Author: Jim Downer

I am constantly slaying horticultural snake oil dragons. There is so much misinformation on the web and even within University/Extension publications. In this blog I turn my attention to compost–a subject that is almost universally cherished by gardeners, gardening groups and horticulturists. Unfortunately there are a lot of misnomers about compost.

Plants are composed of cellulose and cellulose is a complicated polymer of glucose molecules. Compost is made from the decomposition of organic matter—usually plant debris. The composting process can be fast or slow depending on aeration, mixing and pile size. Composting requires a carbon source and enough nitrogen to allow microbial respiration of the sugars contained in the plant material being decomposed. Since the laws of thermodynamics indicate that no chemical reaction is 100% efficient, some of the energy of respiration is lost as heat. Billions of respiring microbes heat the pile creating a very hot environment where thermophilic organisms propagate quickly. As all the available sugars in leaves and other less woody components of the compost decompose the thermophilic organisms lose temperature and the readily available sugars necessary for growth. Other organisms begin to grow and attack the cellulose in the wood fibers, attacking the more recalcitrant carbon in the pile. Eventually most of the sugar bound in plant residues is attacked and only the difficult to decompose materials are left, these contain lignin and form the basis for humus. When the compost will no longer heat after turning it is beginning to mature. Once all the easily broken down carbon is utilized, the microbes die off or form spores and go into a resting phase. The compost is now screened to remove large undecomposed particles and is ready for use in the garden

It’s NOT NATURAL

I have often heard composting touted as a natural process. It is not.
Composting is a process that is “man made”. The alternative is litter fall and mulching which is a natural process that processes organic matter much more slowly. Composting is a process that requires a specific mass of feedstock, sufficient oxygen for respiration, reactions provided by air or by frequently turning the pile, moisture maintained by adding water if needed, and heat which is maintained within the pile itself. These are not natural conditions easily found in nature. They are carefully manipulated by those monitoring the compost process.

The fungi and bacteria on the initial feedstock are part of the ecosystem and are generally not directly manipulated in the process. Fungi and bacteria have the enzyme systems necessary to break the bonds that link the glucose molecules and then utilize the energy in glucose for their own growth.

Composting does not help the environment

As I have discussed, composting liberates carbon dioxide increasing the amount of greenhouse gasses in the atmosphere. On a large scale composting adds many tons of CO2 to the atmosphere as well as oxides of nitrogen which are also potent greenhouse gases. Composting can also release mineral salts into underlying soil and runoff from large composting operations, especially manure composting, can pollute waterways. There is nothing about composting that is helping the environment per se.

Compost is not full of life

Sometimes you hear that compost is “full of life”. Sort of true but not really. The biological processes that break down the compost happen in the pile. As compost matures microbes die, their growth is reduced and they form spores or other resting structures. Once compost is ready for use, it is not particularly biologically active because all the energy has been utilized to make heat and decompose the feedstock. When the energy (carbon, sugar, cellulose) is used up, the microbial activity declines.

What is it good for?

Since compost is a distillation of feedstock minerals it makes an excellent fertilizer. Since compost is mostly fine textured, it is suitable for use in soil as an amendment. The lignin molecules resident in compost help bind nutrients in organic matter and retain them for later uptake by plant roots. Compost can increase the fertility of a sandy soil which has low nutrient binding capacity. Compost is full of secondary metabolites left over from the microbial activity produced when the pile was hot. These compounds can confer disease protection when pathogens are present in soil. Since feedstocks are variable this can not be predicted. Finished composts with a carbon:nitrogen ratio (C:N) of less than 25:1 do not perturb the nitrogen dynamics of most soils and in many cases may be a source of nitrogen in the amended soil. Since compost is mostly broken down feedstock it does not deteriorate as fast when mixed in garden soil. It resides longer than other more labile amendments. Compost is also a great container medium if mixed with coarse materials to assure aeration. Because the compost feedstock is well decomposed, the material has a longer life as a growing medium.

Since composts get hot as the feedstock is broken down, they tend to sanitize the pile of pathogens. Composting kills food-borne pathogens and plant pathogens easily since most do not survive the high temperatures (>140F) found in an active compost pile for more than a day or so. For effective pathogen kill it is important to turn the pile frequently.
Some plants may survive high composting temperatures, e.g, tomatoes are a notorious compost weed. Yellow nutsedge and bermudagrass stolons can also resist the high temperatures found in compost piles.

It’s not a good mulch

One of the amazing things about mulch is undergoes the same processes that make compost and it does have a place in your garden. The microbial processes that decay arborist wood chips on the soil surface happen slowly over months of time. The chips are mineralized but more of the carbon enters the soil rather than the atmosphere because soil fungi, especially mycorrhizal fungi, transform the energetic carbon molecules (labile carbon) into a soil stabile polysaccharide called glomalin. This in turn binds soil particles which increases soil structure. Note: When these processes happen in a compost pile they can not happen again in your garden. The energy is gone.
Texturally fine compost will make greater hydraulic conductivity with the underlying soil and allow for greater moisture loss through evaporation. In some cases compost layers may impede infiltration of water and prevent newly planted root balls from being watered. Compost layers may also impede gas exchange to underlying soils. Depending on the feedstocks, composts may also contain viable weed seeds or other propagules that contaminate landscape soils. Composts make bad mulches.

References

Daugovish, O.,Downer, J., Faber, B. and M. McGiffin. 2006. Weed survival in yardwaste mulch. Weed Technology 21: 59-65.

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.

Chalker-Scott, L. and A. J. Downer. 2022. Garden Myth-Busting for Extension Educators:The Science Behind the Use of Arborist Wood Chips as Landscape Mulches. Journal of the NACAA 15(2). https://www.nacaa.com/file.ashx?id=6c7d4542-7481-4f0a-9508-d8263a437348

I think I have a pruning fixation. I take most opportunities that come along to write about pruning. I have not blogged yet about Spring pruning. It can be a useful way to achieve some pruning objectives. Like all practices it is not necessarily the method or timing of method of choice for all plants. Spring Pruning can have some specific impacts on development of deciduous fruit trees that may help in the home orchard.

Springtime may not be the most obvious time to prune–in fact springtime within the geographic context of this blog requires definition. For this discussion, springtime is the period during which buds are opening, shoots are elongating, flowers are pollinated, and fruit is set or is rapidly enlarging. These are changes in the tree phenology that are critical to fruit production. As you may recall from previous blogs on pruning there are some basic impacts that pruning has. Pruning is growth limiting. Pruned parts will grow less than unpruned parts. Spring pruning is an opportunity to regulate fruit retention.

Spring growth and tree phenology are not timed to be the same. Apricots will flower before or after peaches, plums, pears or apples. This can happen in different months depending on latitude of your garden. Spring is in set time back to another vegetative shootand Spring pruning is thus variable across location and species in your garden.

So why is pruning in Spring at all helpful? The main reason is to reduce the number of fruit that are set on a tree. Reducing fruit count will allow more sugar to enter fewer fruit increasing the size of remaining fruit and improving quality.
Pruning during bloom is risky, we don’t know what the fruit set will be until a few weeks later. Also changes in weather such as spring frosts, wind, or even hail and snow can destroy a crop in its juvenile stages and if you have already pruned, you have lessened your changes for fruit later. It’s best to wait until fruit have set, are growing, enlarging and that you are pretty sure the crop is under normal progression.

With a Spring prune I like to remove about half the set fruit. This would involve trimming the ends of branches (that have fruit) by 50 percent. You may still need to thin fruit later because the remaining fruitful stems you leave on the tree may have too many fruit to ensure quality. Thinning peaches to about one every six inches in late Spring, reducing pear and apple clusters to one fruit per spur and minor thinning of plums will suffice. Apricots usually need little thinning for adequate quality.

Another reason to thin in Spring is to reduce disease incidence. Peach leaf curl is usually well developed even as fruit is setting. The best control of peach leaf curl is with a dormant fruit tree spray prior to bud break. But, if you miss that opportunity to spray, pruning out the infected leaves and shoots will decrease the inoculum for next year. Dispose of the infested shoots in the trash (although correct hot-composting will likely kill the inoculum as well).

Spring pruning is not recommended in areas where there are frequent rains, bacterial diseases such as bacterial canker or when your trees are not vigorous and otherwise healthy. Pruning creates wounds that allow pathogens to enter the tree and a wise gardener will avoid pruning during warm showery weather. If conditions are dry and sunny, Spring pruning can be effectively used to slow growth and increase fruit quality for the coming summer harvest.

For more information on the science – and myths – behind pruning, Dr. Chalker-Scott and I published a peer-reviewed article on this recently.

Companion plants! Great, what a good idea. When you first hear the term and think about the concept it sounds great but there is a lot to not like about it. The term “companion plants”  implies that these plants are partners and they “enjoy” each other’s company.  The term is an anthropomorphism or overlaying of human qualities on non-human organisms.  A more appropriate term may be plant associates or plant associations, a term taken from plant ecology, which has more basis for use.

Plants naturally grow together in groups which are called plant communities. These plants evolved under certain climate, soil, and environmental conditions that allow them to live together in the same place. Coastal sage scrub, oak woodland, and juniper pinyon woodland are some common plant communities where I reside in Ventura County. All of the plants growing in these communities receive winter rainfall and summer drought (Mediterranean climate) to which they are adapted to grow in. Plants growing here either resist drought through specific plant adaptations such as reflective leaf surfaces (white sage), abundant trichomes (sycamore), microphylly (buckwheat), succulent water storage (agaves, yucca and other lily family bulb forming plants), and C-4 metabolism (grasses). Some plants avoid drought by growing in the rainy season, setting seed and then remaining dormant during hot dry weather. Plants can grow in this climate because they have the adaptations to do so.

Plants compete for resources and while doing so may provide a place for other plants to grow. Trees have an advantage over grasses because they can grow above, catching the sun and shade the grasses out. But shade may also provide a place for shade adapted plants to grow. Plants surviving challenges in a specific environment may end up growing together. Woody plants also provide perching birds a place to defecate and spread seeds. This is why unexpected things may grow under other plants. Shade may even be necessary for development of proper form. We have noticed in studying western hackberry (Celtis reticulata) that the tree has no apical control and will not develop into a tree shape when grown in full sun. When grown in shade apical control is present and the plant grows a straight trunk. Birds commonly eat hackberry fruit and likely disseminate it under the canopies of other plants. I don’t think the hackberry minds growing as a blob but its “companion” plants cause it to change form due to changes in light intensity.

Some plants live very closely with others. Mistletoe is a great example. Leafy mistletoe is a hemiparasite deriving its energy from sunlight of its own leaves but utilizing water and photosynthate from its host. Similarly there are free living plants such as Indian paint brush (a member of the Orobanchaceae) that are also hemiparasitic using their roots to extract benefit from neighboring plants. Holoparasites are true parasites deriving all their nourishment from their hosts, e.g., Dodder (Cuscuta spp.). Dwarf mistletoe is also holoparasitic as it largely lacks chlorophyll. These plants are always found on or near their hosts but it is hard to call them true “companions.” The plants clearly associate with each other and in some cases are detrimental as one of the plants stands to gain nothing from the interaction.

One popular example of “companion planting” is The Three Sisters (TTS) polyculture of corn, squash and beans. This agricultural system is said to be synergistic. Corn provides support for beans and shades the squash, and beans provide nitrogen fixed from the air for the other two members of the system. The system was “practiced” by indigenous Americans all across the continent. Soils, rainfall and climate are quite diverse across the United States, and I am sure that TTS agriculture had mixed success. It is an interesting thought that the human diet can be satisfied by these crops and likely the combination was more about ensuring sustained calories and nutrients for those who grew them. In one published study there was no increase in production when comparing TTS to mono-cultures of the component parts, nor was N increased in soil. This makes sense since it’s not available until the plant dies giving up its nitrogen to the next crop which is the basis of legume cover cropping. Continued use of the TTS system is a zero sum game as corn and squash will rapidly use all the nitrogen from the previous year’s legume crop.

Mutualism is the concept that interactions between two organisms benefits both. There are many examples of plants that have a mutual relationship with insects, birds, fungi and bacteria. I found no examples of plants that have mutual relationships with other plants, e.g., “companion plants”, common to the scientific literature. I thought this was unusual so I called a friend who is a plant ecologist and asked her the question. At first she was enthusiastic and pointed to non-plant-plant relationships. As I redirected her to only plant-plant interactions we could not identify anything. My suspicion is I have missed something important or we will discover one day that there are plants evolved to help one another but for now, it evades me.

There is no doubt that one plant can help another but it’s incidental and not a sign of a mutual relationship. Most plants evolved to grow in communities because the growing conditions are suited to all. Knowledgeable gardeners and landscape architects will group plants that grow well together. This is only common sense.
Understanding how plants grow in nature informs gardeners about adaptations and this in turn elevates the practice of horticulture.

References

Martinez, R.T. 2008. An evaluations of the productivity of the native American ‘Three Sisters’ agriculture system in northern Wisconsin. M.S. Thesis. University of Wisconsin-Stevens Point, College of Natural Resources.

Marsh, E. 2023. The Three Sisters of Indigenous American Agriculture. National Agricultural Library (USDA). https://www.nal.usda.gov/collections/stories/three-sisters

I have a very interesting research project on the effects of urban pressure on Coast Live Oak (CLO). CLO is a California native oak and I am interested in seeing if urban cultural conditions prevent the development of mycorrhizal fungi on their roots.  My study is blocked, that means that all the treatments occur in a block and the blocks are repeated for replication.  Blocking allows the statistics to account for variability in field locations.  Its a good thing too, since one of the blocks has never done well.  One tree died, two are severely chlorotic etc.  This was not just the effect of the urban pressure treatments, but way more severe than any other trees growing in other blocks.  It turns out there was a reason…  I had unwittingly planted my sapling oaks in an area of our research farm where  buried landscape fabric was installed.

So most trees that were covered by the landscape fabric were chlorotic. One died and one grew normally. The one growing normally had extended roots over the top of the fabric and then grown into soil beyond the fabric. Note in the picture above a lack of roots despite adequate moisture.

How does landscape fabric hurt trees? Let me describe the mechanisms… First and foremost soil coverings reduce the ability of soil to diffuse gases, both into and out of soil. As we know from other blogs on this subject in the archive Dr. Linda Chalker Scott and colleagues conducted research on gas diffusion rates under different kinds of landscape or soil coverings. It is important to understand that gases go both ways. For roots to remain healthy, they must convert sugar to energy through the process of respiration. During chemical respiration oxygen is combined with glucose and converted into energy (for cell growth) and carbon dioxide is produced. Carbon dioxide must diffuse out of soil and oxygen diffuse into soil for this reaction to occur.

Many of our blogs have touted the benefits of coarse, fresh, arborist chips for woody plants. One of the supreme benefits is the increase in rooting under these mulches. Unlike landscape fabrics, wood chip mulches eventually modify soil actually promoting gas exchange into deeper levels. Also, landscape fabrics prevent soil arthropods and other organisms from transporting organic matter to lower levels. Think of plastics and fabrics as a suffocating blanket over root systems, they deprive roots of moisture and gas exchange and prevent soil modification and organic matter movement.

While thick, coarse organic mulches actually enhance establishment and rooting of landscape plants without limiting gas exchange they can not overcome the impact of landscape fabrics. A common practice is to lay down fabrics and then apply mulch over the fabric. This often results in a “tatty” look years later when the mulch decomposes and the fabric shows through. Landscape fabrics and weed barriers are landscape pollutants. We should be limiting the use of petroleum products in landscapes because they do not break down easily and they have a bad impact on all forms of life.

References

Cahill, A., L. Chalker-Scott and K. Ewing. 2005. Wood-chip mulch improves plant survival and establishment at no-maintenance restoration site (Washington). Ecological Restoration 23:212-213. https://www.researchgate.net/publication/303445066_Wood-chip_mulch_improves_plant_survival_and_establishment_at_no-maintenance_restoration_site

Chalker-Scott, L. and A. Downer. 2022. Garden Myth-Busting for Extension Educators: The Science Behind the Use of Arborist Wood Chips as Landscape Mulches. Journal of the NACAA 15(2). https://www.nacaa.com/file.ashx?id=6c7d4542-7481-4f0a-9508-d8263a437348

Shahzad, K., A.I.Bary, D.P. Collins, L. Chalker-Scott, M. Abid, H.Y. Sintim and M. Flury. 2019. Carbon dioxide and oxygen exchange at the soil-atmosphere boundary as affected by various mulch materials. Soil & Tillage Research 194. https://doi.org/10.1016/j.still.2019.104335

If you live in a place where water falls from the sky during summer this blog is perhaps not so helpful. However, gardeners in much of the western United States will suffer this summer from hot days (sometimes record breaking) and will need to irrigate their gardens and trees in order for them to survive impending drought conditions. The ongoing drought has drained reservoirs and flows of rivers are down or, in some cases, dry entirely. Due to water scarcity, purveyors are restricting water use outside of homes and in some cases curtailing all landscape irrigation. Using water wisely in the landscape has never been more relevant than now. In this blog post we’ll continue to explore saving our gardens from drought and touch on water use, water demand and plant stress.

Good news — Bad news

The good news is that the longest day of the year was last month. That means that the days are ever so gradually getting shorter. As days shorten, plants use less water. Water use is tied directly to photosynthesis and when the lights are out there is no photosynthesis. Shorter days mean less demand for water. The bad news is that we may have record breaking hot days ahead. Plants become susceptible to wilt, sunburn and dieback during very hot weather. The best way to prevent this is to ensure that roots are moist during very hot weather.

Mediterranean Climate

It turns out that in Ventura, CA the longest day of the year is one of the historically driest months (least rainfall) and the shortest day occurs in a month with more rain than average. This is a classical Mediterranean climate, the rain falls when we least need it for thirsty plants. Plants may not use all the water that falls in winter but soil is leached of salts and deeper soil layers are filled with water. Large woody plants can utilize this deeper soil water in drier months.

Stress

What is not obvious is that stress, especially water stress, is not highest on the longest day but occurs and builds later in the summer and fall months. This is because water is slowly depleted from soils over time. Water use is compensated by irrigations during dry months so stress may not build depending on the effective use of irrigation to keep plants hydrated. Plants and gardens that are less reliant on irrigation build stress over time CUMULATIVELY until they are irrigated or rain falls again. So after the spring rains stop, the “stress clock” starts ticking and keeps building until rain or irrigation happens again in the fall or winter. It is no wonder that symptoms of stress such as wilt, sun burnt leaves, leaf fall and plant death occur in late summer when cumulative stress levels overcome plant physiological limits. Late summer often is the time when the hottest days occur and heat stress in addition to drought stress adds to the struggle for garden plants.

Save your plants in the fall from deadly stress

In the late summer or fall months plants are most likely to die of drought induced stress. This can be forestalled by irrigation applied in August which will reset the stress clock to lower levels. Stress won’t return to low wintertime levels because irrigation water is often of less quality than rain water. The salts in the irrigation water raise the osmotic potential of soil water creating another kind of stress. When it rains, these salts are washed from soil and plants are at their lowest stress levels.

Monsoons (see recent blog by Pam Knox) provide arid climates with stress relief when plants growing in hot deserts most need it. Many plants come out of dormancy or germinate only after the onset of summer rains. While monsoonal moisture can be unpredictable usually some rain falls in the desert southwest. In years when the monsoons don’t come or provide only low amounts of water, some trees or other woody plants even cacti, may die from stress.

Monitor your plants for stress as long dry periods or hot weather get longer and hotter. Apply water strategically to take off stress. Irrigate deeply by soaking the root ball(s) of the plant(s) you want to save. Established plants are pretty tough and can survive adverse periods, newly planted specimens not so much. They must be irrigated frequently just like they were in the nursery until their roots are will grown into site soils. For those of us living in a Mediterranean or monsoon climate we should wait until the onset of a wet period is near before installing new plants.

Yikes!

Drought of epic proportion is imperiling many western states this year. For the first time some water districts have proposed curtailment of all exterior irrigation, no applied water will be allowed outside of residences. There are public forums are scheduled with experts and officials giving advice. Of great concern is the certain loss of turfgrass swards but far more concern is being expressed for the loss of trees.

Don’t assume your trees will die of drought!

Most established trees are resilient, they have built in drought avoidance and tolerance strategies. It helps to understand these processes and know the symptoms of drought injury in trees. Almost all trees will stop growing when they enter drought conditions because there is not enough water to produce the turgor pressure necessary to expand cells. While growth may slow, trees have root systems that help prevent them dying of drought. The root systems, while mostly in the surface layers of soil, also explore greater depths where they can extract water from larger volumes of soil. So even though soil may be dry on top, trees have greater access to moisture than is obvious from above. Trees also have mycorrhizal fungi that help them extract water bound tightly to soil. When these strategies become limiting, tree roots produce abscisic acid that flows to the pores in leaves and closes them to reduce transpiration. If drought continues, many trees will drop leaves entirely to help stem and root tissues survive, thus avoiding drought. These mechanisms are all controlled by tree genetics and their ability to ameliorate drought effects is variable. Some trees are just more drought resistant than others.

Don’t plant new trees in the Summer!

Now is not the time to plant trees. As we near June 20th (the longest day of the year) demand for water is also greatest as sunlight drives photosynthesis and thus water use by trees. Newly planted trees all need irrigation to help them establish. During drought years, wait until later fall months (when rain is more likely and day length is decreasing) to schedule planting.

Don’t fertilize trees during drought!

Fertilization is the last thing you want to do during or at the onset of drought! Fertilizer (organic or inorganic) contains salts that increase the osmotic potential around roots. This alone can create “physiological drought” as water is drawn out of roots into soil solutions. Fertilizers should only be applied when known deficiencies are present and water is abundant enough to dissolve the applied materials.

Don’t prune trees during drought!

Pruning removes terminal buds that regulate growth of the canopy. When they are removed by pruning, lateral buds are released to grow. Stimulating new growth during drought is a disaster for trees. Don’t do it! You may falsely think removing branches in a tree canopy will save water. Most trees can regulate their own water loss as discussed above or by dropping leaves as necessary when dry conditions ensue.

Do not install artificial turfgrass

Artificial turfgrass is not a solution for hot dry conditions. In some cases it may exacerbate the situation. Artificial turfgrass does not allow percolation and capture of water since it covers soil. Artificial turfgrass does not transpire, so landscapes will not be cooled by it. Trees adjacent to artificial turfgrass have less ability to access water than those adjacent to a mulched area.

The longest day in June may not be the most stressful for trees

When water is scarce, it is important to apply it strategically to reduce tree stress. Even though June has the longest day and potentially tress will transpire the most, it may not be the most stressful time for trees because water may still be available at lower soil levels in June. As we enter later summer and early fall, stress builds as available water is depleted from tree root zones. Deciduous trees will lose leaves but evergreen trees or trees that can’t shed their canopy may begin to enter their permanent wilting points. This is usually proceeded by wilt, dieback, and loss of color. This is a critical point where strategic water applications can help trees through a critical period.

Do apply Arborist Chip Mulches

Mulch is a critical drought survival tool for trees. It is best if mulch is already in place but it is never too late to apply it. Mulch changes soil structure allowing for more water storage. Over time, mulched soils become more drought resilient. In the short term mulch prevents evaporation from soil surfaces so that applied water stays applied in the soil and is not lost. Coarse wood chip mulches prevent weeds that use water thereby keeping more moisture in the soil. Wood chip mulches support the mycorrhizal fungi that help trees survive.

Do “top up” existing mulched areas around trees

Mulch breaks down as it is supposed to. It is important to keep mulch layers intact by occasionally adding to mulch layers. If you have not done so, add mulch before summer gets too far along.

Do remove lawns or shrubs that are no longer sustainable in the landscape with care.

Water restrictions, hot weather and dry soils culminate and can greatly damage landscapes. If landscapes are over planted or there are unwanted/unnecessary plantings they can be removed to save the water they would use. Be careful not to expose existing plants and trees to bright sunlight as this may cause them harm from sunburn. Be careful removing turfgrass swards and the irrigation that accompanies it because adjacent trees may be reliant on the excess applied water. A slow dry down and mulch over process may be the best approach to save valuable perennial plantings near unsustainable turfgrass.

Do monitor your trees for signs of impending drought stress. and apply water in a timely manner

Wait until leaves start to turn yellow prematurely or canopies show wilt symptoms to apply water. As drought symptoms develop, consider a slow application of water by a dripping hose (moved frequently) or a low flow sprinkler that applies water only as fast as the soil can take it in. Apply water at night to cut down evaporation loss. Continue to monitor for further drought symptoms and spread out irrigation as needed to conserve water.

Do turn off your valve controllers to avoid over application of water.

Don’t let electronic devices make irrigation decisions for you during water restrictive drought periods. No electronic system completely understands the stress conditions around trees and will not be able to accurately predict when to irrigate. It is best to make these decisions based on your own assessment of conditions and resources available. Many “irrigation clocks” rely on regular frequent applications of water to keep soil moisture supplied. Frequent short run cycles replace water used by plants. During drought restrictions, controllers need to be reprogrammed to apply less frequently but for longer runs (to the point of run off) or not used at all if the sprinkler emitters put out too much water. Targeted water applications will likely be necessary and valve controllers will need to go “dormant”, i.e., turned off.

Be hopeful

Droughts come and go, right now they keep coming. But there are many examples of trees that only receive rainfall, no applied irrigation and yet survive well. Don’t assume your trees will die of drought. This may be a time to remove trees that are not adapted to growing in your area and drought conditions will reinforce this. European Birch are certainly disappearing from many landscapes this year in Southern California. Increase the use of mulch, apply water strategically, and consider planting more adapted trees late in fall or winter when water is available to support establishment.

I’ve had this funny feeling that something is just not right in my garden. Can’t put my finger on it, but something is amiss. OMG everything is dying! Help! Garden Death is rampant! Well, a bit of hyperbole perhaps, but over the years I have had many calls from gardeners with great concern for plants or their entire garden based on things they perceive to be going on. I have helped them by trying to diagnose their problems. Thought occurs though that most gardeners should be able to diagnose their own garden problems with guidelines and framework that informs their decision making processes. The problem with solving problems is that often gardeners don’t notice a problem until it has advanced quite far often to the point of no return. So, the trick is to “see” things early so they can still be fixed.

Patterns

The first step to solving garden problems involves looking for patterns in the symptoms that are presenting as the “that does not look right to me” situation. The redwoods and boxwood in the image above are all performing badly and the symptoms are uniform. Uniform symptoms that occur across a population of plant often suggest an abiotic cause. In this case the use of recycled water high in salts has impacted the landscape plantings.

Symptoms

…are plant responses, changes in physiology such as chlorosis, and necrosis, spots, coloration or discoloration etc. Foliar symptoms often form when a plants ability to make or utilize chlorophyll is compromised. Symptoms also occur on stems in the form of cankers or dead spots that can ‘girdle’ the stem leading to foliar symptoms in the shoots on that stem. When diagnosing garden problems it is important to look at symptoms carefully and early. This involves understanding what is normal for the plants being grown. Plants exhibit a variety of growth patterns and changes throughout the season so some changes are normal. The trick is to see the early onset of “not normal” symptoms.

Signs

…are the cause of disease. Signs often confirm a diagnosis and give way to control options once a pathogen or other disorder is identified. Finding signs is of the confirmation needed to take some action to fix the problem in the garden. Often fruiting bodies of pathogens don’t form until the host has died or shed leaves that fall on the ground. Many signs are microscopic, but some spores can be seen ‘en-masse’ when inocculum builds up to visible levels. And sometimes symptoms and signs occur together helping to solve the diagnostic problem.

Canker diseases cause a variety of symptoms and signs. Most cankers only form signs after the stem has died. Early in the progress of disease plants may appear discolored but it is not until later that the signs will form usually after the plant is visibly necrotic

Time

…is an important factor in disease progression. Diseases do not happen instantly but form over time. Diseases, if they result from pathogens, have a “life history” where the various stages of the pathogen are formed or survive and accompany symptom development in the host. Early symptoms may be innocuous or subtle. The problem is we notice problems at a single point in time but the problem is often well along or has been developing long before we notice it. Understating the time line of disease or pest formation is important in diagnosing the cause.

Insects

—are often confused with pathogens because they can cause some of the same symptoms as plant pathogens or abiotic disorders. Insects cause an array of symptoms that can be used to diagnose their presence. Some insects related symptoms are: foliar stippling, bronzing and bleaching, leaf spots, chewed foliage, wilting and death of branches or entire portions of a plant or tree and galls. Insects also create signs of their activity such as frass, galleries, honeydew, cast skins, and excrement. Of course the ‘gold standard’ of insect signs is the insect itself which can take the form of adult insects or larval insects, both of which may look very different and affect different parts of a plant.

Diagnosis of garden enemies is just the first step in finding a solution to a garden plant malady. Often determining the cause requires some expert help. Your local Cooperative Extension advisor often has experience in diagnosing the most common problems or can find assistance getting answers. With the advent of smartphones that have great cameras we can diagnose many issues remotely with images. Regional expertise is best as pests vary by state and region. The diagnostic prowess is often local–in the county where you live. Start there and widen your research until you feel you have the identification you need to research possible cures.

You may have heard about these fungi or perhaps not. But if you look carefully on bags of potting mix and on some fertilizers you will see that they are marketed as “essential” to your garden plants.  Claims on mycorrhizal products suggest dramatic growth increases.  These claims like many “snake oil” products can be extreme and are based on science that supposedly bolsters their efficacy.  Mycorrhize are responsible for tremendous growth increased when compared to plants denied access to the fungi.  This has been known for many decades.

The disconnect between mycorrhizal claims and garden efficacy is that there are usually mycorrhizae present in most gardens. So adding more won’t necessarily improve the growth of plants. There are also some other concerns. Mycorrhizal products are not all the same. Research on product efficacy suggests that about half the retail products available contain no viable inoculum. Spores of mycorrhizae have poor germination viability and do not last long on the shelf although some products contain hyphae as well as spores and these may last longer. So even though products are out there they might not not infect plants.

While mycorrhizal products may or may not hold value for gardeners, mycorrhizal fungi are essential for almost all plants. A few, such as brassicas, do not form mycorrhizal partnerships but all trees, other woody plants and most annuals do become infected by and benefit from these fungi. Plants and mycorrhizal fungi are symbiotic and each receive reciprocal benefits when each partner is well established.

There are two broad categories of mycorrhizal fungi the VA (formerly VAM) or vesicular-arbuscular mycorrhizae and the ectomycorrhizae, (EM). VA mycorrhizae are fungi in the class Zygomycetes related to the common bread mold fungus. They inhabit 80% of the worlds plants. They can not be seen without staining and careful microscopy. Ectomycorrhizae are the other form and they are exclusively from the Basidiomycete or mushroom forming class of fungi. Many of the mushrooms that grow in forests are actually supported by tree roots they affiliate with. Ectomycorrhizae change the shape of roots giving them a stubby appearance. This is because ectomycorrhizae form a mantle around the root of hyphae called the Hartig Net.

So why the big deal? What are the benefits that plants share with mycorrhizae and how do the fungi benefit from their plant hosts? Early studies showed that mycorrhizae make minerals, especially phosphorus, more available to their plant partners. Studies show that mycorrhizae increase the efficiency which plants use many fertilizer elements, even nitrogen. Fungi become “sinks” for plant carbohydrate or sugar. Mycorrhizal hyphae replace root hairs in most infected plants and vastly increase the surface area of roots. This gives roots the ability to withdraw water from very dry soils since mycorrhize can access water held at higher pressures on soil particles than roots can. Thus mycorrhizae infected plants especially with EM, have greater drought tolerance.

Mycorrhizae are an integral part of the carbon cycle on earth and are the reason why there is roughly 2X the amount of carbon stored in soil than in all the plants growing above the soil. This is because up to 20% of plant photosynthate is excreted into soil as a stable polymer called glomalin. Glomalin is responsible for binding soil particles and creating micro-aggregates and soil with water soluble aggregates does all kinds of good stuff. It increases soil moisture holding capacity while improving porosity and drainage. All of this helps reduce root rot hazard.

Mycorrhizae also affiliate with microbes. The hyphae of mycorrhizae cultivate bacteria which produce antibiotics that protect the host plant from pathogens. Linderman coined the term mycorrhizosphere to describe the microbial community that affiliates with these fungi. Plants are also protected by the Hartig net of EM mycorrhizae because it provides a shield or barrier so that pathogens have a difficult time invading the plant root. So, mycorrhizae greatly benefit plants by defending their roots from pathogens.

How do we keep the mycorrhizae growing with our garden plants? Most gardens are well inoculated with mycorrhizae at least the AM kinds. To get more access to EM it is necessary to also provide the organic carbon that they affiliate with. While EM absorb sugar from plant roots, their hyphae also grow into woody mulches helping to solubulize the nutrients contained in mulch and bring them back to their tree hosts. The litter and woody debris that fall in forests (litterfall) are essential for these organisms. We can simulate litterfall in gardens by applying fresh arborist chips and nourish the EM fungi as well as our woody garden plants at the same time.

References

Corkidi, L., Allen, E.B., Merhaut, D., Allen, M.F., Downer, J., Bohn, J. and Evans, M. 2004. Assessing the infectivity of commercial mycorrhizal inoculants in plant nursery conditions. J. Environmental Horticulture 22:149-154

Corkidi, L. Allen, E.B., Merhaut, D., Allen, M.F., Downer, J., Bohn, J and Evans, M. 2005. Effectiveness of four commercial mycorrhizal inoculants on the growth of Liquidambar styraciflua in plant nursery conditions

Linderman, R.G. 2005. Bio-based strategies for the management of soilborne pathogens. Presented at the Landscape Disease Symposium, University of California, Santa Paula.

Linderman RG. 1988. Mycorrhizal interactions with the rhizosphere microflora: The mycorrhizosphere effect. Phytopathology 78:366-371.

Ukraine is all in the news these days as Russian troops are amassed along its borders in Belarus and neighboring Russia. I have some knowledge of Ukraine having visited the Chernobyl nuclear exclusion zone (the “Zone”) four times in 2012, -15, -16 and 2018. I had planned more visits but the global COVID-19 pandemic prevented my return to Ukraine and the Zone. The accident at Chernobyl was the worst nuclear accident in the history of mankind releasing more radio isotopes than the event at Fukashima and had long ranging impact on Ukraine and the then Soviet Union. Some say that the event precipitated the down fall of the former Soviet empire.

Today the Chernobyl Nuclear Power Plant (CNPP) remains one of the most radioactive places that you can safely visit in the world. It was also the source of most of the world’s background radioactivity. When the disaster occurred in 1986, it temporarily raised the background gamma radiation of the entire planet by two percent. This rapidly declined as the half life of the released gases is very short and their radioactivity went away a few days and months later. Some of the elements, like radio Cesium 137 and Strontium 90, have longer half lives (around 30 years) and there was enough of them released to maintain high levels or gamma radiation where the fallout was most concentrated around the power plant. Elements such as plutonium remain radioactive for thousands of years but the amount of plutonium released was much less than that of strontium or cesium. Today the background gamma radiation near the CNPP remains up to ten times greater than the normal background levels found in the Ukraine capitol of Kiev. So how has this affected the gardens of Pripyat, the workers town not less than a km from the CNPP? To get to that let’s first talk more about the worker’s town and the disaster and then move on to what happened to the horticulture years later.

When the reactor exploded in 1986 (and yes it actually exploded) the area was surrounded with an exclusion zone complete with military checkpoints at 10 and 30 km circles away from the CNPP. This was an effort to keep people away from the radiation. The town that held the CNPP workers is Pripyat. It was a modern city of about 50,000 residents. It was considered the model city of its day as Chernobyl was considered the pinnacle of energy producing technology. Pripyat had a performing arts center, sports stadium, nightclubs, libraries several schools, public pool, and an amusement park. The entire population of the town was evacuated by bus in April 1986 in a few days. Although they were told they would return, most never did. Prypiat fell into ghost town status and remains that way to the present day, and like most ghost towns it was heavily looted. Some recent reports suggest that the Ukraine military has been knocking down buildings recently. Like any well planned city, Prypiat had an urban forest plan, street trees on every avenue and boulevard and gardens with ornamental plants. All were abandoned in 1986 and left to rainfall, radiation and the animals that remain there today.

The forest returned vigorously to Pripyat and animals roam the streets. The ecosystem recovery in the Zone has been dramatic over thirty years. Remnant street tree populations remain along the avenues but many more wild and non native exotics have invaded the spaces. The once athletic stadium playing field is now a small forest.

The forest encroachment has had a devastating impact on the architecture of the remnant buildings. Trees grow everywhere and when they attack the buildings they are able to collapse the floors and walls effectively demolishing the structure.

People have great impacts on the health and structure of trees. When left alone they develop their own natural structure according to their genetic code. Over several visits we measured growth of trees in the Zone and took pictures to analyze their structure. We found that trees of similar age growing in Pripyat were smaller in size but had better branch structure due to LACK of pruning for 30 years.

It is hard to imagine what the gardens of Pripyat looked like at the time they were in cultivation because we have so few records of the city to review. There are the remnant street trees which my friend Igor Lacan studied extensively. Garden plant remnants are mostly gone except for extant rose bushes which can still be found around the city.

It is hard to know what the gardens could have become before the forest invaded the city. We can look at landscapes in Kiev that exist today and see the overall gestalt of Ukraine gardens. They are kind of wild not meticulously maintained in public spaces but they also have charm, character and beauty.

As Ukraine is on the brink of uncertainty there are a few things that are certain, the radiation in the Chernobyl nuclear exclusion zone will continue, people will likely not be allowed to travel freely there, and the trees will continue to grow. The fate of gardens and trees in the capitol city of Kiev is less certain.

References:

Lacan, I., J.R. McBride, and D. De Witt. 2015. Urban forest condition and succession in the abandoned city of Pripyat, near Chernobyl, Ukraine. Urban Forestry & Urban Greening 14.4:1068-1078.

Burlakova, E.B. and V.I. Naydich (eds). 2012. The Lessons of Chernobyl: 25 Years Later. Nova Science Publishers, N.Y.

Downer, A.J. and J.F. Karlik. 2019. “A Comparison of Two Horsechestnut Street Tree Plantings in Kiev and Pripyat, Ukraine.” Open J. Forestry 9: 255-263.

Karlik, J.F. and A.J. Downer. 2019. Comparison of Gamma Ray Dosimeters in a Field Study in the Chernobyl Exclusion Zone. J. of Air and Waste Man. 11:1361-1367