Advancing the science of gardening and other stuff since 2009
Author: Jim Downer
Dr. Downer has 34 years of experience as a horticulture and plant pathology Advisor with the University of California Cooperative Extension in Ventura County. Dr. Downer’s academic training is from California Polytechnic Univ., Pomona, (BSc. horticulture & botany, 1981; MSc. Biology, 1983;. In 1998 he earned a Ph.D. in plant pathology, from University of California, Riverside. Dr. Downer’s research is focused on mulch, soil microbiology and disease suppression in mulched soils, diseases of shade trees and cultural practices to maintain landscape plants. Dr. Downer is a member of the American Society of Horticultural Science, the American Phytopathological Society, The International Soc. of Arboriculture, and the Western Chapter of the ISA, and the International Society for Horticultural Science. Dr. Downer is an Adjunct professor at California Polytechnic University in Pomona. Dr. Downer serves on the Board of the John Britton Fund for tree Research as the chair of the research advisory committee, and currently chairs the regional conference committee for WCISA. Dr. Downer has a love of shade trees, Shinrin roku (forest bathing/walking) tree work, wood working, horses, gardening, horticulture and the study of plants and their biology.
As the climate warms the value of trees for cooling the environment around buildings, especially in cities, drives tree planting programs. Planting trees is just the first step in growing a tree in a sustainable landscape. Successful plantings require evaluation and guidance of the new tree’s current and future branch architecture. In almost every case, nursery grown trees will require some structural pruning so that a shade tree can develop strong and effective branch attachments that will support the canopy for the coming decades without failure. In this blog I cover maintenance of the newly planted tree including how to structurally prune young trees so that they develop strong and sustainable canopies.
As mentioned in earlier pruning blogs, trees do not require pruning. This is predicated on the assumption that trees are allowed to grow in the way they are genetically programmed to grow without damage. Unfortunately many container nurseries prune trees with a heading cut to the central leader in order to create branches that can further be pruned to make a “lollipop” canopy that mimics the form of a large tree. Consumers have become accustomed to this “in-pot” miniature version of a shade tree and nurseries are accustomed to producing them. Low branches are removed to enhance the tree lollipop shape. Nurseries often stake trees tightly to provide a way to keep them from being blown over in wind events and since all the temporary branches are removed from the low trunk they are top heavy and require rigid staking usually with a stake taped to the trunk. Tightly staked trees grow taller than unstaked trees and their trunks may lack caliper or taper (increase in trunk diameter lower on the stem). This requires that when these trees are planted out that they continue to be staked, otherwise they would fall over. This creates another burden in getting the newly planted landscape tree to survive—helping trees stand on their own.
Nursery pruning creates two kinds of branch faults that if left in the tree canopy will lead to failure later. These result from heading the main leader of the young tree. When buds grow from the pruned tree, they often produce too many branches from the same place or two branches or new leaders that are the same size. We call these faults: too many branches from one point and codominant stems respectively. If the nursery tree retains these branches and they are allowed to mature in the landscape tree, one or more branches may break loose. Almost all structural pruning seeks to correct these faults at some point in the life of a nursery-grown landscape tree. The approaches are different depending on how long the branch fault is left in the tree after planting. Branch faults of newly planted trees are best corrected in the first year–they are easy to correct in the first few years and problematic after that. This is because when poorly attached branches grow well and attain greater size over time, they will pose a problem upon removal as pruning will leave behind a substantial wound which provides an entry point for wood decay. Structural pruning is best done in the nursery or if in the landscape, in the first year after planting.
There are several goals of early pruning (1-3 years post planting): -Retain temporary branches on the stem to assist trunk growth (but keep them pruned) -Remove competing leaders (remove a co-dominant stem) – Thin clusters of branches (fix the all branches from one point fault) -Leave the first permanent branch unpruned -Subordinate all other branches to “temporary” status by heading them back – Leave unpruned branches along the stem that will take a permanent place in the crown of the tree. -Leave enough space between permanent branches to support their sustained growth over the life of a tree -Permanent branches should be spaced vertically and helically around the main or central leader
Most trees will do all of this without any pruning if they are unpruned from the seedling stage. They will shade out their temporary branches and permanent large branches will form strong attachments and uniform spacings. Heading cuts on young trees destroy their form and this should be avoided. In the next blog I will cover pruning young to mature trees.
As we head into Fall garden routines and leaves start to turn color, the smell and feel of the Fall weather is in the air. Winter is just around the corner and with those horticultural routines comes the urge to prune stuff . Both fruit producing and shade producing trees often get a hair cut during fall and winter months, herbaceous perennials are often cut back in the fall after bloom and before their winter rest so it seems a good time to blog about pruning before you get the urge! After years of pruning demonstrations for Master Gardeners and the public I have noted a common thread in how gardeners think about pruning. Pruning is a mysterious process. How we take that tangled mess of a plant (tree) and fix it? What do we prune? And the less frequently asked question: What do we not prune? To add confusion, some plants such as roses seem to have their own pruning “culture”. In this blog post I will cover the basic principles that apply to pruning all plants and then expand into specifics in upcoming blogs.
Plants don’t want to be pruned! The first point is that no plant wants to be pruned. Gardeners prune plants because they think it is necessary for horticultural, aesthetic or safety purposes. Gardeners should temper their pruning by understanding plant responses that result from pruning. Generally plants don’t respond much when dead portions are pruned away. In some cases removing large dead portions of a plant will allow more light to enter and some response can occur, e.g., damage to portions of a plant not used to such sunlight intensity. So even a dead plant part may be doing something that you don’t understand. Also dead wood or dead plant parts may be part of some other organism’s home. Owls and other birds nest in cavities, some kinds of bumble bees will reproduce in old flower stalks of desert plants such as Nolina, etc. So dead plant parts are not always useless. If you are pretty sure that nobody else is using dead material go ahead and remove it if bothers your garden aesthetic.
There are two physiological responses to pruning
The two principles of pruning can be used to train plants and in the case of trees, produce a strong architecture that will not easily fail (drop branches). To achieve pruning goals two kinds of pruning cuts are used: the heading cut and the thinning cut. Heading cuts are often made in the middle of stems and do not have a branch that can take over the terminal role of the removed portion. Heading cuts are often used to reduce size or volume of plants. Thinning cuts remove branches at their origin. If thinning cuts are not too large and don’t allow excessive light into a canopy the plant will not respond by invigorating buds. Thinning cuts are used to maintain the natural form of a plant but can be overdone. Over-thinning results in plants that have so much light now entering that buds are invigorated and new shoot form in overwhelming and unnatural locations just as when heading cuts are made. Excessive use of thinning cuts can also produce trees that are “lion-tailed” where all the leaves occur at the end of Pom Pom branches. Remember from a tree or shrub point of view they don’t need or want to be pruned.
Back to plant responses. There are two responses that most plants have to pruning. When living portions are pruned the remaining portions are then invigorated. This implies that dormant or “latent” buds will grow that would not ordinarily grow so plants will produce flowers or foliage in new places. In this way we can re-direct the growth of plants to achieve pruning goals we may have. This is how we can pleach a tree to grow flat along a wall or produce topiary shapes with shrubs. These kinds of pruning that dramatically alter form of a plant will require successive and significant pruning to maintain the altered form or shape. Not all plants can tolerate this and even those that do can be subject to sunburn or other processes that cause them injury. The second common response to pruning is that the more a plant is pruned, the less it will grow—pruning is a growth reducing practice. Even though buds are invigorated through pruning they can’t make up for the lost leaves and buds taken away without utilizing stored energy. The overall effect of having leaves removed is to slow the growth of the entire plant. Pruning when used as high art results in Bonsai plants that are really stunted individuals with highly stylized forms.
Pruning devigorates plants
Since pruning removes leaves and buds (which make more leaves) it is a devigorating process. You are taking away a plant’s ability to harvest light energy and convert carbon dioxide and water to sugar. All this happens in leaves. The fewer leaves a plant has the less sugar it can accumulate and then the less work it can do in terms of growing. On old or slow growing plants pruning removes energy needed for growth and also the energy needed to make secondary metabolites or chemicals which fight insect and pathogen attacks. This is why old trees pruned hard often died not soon after or become susceptible to pathogens they may have been able to fight before the pruning happened. Whenever you prune something think about how you are taking away photosynthate and what it might mean to the plant.
Fruit Trees and Roses
We have to prune fruit trees to make them fruitful? NO. Fruit trees produce lots of fruit when they are not pruned. The goal of pruning fruit trees is to modify trees so fruit is: • in an easy to pick location, • so there is less of it • and so the fruit that forms is of higher quality. An unpruned tree will make the most fruit but it may not be the quality or size you desire or where you want it in terms of picking height.
The same goes for roses. There are many pruning schemes for roses, but the most flowers will be found on the least pruned shrubs. Flower size is mostly determined by genetics. Shrubs that are severely pruned will have fewer flowers than their unpruned counterparts.
Pruning and Disease
Pruning to remove diseased parts is often cited as a common garden practice. With some diseases like cankers and blights it is a good idea to prune out infected portions before they make spores or other inoculum to further infect the rest of the plant. In most cases it is important to prune well beyond the diseased portion so all of an infection is removed. Some diseases are “systemic” such as wilt diseases and while pruning will remove a dying portion it will not rid the plant of the infection. It is always best to identify the cause of disease even before pruning it from the plant. As we will learn in an upcoming blog I rarely recommend sterilizing your pruning equipment with disinfectants. A stiff brush and water is all that is needed when removing most diseased plant parts.
Pruning is a useful tool for gardeners. To get the most from the practice it should be conducted with knowledge of the effects it will have on the plant that is being pruned. This is quite variable and in some cases pruning is really contraindicated. While some plants like herbaceous perennials will be pruned to the ground either by the gardener or by frost, others maintain above ground architecture and pruning choices make permanent impact to many woody plants. In the next blog I will write about pruning young trees to create strong structure.
Downer, J., Uchida, J.Y., Elliot, M., and D.R. Hodel. 2009. Lethal palm diseases common in the United States. HortTechnology: 19:710-716.
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
In this blog I continue to examine maladies caused by environmental conditions in the absence of a disease agent or insect.
Salinity Salt in soils or water is simply the presence of too many soluble ions in the soil-water solution. This tends to happen in dry climates where evaporation rates exceed precipitation rates. In these climates salts accumulate in soil when surface waters pick up minerals from soil that is high in precipitated salts. In wetter climates water leaches salts from soil so surface waters (rivers and lakes) have fewer dissolved salts. Also, in dry climates irrigation is often a must and irrigation sources usually have high amounts of dissolved salts. In high salt environments plants must use energy to increase their own salt balance at the root interface to make uptake of fresh water through their membranes possible. This energy is thus not available for growth. Salt affected plants are often smaller, even stunted depending on salinity levels and are more susceptible to root pathogens as their roots are more likely to be “leaky” giving pathogens chemical signals of their susceptibility. Salt damaged leaves often show “edge” necrosis or burning on the oldest leaves.
Salt affected soils should not be allowed to dry out as roots will be damaged. Leaching to dissolve salts and move them below the root zone is one approach to prevent further symptoms.
Soil compaction Soil compaction is the increase in soil bulk density beyond a point where roots function and grow. Bulk density is a measure of soil compactness and is calculated as the weight of dry soil per volume. Optimal and harmful bulk densities vary by soil texture. Sands have higher bulk densities than loams which are higher than clays in their growing range. Normal bulk density for a sand will be a compacted value for a clay. Values above 1.1, 1.4 and 1.6 g/cm3 can be restrictive for roots growing clays, loams and sands respectively. Compacted soils of any texture restrict plant growth. Stunting, poor growth and nutrient deficiencies due to loss of root function are common.
Extremes of light Light is necessary for photosynthesis but it is also a radiation source that can include damaging light energy when it reaches tissues that are not accustomed to it. This happens frequently on over-pruned or damaged trees, where the canopy is suddenly reduced and stem tissues receive intense sunlight. On thin or green barked trees this can cause sun scald. Apples are particularly sensitive and will develop large cankers on upper branch surfaces if too much light is allowed into the canopy during summer. Canopy loss compounds light injury because the tree is not cooling itself as efficiently with fewer leaves. Infrared energy (heat) builds up on branch surfaces and can kill underlying stem cambium layers.
Low light levels also harm plant productivity. All trees tend to lose interior branches as normal growth increases canopy density and light levels decrease in the innermost canopy. Inner branches store less and less energy and essentially die due to light starvation. The same thing can happen to entire trees if they are overgrown by vines, other trees or shaded by buildings. While canopy thinning will preserve inner branches, it is not absolutely necessary as branch dieback is a natural process in most trees.
Effects of Herbicides Sometimes herbicides cause damage to non target plants. This happens when herbicides are applied unknowingly, such as residues in composts, drift from off-site applications, or by choosing the wrong herbicide to use in a garden setting. Herbicides affect plants in different ways: some only affect tissues they contact, others are systemic, and some affect seeds as pre-emergent herbicides and have activity in soil over time. Diagnosing herbicide damage often requires sleuthing and inquiry of what has happened in the past and what materials your neighbors may be applying. As with any pesticide, herbicides should be applied according to label instructions.
In conclusion…Disease diagnosis can be a challenge for the gardener and dysfunctions caused by abiotic factors are no different. Carefully considering the symptoms that the plant presents is the first step to recognizing an abiotic disorder. Uniformity of symptoms is often indicative of disorders not caused by biotic pathogens. As with any plant health issue, figuring out the cause is the first step in helping plants succeed.
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.
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.
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 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.
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.
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
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.
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.
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.
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.
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.
A sustainable landscape should not consume excessive amounts of
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.
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
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.
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 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.
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.
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.
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
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.
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.
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.
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.
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.
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
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.
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.
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.
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.
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.
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.
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
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.
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.
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.
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.
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.