Myth III Roses require rose specific fertilizers

Roses need the same mineral element as other plants. There is no evidence that increased magnesium (Epsom Salts) benefits roses in any way. Prescriptive fertilization is not appropriate for rose culture or any landscape setting. Fertilizers should be applied on the basis of soils tests that determine the necessity of minerals that may be missing from the soil.

Rose varieties respond widely to field conditions. In the same field some varieties consistently thrive and others grow poorly. Rose varieties have variable vigor, tolerance of soil conditions and pest resistance.

Myth IV Prune rose canes at 45 degrees that is with angled Cuts

There are many pruning strategies for roses. One of the most consistent myths is that roses should be pruned with angled cuts so water is shed away from the cut end. There is no scientific basis for this and therefore it is not recommended. Pruning back to an outward facing bud is a good idea as it maintains a less tangled rose canopy and helps to promote a more organized architecture in the shrub. Various sources recommend more or less severe winter pruning for roses. Our research shows that the less severely you prune major canes the more flowers that will result. Severe pruning did not increase rose flower quality or quantity. The best rose shrubs (most flowers) are pruned to maintain their shape and reduce tangle while maintaining shrub size.  I almost forgot–Don’t seal pruning wounds made to rose canes.  Leave cuts to dry.

Myth V Mounding soil around the base of roses should be done every winter

Some rose experts, especially in places with cold climates have advocated mulching with manure or soil over the crown of the rose before freezing winter temperatures set in. Most rose varieties survive the cold winters without this treatment if snow is present. If temperatures fall rapidly without snow, a covering of leaves or straw may be helpful.

Myth VI Grafted roses are better than non-grafted roses

The recent advent of landscape or shrub roses has proven that this myth is incorrect. Non-grafted roses have the advantage of not producing annoying suckers that need to be removed frequently as on some grafted varieties. Many of the landscape roses growing on their own roots are more disease resistant, more vigorous, and produce more flowers consistently than their grafted counterparts. Not all scions are perfectly compatible with their rootstocks so some grafted roses are less vigorous due to graft incompatibility.

Roses are easy to grow once they are established. In recent years, I have had trouble with roses purchased from garden centers that would not grow when planted out. This may be because the plants were held too long in storage before coming to market. It is also imperative when first planting roses to frequently sprinkle the canes to avoid them drying out. Desiccation is a common killer of freshly harvested roses. Once buds “pop” and shoots emerge, culture can continue as with any garden plant providing appropriate moisture as needed. Fertilization should follow recommendations of your soils analysis.

Reference:

Downer, A.J., A.D. Howell, and J. Karlik. 2015. Effect of pruning on eight landscape rose cultivars grown outdoors. Acta Horticulturae 1064:253-255.

Why Fresh is Best—when it comes to mulch?

Fresh wood chips!

One of the most misunderstood gardening practices is mulching. There is much mulch misinformation in horticulture books, web pages and even extension leaflets. First,what is Mulch? Mulch is any substance the covers the soil surface. Mulch can be inorganic (rock), hydrocarbon (plastic) or carbon based (chips, bark etc.) While any material applied to the soil surface could be considered mulch, the benefits of mulching especially to woody plants are greatest from fresh woody chippings of tree trimmings–so called “arborist chips” applied fresh—not composted. Annual plants such as vegetable plants are often mulched as well but usually with materials that rapidly break down such as straw or some mixtures of shavings and manures. These materials are easily incorporated later when the next crop is planted. For woody plants such as trees and shrubs, mulches that persist for a longer time are desirable. Plastic mulches used in agriculture are not suited to shade trees or other landscape uses nor are landscape fabrics. Each of these deteriorate into landscape trash rapidly and do not benefit soils under the mulch layer. Stone mulches while used extensively in the South west US are not as beneficial to soils as arborist chips.

Why use mulches anyway? Mulches support healthy tree and woody plant growth in landscapes around the world. They increase soil organic matter, the diversity and functionality of the soil food web (particularly saprophytic fungi), support mycorrhizal partners of woody plants, supply nutrients and suppress weeds. Thick mulch layers increase root development, and help to suppress soil borne plant pathogens. The breakdown of woody mulches on the soil surface encourages development of soil structure, increased water infiltration, water holding capacity, and nutrient holding capacity of underlying soil layers. Well mulched trees and shrubs grow healthfully without fertilization.

So why not mulch with compost? Compost is not suited for use as a mulch around trees and shrubs. Compost is often screened and is of fine texture. Fine texture presents a few problems. Fine compost will make hydraulic conductivity with soil and allow for water to evaporate through the compost/soil interface. Thus the moisture savings we see under arborist chips will not be the same under compost. Compost is also able to allow weeds to germinate in it so the weed suppression effects of a mulch will also be lost. Composts applied as mulch can make an interface between the soil surface and the mulch layer which should always be avoided as it will impede water movement through the interface.

Another important reason for not mulching with compost is that compost is poor nutritionally for soil microbes. Composts have most of their active or labile carbon burned away during the composting process by the rapid respiration of microbes. The compost is turned aerated and kept moist until the process stops at this point it has some level of maturity. It won’t reheat when turned. The microbes have consumed most of the available carbon for their own growth and respiration in the compost pile, none of this remains for microbes in the landscape. Fresh arborists chips are full of labile carbon. When laid over the soil surface spores of fungi invade and they begin to uses this carbon for their own growth as an energy sources. Placing fresh wood chips on the soil surface is feeding the soil microbiology at the soil-mulch interface. In time (a few years) these processes go deeper in the soil and begin to feed the soil food web beneath the mulch layer. The diversity of fungi increases, mycorrhizae begin to transfer mulch nutrients to their woody hosts and pathogens are destroyed by enzymes that leach from the fungi infested wood chips. While composts supply minerals (all that is left of the feedstock after composting) they can’t supply the labile carbon as a source for microbes. Fresh arborists chips do all this and are thus the best mulch for woody plants.

Fungi eventually invade fresh mulches releasing nutrients and enzymes to underlying soils

There has been some concern lately for using mulches that are recycled as yardwastes. This concerns me as well because gardeners may be disposing of dead plants in their greenwaste cans. In theory, pathogens could be coming through the greenwaste stream to gardeners. Getting tree chips is best because there is little likelihood for soil borne pathogens since the materials are chipped branches. There is some possibility of wilt diseases (Verticillium spp.) surviving in arborists chips but little research has established that the pathogen can infect especially if the chips are stockpiled for a short time. In my own research we showed that pathogens, weeds an insects had very short survival times in stockpiled (not turned) piles of greenwaste. There is very little chance of pathogens coming to your garden from arborist chips and the benefits to your soil and perennial plants are worth the effort to get a “chip drop” from your local tree care company.

Pathogens buried in fresh yardwaste do not survive for very long

Literature

Chalker-Scott, L. 2007. Impact of Mulches on Landscape Plants and the Environment — A review. J. Environ. Hort. 25(4) 239-249.

Chalker-Scott, L., and A. J. Downer 2020. Soil Myth Busting for Extension Educators: Reviewing the Literature on Soil Nutrition. J. of the NACAA 13(2): https://www.nacaa.com/journal/index.php?jid=1134&fbclid=IwAR0cPfBl3V-3car-RPeEmlqzwW8bPEOPgND07xMTNgCOa5GkuSWtdD5WzF8

Downer, A.J., and B.A. Faber. 2019. Mulches for Landscapes UCANR publication #8672

Downer, A.J., D. Crohn, B. Faber, O. Daugovish, J.O. Becker, J.A. Menge, and M. J. Mochizuki. 2008. Survival of plant pathogens in static piles of ground green waste. Phytopathology 98: 574-554.

Downer, A.J., J.A. Menge, and E Pond. 2001. Association of cellulytic enzyme activities in eucalyptus mulches with biological control of Phytophthora cinnamomi Rands. Phytopathology: 91 847-855

Downer, J. and D. Hodel. 2001. The effect of mulching and turfgrass on growth and establishment of Syagrus romanzoffiana (Cham.) Becc., Washingtonia robusta H.Wendl. and Archontophoenix cunninhamiana (H.Wendl.)H. Wendl. & Drude in the landscape. Scientia Horticulturae: 87:85-92

Pruning Paints Debunked

When my turn comes up to blog for the Garden Professor site I like to reflect on the horticulture in my own gardens and orchard. Right now I am focused on pruning my old apple and stone fruit orchard. It has suffered bear attacks, drought, and mismanagement before we arrived in 2018. The previous owners were very aware of the need to treat pruning cuts large and small. The remnants of tree wound dressings are found all through our orchard and range from white latex paint to silicone caulk. Unfortunately there has never been good research evidence to support pruning paint use. Despite the lack of any published evidence, for their usefulness, pruning paints are still available in garden centers and there are no end of do it yourself preparations that gardeners continue to use on pruning wounds.

Wound dressings did not protect this apple branch from decay fungi

So why paint the cuts on your fruit trees after pruning? One idea is to keep the surface protected from infection by pathogens. Plant pathogenic fungi and bacteria can cause disease that may lead to blight, cankers, or wood decay.

Laetiporus gilbersonii (chicken of the woods) is a common brown rot wood decay fungus that destroys cellulose in wood.

Wounds are often implicated in pathogenesis or disease development. Many horticulturists believed that wound dressings provide a barrier to entry of pathogens and insects. Fruit trees are easily decayed by a number of fungi which cause white and brown rots in their wood. Wood decay organisms enter through wounds created when branches break from excessive fruit loads or when pruning wounds expose heartwood or significant amounts of sapwood. So painting cuts became a very common practice advocated by gardening columns and various books over the last century.

Wound dressings used in Ukraine for many years on this shade trees did not stop decay fungi from fruiting under the wound dressing! Photo courtesy Igor Signer, Kiev, Ukraine

Wood contains cells that store starch. Here, parenchyma cells in the wood ray tissues have been stained purple to show their starch content. Fungi that invade wood use this stored energy to grow, invade and degrade wood. Fungi invade both the heartwood (non-living) and the living, water transporting sapwood. Sap-rotters typically lead to the decline in tree vigor and canopy density.

Over one hundred years ago Howe (1915) recognized that pruning paints did not help wounds to close, in fact, they retarded the development of callus wood especially in peaches. Howe called into question the necessity of using wound dressings at all. Still the use of wound dressings has prevailed to this day.

Shigo and Shortle (1981) showed that wound dressings do not prevent decay nor do they promote wound closure. If the poor pruning practices that harm trees are abandoned, then wound dressings are unnecessary (never mind that they don’t work). Shigo often maintained that tree genetics determine the extent of decay forming in a given species. His work conclusively showed that flush cuts would lead to more decay than cuts that were made outside the branch collar or bark ridge.

Expanding foam? As far as I know there is no research on expanding foam but lots of anecdotes and observations of how it is often used to fill tree cavities. Filling cavities with cement to prevent or limit decay is a practice that subsided some decades ago and is generally not recommended as part of modern arboricultural practice. By the time decay has caused a cavity it is usually well entrenched in the wood of a tree and is not controlled by filling in the void. The best way to limit decay in trees is to prune them frequently so cuts are never large and the tree (fruit or shade) develops a strong structure that is unlikely to fail.

Literature:

Chalker-Scott, L., and A.J. Downer 2018. Garden Myth Busting for Extension Educators: Reviewing the Literature on Landscape Tree. Journal of the NACCA 11:(2) https://www.nacaa.com/journal/index.php?jid=885

Howe, G.H. 1915. Effect of various dressings on pruning wounds of fruit trees. New York Agricultural Experiment Station, Geneva, N.Y. Bulletin No 396.

Shigo, A.L and W. C Shortle. 1983. Wound dressings: Results of studies over 13 ykears. J. or Arboriculture 9(10): 317-329.

Shigo, A.L. 1984. Tree Decay and Pruning. Arboricultural J. 8:1-12.

The worms crawl in and the worms crawl out but these worms kill your plants

Our first major frost hit my part of Arizona a month ago, killing all tomato vines. I did my thanksgiving cleanup chores–removed all the vines and ground them into mulch. I noticed an ominous symptom on one a few of the heirloom varieties (Prudence Purple) that I removed—galled roots. This symptom when seen on tomato is evidence of Root Knot Nematode (RKN). More about RKN shortly. Nematodes are non-segmented worms, mostly free living in soil and feed on bacteria, fungi, small animals or each other. Nematodes are small, barely perceived without magnification but easily observed under low power microscopy. Most nematodes are principal components of the soil food web and are vital to its health and functioning. A few kinds (>30) are opportunistic plant feeders. Plant pathologists consider nematodes plant pathogens because they evoke complicated responses in plant physiology leading to the development of symptoms.

Root knot nematode (Meloidogyne spp.) forms extensive galls on Prudence Purple tomato by the end of a growing season.

Plant parasitic nematodes have some common features and some rather diverse feeding habits and lifestyles. All plant parasitic nematodes have a stylet or spear at their mouth end that is used to puncture plant tissues and such the sap from their host. Looking under a dissecting microscope you may not be able to identify the genus of a nematode but you can tell if it is bad for plants by seeing the spear just behind its mouth. Plant Parasitic Nematodes (PPN) are either migratory or sedentary. All PPN reproduce by eggs and molt once inside the egg emerging as a second instar juvenile nematode. After a couple more molts the juveniles become adults. Male nematodes are less common than female worms. As adults they can keep feeding from plant to plant if they are ectoparasitic (feeding outside of the root) or they can settle down and make eggs inside a cyst or gall. Some nematodes are endoparasitic and once inside the root never leave it until their eggs hatch and juveniles swim off find another host.

Even though these marigolds are heavily galled by root rot nematode their only above ground symptoms are dwarfing or slowed growth

Gardeners should be on the lookout for PPN by noticing symptoms of infection. The most common symptom caused by nematodes is stunting or reduced growth. There may be no other symptoms observable. When the number of PPN is quite large, yellowing or chlorosis can occur as the worms shut down a plant’s ability to take up water and minerals. RKN is the most common destructive plant parasitic nematodes for many gardeners. The gall symptoms on roots are indicative of an infested host. Galling can be light or complete, occurring on every root the plant has. RKN survives in soil for years even without a host because the eggs enter a dormant stage called cryptobiosis. Hatch is snychronous with susceptible roots that grow nearby. Root knot nematodes can build huge populations in a single growing season. Gardeners get nematodes by introducing contaminated soils that come with plants to their gardens. Since symptoms don’t show on plants with minor infections, people think they are buying healthy stock. Even with RKN, there may be juveniles in the soil that have not formed galls yet and when introduced to your garden they will develop later on susceptible plants.

RKN has a very wide host range. Fruit trees, impatiens, calendulas, and tomatoes are a few of its common hosts. Perennial plants can really develop high populations of RKN because the host is undisturbed and provides many seasons for the pathogen to develop. Once detected as galls on roots the plant should be removed and destroyed. RKN is particularly horrible for tomatoes and other annuals when it combines with fungi that also cause disease. RKN forms disease complexes with Fusarium which causes wilts. When tomatoes are infected with both RKN and Fusarium the symptoms are severe, and the plant will die relatively early in its life cycle often before a crop can develop.

Chipping or grinding and composting will kill most nematodes if you want to reuse your greenwaste. More likely RKN will survive as eggs in the soil. Soil samples that find just one RKN per gram of soil sample are considered hazardous as the worms can rapidly develop from these low populations. You may have heard that Marigolds will control RKN. Switching gardens to a non-host (crop rotation) does help decrease populations. And French marigolds and crucifers if tilled into soil as “green manure” will decrease RKN but these methods will not eliminate them from soil. There is a dose response to tilling in mustards so the more you incorporate the more RKN will be harmed. Some varieties are better than others. Fumigation provides a good level of control but is not feasible outside commercial agriculture. Soil solaraization with plastic tarps also controls nematodes in the upper regions of soil but there are usually many eggs that survive in lower soil profiles. The best control is not to plant susceptible plants.

Some tomato varieties are resistant to RKN. In fact VFN (Verticillium Fusarium and Nematode resistant) varieties should be chosen to avoid recurrent problems. The resistance to RKN in tomato is not complete and under high nematode populations and/or high temperatures the resistance can break down and even resistant varieties can develop galls and symptoms. There are no pesticides that home gardeners can use to kill nematodes. However there are biological controls of nematodes and since they are soil food web opportunists, increasing the diversity of organisms in soil tends to cut down on PPN. As always, fresh arborist chips applied as mulch will build a resilient soil food web and will slow the development of PPN harmful to garden plants.

Fall is for fungal fruit

Summer is done. The last apples are coming off my orchard trees now and persimmons are ripening fast. Some fruit remains to be picked but most is off. As garden productivity subsides we turn our tasks to winter. In Southern California it means planting the winter vegetable garden, in Northern Mn snow has already flown so gardens are shut down now. For fungi that may be pathogens in our gardens, it is a time for reproduction. Fall is the time for fruiting and for gardeners a time to reckon with next year’s disease cycles.

Most fungi are saprophytic, that is they live on dead or decayed organic matter. Fungi are largely responsible for recycling forest nutrients from litterfall (leaves, branches and whole trees) back to soil minerals. Without fungal decay, mulch would never break down and organic matter would pile up. If you use fresh wood chips (often advocated in this group) you may notice that after some time they are full of fungal mycelium or cordons (rhizomorphs). This is normal and healthy—a good sign that your mulch is decomposing and improving the underlying layers of soil.

Furngi survive as fruting bodies in cankered branches, dead wood and leaves

Some plant pathogen fruiting bodies are edible. The mushrooms formed by Armillaria are often collected and considered delectable by many. Most edible fungi are saprophytes or mycorrhizal fungi. Truffles and other edible mushrooms like Chanterelles are plant symbionts often benefiting oaks and other northern temperate trees. Some wood decay fungi are also considered a delicacy such as the Oyster mushrooms (Pleurotis spp.) or the sulfur mushroom (Laetiporus gilbersonii). I don’t recommend harvesting wild mushrooms for food unless you are able to accurately identify what you collect, even then, second opinions of mycologists are a good idea. Also, not everyone reacts the same to fungi when they consume non-commercial mushrooms, so moderation is best or just get your fungus from commercial sources.

The sulfur conk (Laetiporus gilbersonii) is an edible wood decay mushroom

Not all fungi are beneficial. Some have evolved life histories that allow them to gain energy not from organic matter or dead plant materials but from living plants. These are parasites. Fungi have been evolving their lifestyles for about 400 million years and in that time have developed several strategies involving plant hosts to live and reproduce. Sixty five million years ago, after the Cretaceous-Paleogene extinction event that famously destroyed dinosaurs, fungi bloomed on earth and increased in importance. As land plants diversified, so did fungi developing many forms and parternships, many of them becoming essential to plants such as mycorrhizae. A few fungi specialized as plant pathogens.

Fungi use their reproductive structures to survive and ready themselves to attack susceptible plants. The most common fungal fruiting body the mushroom may not seem like a survival structure. But mushrooms can produce millions if not over a billion spores. Massive spore production ensures that some of those spores will find a place for the organism to survive. Also some mushrooms found on trees (sometimes called conks or bracket mushrooms) are perennial, and live for years—each year they add a new spore bearing surface over the last one. Many of the pathogenic tree fungi that produce conks fruit in the fall or winter.

Mushrooms help fungi survive by producing millions of spores. Don’t attempt to eat this kind though as it is an Amanita and is poisonous! Never eat wild or collected mushrooms without proper identification and study.

Many fungi form their fruiting bodies as small melanized structures that contain their spores. These are often formed in dead host tissue, such as dead twigs or branches. The spores are protected until they are splashed by water onto tender or susceptible plant tissues such as shoots. In soil, fungi can form hyphae that are very concentrated and melanized in to long lasting structures called scleortia. They lay dormant in soil for years until a susceptible root grows into them. Crop rotation often helps to limit disease but some fungi can last decades between crops and remain viable by producing thick walled spores called chlamydospores or sclerotia. The wilt fungi (Fusarium and Verticillium) survive in this way.

Another key strategy that fungi use is a kind of timing called phenotypic synchronicity. Fungi often have their spores ready to be dispersed exactly when new growth or susceptible plants are available for infection. The timing also often aligns with weather conditions that favor spore dispersal or arrival at the intended plant growth stage or phenotype.

Fungi evolved with land plants to take advantage of the environmental conditions and phenology of their hosts. We can interrupt the process with a bit of diligence as gardeners. As fall continues and winter approaches, it is a good time to remove dead twigs and branches from perennials that are prone to disease, clean up fallen or dead flowers from plants like Camellia that are attacked by petal blight because the flower mummies contain sclerotia that start the disease in the Winter. Unfortunately removing conks from trees does nothing to stem the progress of wood decay fungi in the tree they formed on, or their further spread, because so many spores are formed that the few mushrooms we remove will not stop those diseases. Some evidence suggests that increasing soil organic matter will over time reduce soil-borne pathogens, but once a pathogen has affected a perennial, there is often little to be done about it as in the case of Verticillium wilt of shade trees. No matter how fungi survive, its always a good idea to apply fresh tree trimming chips around perennials in the garden….

Extremes

Extremes

On September 06, 2020, it was 122F in my yard in Ojai, California. A new all time high never before recorded in Ojai, Ca.

Here in California we had an extreme heat event on September 6, 2020. In my yard temperatures peaked at 120 degrees F. This also happened back in 2018 earlier in the summer where we reached a similar peak temperature. It is not supposed to get to be 120 degrees F. in Ojai. This year new high temperature records were set all over southern California for the month of September. Following these heat extremes, wildfires have spread from border to border (Canada to Mexico) in western states. As we suffer through heat and flames here in Western US states, we are also now told that this is a la Nina year so Southern California will continue with drought conditions into 2021. Extremes in climate bring hot dry weather to the Western United States and hurricanes and drenching rains to the eastern United States. Plants in landscapes may or may not be adapted to these extremes.

Damage from September 6, 2020 heat day showing damage to foliage on the tree on the right; a native Coast Live Oak (Quercus agrifolia), but not on the non-native Peruvian Pepper (Schinus mole).

My poster child heat monitor is the coast live oak, Quercus agrifolia. When temperatures exceed triple digits >110F, foliage on this native oak turn brown and burn on the south exposed canopies. They are not adapted to these record temperatures. This can be evidenced by looking at the damage throughout many California communities. Coincidentally other non-native plants are better adapted to high temperatures. The California pepper or Peruvian Pepper (Schinus mole) does fine in 120F weather with no irrigation. Eucalyptus of several species also have tolerated these increased temperatures. Trees that are drought stressed from lack of irrigation after a long dry summer will sunburn more severely than the same plants under consistent irrigation. If you see this kind of damage, its best to leave it alone until the plant responds by growing new shoots.

Damage to the tender new growth and leaves of Cherimoya. Sunburn symptoms usually show in the middle of leaves.

While study of “climate ready” trees is giving us tree selection options for hotter climates, the research is still new and we have many other species to consider beyond what has been recently reported. Of the species I have in Ventura County few of our study trees showed any damage from the extreme heat, and only the very youngest leaves were damaged on western hackberry and Catalina Cherry. Pistache, Island Oak, Palo Blanco, Tecate cypress, Arizona madrone, and Ghost Gum were not affected by triple digit weather this September. Other ornamental species that were damaged all over Southern California include the following: Avocado, Camphor, Privet, Magnolia, Coast Live Oak, Sycamore (especially the native Platanus racemosa), loquat and ornamental plum.

It our recent heat damage surveys I have observed that Coast Live Oak and Western Sycamore, two native trees that enjoy widespread tree ordinance protections were consistently damaged by our hot day early this month. If we continue to have extreme hot days, poorly adapted native trees will be injured more frequently, and possibly become more susceptible to damaging insects or native pathogens. This tends to restrict the range of natives to areas they are still adapted to growing in or grow into a new region where they are more successful. A time may come when a native tree is not the best choice for your area.

McPherson E.G., Berry, A.M., van Doorn, N.S., Downer, J, Hartin, J., Haver, D., and E. Teach. 2020. Climate-Ready Tree Study: Update for Southern California Communities. Western Arborist 45:12-18.