Author: Jim Downer

Arborists are trained in seminars and texts that rot in trees is bad. Wood decay can constitute a “hazardous condition” which when accompanied by the tree being in a place that has a target and the tree is large, can create a “hazardous tree”. The notion of hazardous trees is a uniquely human construct that has little to do with the ecology of trees, the variety of organisms that utilizes large declining trees, and does not consider what the various defects in trees may be contributing to the environment or forest around that tree in terms of organism habitat.  Humans require that trees living near them must perform appropriately otherwise get out the chain saw and make them comply.   In the last decade tree care for birds and  wildlife has become a popular training subject for arborists in the western United States.  In Europe researchers have been popularizing the notion that large trees can become centers of biodiversity because they have many microhabitats that support numerous organisms not found on younger trees.  This concept is abbreviated TreM or Tree related Microhabitat.

As trees mature and then decline, they accumulate deadwood, cavities, epiphytic organisms, excrescences, exudates, fungal decay organisms, and even accumulates of soil or pockets of water in branch crotches. Arboriculture practice tends to regard tree defects as having no value, thus we remove dead wood, cut down trees with cavities and condemn trees with wood decay sporophores. It is now accepted that the more “defects” a tree accumulates the quantity and diversity of organisms associated with that individual tree also increases. In this sense old trees become centers of biodiversity within both managed and unmanaged forests.

The health of a forest is not measured only by the quality of the wood it can produce or the number of board feet it can supply, but also by its connections to other organisms that ensure its health over time. Forests are ecosystems and require connections between organisms and diversity of organisms in order to be resilient. These organisms utilize not only living but also dying and declining trees. Ancient trees are often rich in defects and have many TreM’s.

Tree injuries such as storm damaged branches, lightning scars, frost cracks, branch failures and and other damage are all considered TreM’s. While these are functional habitat in forests they may be quickly removed from the urban forest even if they do not pose a hazard. Now that they have apparent value, perhaps we can rethink their removal where and when appropriate.

The TreM concept is derived from trees growing in forests not those in cities. TreM’s may not become a management tool for urban forestry, however there are many lessons to be learned from the TreM concept. The greater the number of microhabitats, the more organisms and connections between organisms there will be. This provides resilience even to urban ecosystems. There is strong evidence that TreM’s serve as a reservoir of organisms in forests helping to maintain their health. Using the TreM concept for non-forest trees will not change how trees are managed for many situations. Risk tolerance often trumps ecosystem services. Greater understanding of TreM’s will perhaps allow us to save trees that do not pose hazards where they would otherwise be disposed of. Some tree managers have tried to create defects in trees to enhance habitat for wildlife. This is not based in science and I do not advocate creating TreM’s for the sake of having them in trees. Fungi and other organisms find their way into trees all too easily. Until we have some science based evidence for the creation of TreM’s, I recommend against it. It’s the whole do no harm thing we have going as plant pathologists. Being aware of TreM’s and evaluating their usefulness in the urban forest is a new area of study.

In their field guide, Butler et al., 2020 describe 47 TreM’s that they further break down into 15 groups and 7 types. The field guide is available on line if you want to find out more about TreM’s. The research on TreM’s is nascent, and restricted mainly to Europe and Canada. This fall we will collect data in the Chiricahua Mountains to add to that body of research as part of the South Western Research Station’s Trees Course to be held the last week of September into early October.

References

Butler, R., T. Lachat, F. Krumm, D. Kraus, and L. Larrieu. 2020. Field guide to Tree-related Microhabitats. Descritpions and size limits for their inventory. Birmensdorf, Swiss Federal Insitute for Forest, Snow and Landscape Research WSL. 59 p. www.wsl.ch/fg-trems

Larrieu, L., Paillet, Y., Winter, S., Butler, R., Kraus, D., Krumm, F., Lachat, T., Michel, A.K., Regenery, B., and Vanderkerkhove, K. 2018 Tree related microhabitats in temperate and Mediterranean European forests: a hierarchical typology fr inventory standardization. Ecologial Indicators, 84: 194-207

January is here with its resolutions, cold long nights and not that warm days. Winter is a season of rest and survival. The cats and horses have long furry coats, the resident song birds eagerly clean out the feeder every day and the garden beckons. For me Winter is a special season when I can do a lot of fruit tree pruning, especially enjoyed with my daughter. Father-daughter pruning bonding is not to be missed if it’s an option for you. Gardens are tuned to winter as period of rest but the promise of longer days that will initiate the changes that happen in Spring will soon be upon us. In this post I’ll reflect on how plants survive winter and what we can do to help them.

Winter is actually a very dry time of the year in many places and the winter cold that freezes soil leads to dehydration. Plants installed just before winter will not emerge in spring alive w/o moisture in their systems. Mulch is an essential and natural part of winterization for many North American temperate plants. Protecting the root ball of a newly planted perennial is a must do for winter survival. In nature this is accommodated by the deciduous habit of many trees and shrubs, falling leaves are a big part of winterization. In our gardens we can do this with mulch.

Deciduosity

I know deciduosity is not often used but I like to use unusual words so here we go. The deciduous habits of many north American temperate trees enable them and other plants to survive cold, dry, freezing winters. Environmental cues (photoperiod and cooling temperatures) signal trees to drop their leaves (Fadon et al., 2020). Cold temperatures are also required by temperate perennials to invigorate buds and make starch into soluble sugars for strong spring growth. Deciduosity also leads to abundant mulch on the forest (or garden) floor. This protects soil and surface root systems, seeds, perennial herbaceous plants and bulbs and provides an insulating layer under snow, if snow is a thing where you are. When warm temps arrive in Spring the leaves quickly break down as growth under them emerges.

Solutes

Deciduosity brings certain challenges to woody perennials that donate their canopy to the soil each year. Trees in spring have no photosynthetic organs to supply the energy of growth. That energy has to be stored in the wood and roots as carbohydrates, mostly as starch, at the end of the growing season and before leaf fall. In spring at the end of dormancy when buds grow, these stored carbohydrates convert to soluble sugars and fuel the rebirth of a a new canopy. Having all that stored sugar in cells throughout the plant also reduces the freezing point of water in the cells so that subzero temperatures do not lead to ice crystal formation (and cell death) of the dormant plant.

Seeds

Another way plants survive Winter is by forming seeds. The strategy of annual plants is to “go to sleep” as seeds and “wake up” by germinating. To ensure that seeds don’t germinate too early, they often have inhibitors that need to be washed away by water (Spring thaw), burned by fire (usually summer time), or by scarification (tumbling in the creek etc). Many seeds germinate better after a cold winter than if they were sown without cold chilling. Not all seeds will germinate at the same time as inhibitors delay germination. This ensures that conditions will be right for some of the seeds and thus the species will survive, even thrive in the right place.

Roots

While the above ground part of gardens can be in a dormant state in January, the situation underground is different. Roots respire (break down sugars to get energy for growth) during winter and may grow continuously depending on climate, depth and soil coverage conditions. Roots, just like buds, utilize stored carbohydrates to fuel their growth. If temperatures remain more moderate under the soil they can continue to respire well into winter months. Soils freeze when they lack snow cover or mulch, Reinmann and Templer (2016) propose that roots in frozen soils are less active. Leaf mulches help protect soils from hard freezes.

Am I crazy or What?

I know that a leaf dump on the garden every year is not what many gardeners want to deal with. That is what leaf blowers are for right? Some municipalities even have line items in their budget for disposing of fallen leaves which are some of the most disposed of green waste. Leaves that accumulate on hardscape can be a pollution source accounting for up to 80% of phosphorus pollution in one study (Bratt et al., 2017). It’s best to utilize leaves around perennials and keep them away from streets, gutters and sidewalks.
Trees evolved to drop their leaves on the ground and for them to stay there. Finding ways to accommodate this in gardens will lead to a healthier garden and less waste in landfills. Leaves can be mown on turf areas and the biomass will be incorporated into the turf sward (Nektarios et al., 1999) without loss of turfgrass quality. In gardens they can become part of the surface mulch. If you are really crazy, you can grind them in a shredder to make really high quality micro mulch to be used around certain plants or vegetables (we do this with coast live oak leaves of which we have an abundance in California). Stavi, (2020) encourages us to think of fallen leaves as a resource not a waste product. Your garden will benefit.

For more information on leaves please see the other blogs at this site:

• https://gardenprofessors.com/noraking/
  
• https://gardenprofessors.com/leaves-for-lawn-fertilizer/
  
• https://gardenprofessors.com/one-trees-leaves-over-400-kinds-of-bacteria/

References

A. R. Bratt, J.C. Finlay, S. E. Hobbie, B. D. Janke, A. C. Worm, and K.L. Kemmitt 2017. Contribution of Leaf Litter to Nutrient Export during Winter Months in an Urban Residential Watershed. Environ. Sci. & Technol. 6: 3138-3147
https://pubmed.ncbi.nlm.nih.gov/28215078/

Fadon, E. E. Fernandez, H. Behn, and E. Luedeling. 2020. A conceptual Framework for Winter Dormancy in Deciduous trees. Agronomy 10(2), 241; https://doi.org/10.3390/agronomy10020241

P. Nektarios, A.M. Petrovic and D. Sender 1999. Tree Leaf Deposition Effects on Kentucky Bluegrass (Poa pratenses L.), J. of Turfgrass Man., 3:(1) 69-74. DOI: 10.1300/J099v03n01_06

Reinmann AB, Templer PH. 2016. Reduced winter snowpack and greater soil frost reduce live root biomass and stimulate radial growth and stem respiration of red maple (Acer rubrum) trees in a mixed-hardwood forest. Ecosystems. 19:129- 141.
https://www.jstor.org/stable/48719251

Stavi, I. 2020. On-Site Use of Plant Litter and Yard Waste as Mulch in Gardening and Landscaping Systems. Sustainability 12(18), 7521; https://doi.org/10.3390/su12187521

Facebook and other social media attempt to help us solve problems.  This group and others seek to inform gardeners and solve problems they are having growing plants.  Looking at queries and posted responses there is so much information missing, leading to wrong and misleading comments in many of these discussions.  I think it is a good idea to reexamine the diagnostic process and how gardeners can solve their own diagnostic questions.

Diagnosis is always the precursor to solving a plant problem. In the world of plant pathology, palliative care (treating symptoms) is often ineffective if the cause of the disorder is unknown. It is amazing how on social media so many cures, fixes, MacGyvers, or treatments are suggested even before a diagnosis is made. The diagnostic process has many components so its good to be familiar with some of the steps in this process.

Identify the plant

Host identification comes as the first step in diagnosis. It sounds simple or silly, but knowing the host name is the first step in diagnosis. Find the scientific name of the plant and then specific disorders of that taxa can be sought out in a web search. Common names are misleading and it is critical to associate disorders with the exact plant you have a problem with. If you are diagnosing remotely (as I am often forced to do), knowing the location is the next question as many disorders are regional. For instance we don’t have black knot of plum in southern California while in southern Ontario, Canada and New York state that is a big problem.

Look at the whole organism

So many gardeners only focus on where they see symptoms. A leaf, shoot, or branch with something that does not look right is a good place to start looking, but always consider the entire plant. It is important to see the entire plant and what the distribution of above ground symptoms is. Don’t forget the “whole organism” includes its root system which is often neglected in diagnosis.

Look at all the components of the plant

Symptoms which are plant responses to a disease or disorder often occur on leaves. The problem, however, may be in the roots. Root rots may go undetected until almost the entire root system is decayed; only then do symptoms start to appear on the distal or far portion of the plant. These rapidly or slowly spread until the entire plant is affected. Whenever there is uniform symptomology of the foliage, always check the root system. Symptoms on only a single branch of a perennial may be localized to that branch, so follow the symptomatic branch back to its attachment point to locate any damage or disease along its stem.

Look closely

It often helps to use a hand lends to closely observe insects, insect products like webbing, eggs, pupal cases, or frass, or just to validate that there are no insects or their products present. Many many fungi form fungal fruiting bodies in dead stem portions and these look like tiny grains of pepper under a hand lens. A closer view is often helpful in deciding if a problem is localized or system in the plant.

Look for symptoms and signs

Symptoms are plant responses to attack from pathogens, insects or abiotic causes such as herbicides, toxic salts, high and low temperatures etc. Symptoms alert the gardener that there is something wrong but may or may not point the way to the cause of the problem. It is also important to look for signs which are parts of the biology causing the problem. Fungal growths, spores, fruiting bodies insects adults and immatures stages of insects and the products they produce and leave behind are all signs. Signs give more direct evidence of the cause of a problem.

Look around

You may not be the only gardener with a plant problem. Look to see if other plants in your garden are similarly affected. If only a single taxa is affected it could possibly be a disease or insect problem. If many different kinds of plants are affected it may be from a non-biological cause–an abiotic disease or environmental disorder. Solving these diagnoses often requires lab work and specific soil or plant sampling

Seek confirmation

Once you have collected all the the information (symptoms and signs) over the entire plant (including if necessary root symptoms), it is time to put the information to work. Searching on your own, on the internet, is daunting because there is so much misleading information. If you have the scientific name, you can put that in a search engine along with the symptoms and tentative ID of insects or pathogens and then look at all the images that match what you have. Click on the image and check the source of the file. If from an .edu or educational source, it is likely a higher quality of information. Read these first.

Taking samples to a lab or University Extension office is of limited value because they can’t see the entire plant. It’s best to take samples to an expert when you have a good hunch what is going on and you want to confirm it (you should include images of the whole plant if you can). So much money is spent sending random leaf or twig samples to labs and they end up sending information that is misleading or just wrong as far as the diagnosis goes. Thousands of fungi grow on plant surfaces and labs will isolate these, some are pathogens but may not be on your specific plant as pathogens can be quite specific to plants they have a disease relationship with. The lab report comes back with a finding Alternaria spp. This is indeed a pathogen of tomato and many other plants but it is also a very common saprophyte often found growing on dead plant tissues. So lab findings are helpful when they confirm your own suspicions, but often unhelpful when random plant tissues are sent by a gardener that has no idea what is happening. This is true of any lab, university or private. The more information the lab has, the more helpful they can be. And all labs everywhere would prefer to have the entire organism for diagnosis.

Diagnosis is hard. The best diagnosticians are correct (solve the diagnostic problem) about 2/3 of the time. Sometimes diagnosis of a problem can take years. Some diagnoses are never solved. But for most common plant problems you can find answers by intelligently searching the internet and with some help from the “ologists” of University and private diagnostic firms.

Fall is for planting they say when folks talk about shade trees. But is it? When is the best time to plant a tree? In this blog I will cover tree planting times and other particulars, the drawbacks and good points of these decisions.

So is fall the best season to plant a tree? Of course like so many questions it depends on many factors. Where you live (latitude) is a big part of this equation. I reside in Southern California and Southern Arizona. Both mild climes by any standard, but the Arizona property is at a higher elevation (4500ft) and gets cold sooner than the Southern California location. In Alaska, for another example, the planting seasons are much shorter or narrower as the onset of cold weather can be sooner in the calendar.

I think it is important to consider things from a “tree perspective”; or when is it best to plant from a tree’s perspective. Planting is not only the act of installing the tree correctly (see other GP blogs posts for correct planting technique) but it is also an acclimatization process. It is a good idea to purchase your tree beforehand and give it time to get used to the temperatures, light levels and water in the new site. Most fall planted trees are in containers or B&B which requires we do root inspections in order to not plant a tree that has root defects. These root inspections include removing all the old growing medium and root washing (search the GP blog for root washing). Locally sources fall planted trees will automatically be acclimated to reduced light and cooler temperatures. In fact if you plant a deciduous tree it may already be preparing to drop leaves. Fall planted trees still need root ball moisture to establish and thus will need some irrigation, but fall is also a time of reduced water use. One benefit to fall planting is that the trees will grow some roots over the winter and be ready for a big growth push in the spring. They will be partially established and take full advantage of longer days, moist soil and warming temperatures.

What about winter planting? I have a colleague that described in great detail his ambition to move to California and seek academic employment after not getting a $.50/hour raise at his landscaping job during a long stint of chipping ice in Minnesota to plant conifers in frozen ground. My colleague just retired from a nice career in Cooperative Extension, but that winter planting helped him make the move. You can plant trees in frozen soil but winter kill is a thing and the success rate of such efforts is less than for fall planting. In Southern California and other areas of the southwest and southern USA, winter planting is preferred for fruit trees because you have great access to bare-root stock (only in Winter actually) and we don’t contend with frozen soils. If it ends up being a super wet winter (see the previous blog by Pam for insight on that) it can be a problem when newly planted trees sit in saturated soils for weeks on end.

Spring planting is a thing because Arbor Day is in spring and everyone wants to plant trees on Arbor Day right? Spring planting is sometimes limited by availability, bare root stock is usually sold out or moved into containers. I don’t like buying left over stock because the leftovers are often not the best. And, trees may be in a new growth or flowering phase and their root systems are activating.

This leaves summer. In Southern California shade trees are planted all year. Fruit trees planted in summer will be the left overs from winter and again I don’t like left overs, so I generally don’t plant fruit trees then. Subtropicals establish well in warm weather so mangoes, avocados and citrus are easily planted in summer if irrigation is assured.

So the harsher the climate, the more restrictive the planting dates, but Fall is still best. In mild climates of southern states you can plant when you want in most cases. But avoiding months with frost is usually helpful as nursery stock often has tender growth. In almost all cases follow your plantings with a generous ring of arborist chips, avoid planting directly in turfgrass and irrigate your tree like it is still in the nursery for the first few weeks until it roots into the native soil. Do not amend the backfill and PLEASE remove the nursery stake at planting. Provide whatever the tree needs to stand upright with loose ties to poles outside the rootzone. Plant trees where they have room to grow and access to sunlight for most of the day. Plant HO!

The hot weather that stimulated the last blog is still with us! Keep up the mulch and occasional watering to help shade trees. Today I want to cover a topic that seems like a garden myth but actually has considerable science behind it. Bicarbonate! The miracle cure for all garden pests? No. My wife came across an article in her news feed about a ‘garden guru’ who touted baking soda as a miracle cure for powdery mildew and other “blight” diseases. In this blog I will review the efficacy of the bicarbonate anion in disease control.

Bicarbonate as a disease control agent is not a new concept.  Many studies going back a few decades documented the efficacy of this molecule in controlling foliar diseases of plants.  Most studies are on powdery mildew of many crops and ornamental plants as well.  Bicarbonate is typically available with three cations: ammonium, sodium and potassium.  Most of the studies and efficacy are with the potassium salt of bicarbonate (not baking soda which is the sodium salt).  All the salts are more or less efficacious against powdery mildew and a few other fungi like apple scab. 

The mechanism of action of bicarbonate control of fungi is not clear, however Monterey Chemical that manufactures a Bicarb. product (Bi Carb Old Fashioned Fungicide) indicates on its label that the mechanism is the disruption of the potassium ion balance in the fungus cell, causing the cell walls to collapse 

One frustrating thing with internet-based articles by garden “gurus” is that everything is some kind of “garden hack” Like we are getting something done on the sly or with common household materials. Using chemistry to control diseases is using pesticides. Companies test and approve these materials based on efficacy data as shown in the references at the end of this article. The benefit of using a registered product is that there are instructions that indicate how to spray, what the target organism is and the environmental conditions under which the product will work. Much of this is never mentioned in the “hack” articles.

Bicarbonate anion is an effective control of powdery mildew often as good as commercial fungicides with very different chemistries and methods of action. There does not appear to be resistance to bicarbonate. It is a contact fungicide (not systemic), the material must contact the fungus in order for it to work. In order to get good contact usually a wetting agent such as an ultrafine oil is combined with the tank spray solution to increase control.

It is best if bicarbonate is applied early in the disease cycle. Powdery mildew organisms are obligate biotrophic fungi. This means that the mildew fungus must grow in living plant cells. So when the mildew is killed the living plant cell is also killed. Powdery mildew is a disease of the epidermal tissues of plants so cell death is superficial but if late stage mildew is control by bicarbonates there can be phytotoxicity (plant damage) as most of the epidermal cells of leaves and flowers will collapse and die.

Powdery mildew fungi are in the Ascoymcete group of fungi and most have two spore stages. The first infections are caused by ascospores (sexual spores) that are released in the springtime. The primary infections develop and then asexual spores develop on plant surfaces that we recognize as powdery mildew. Stopping the primary infection by applying control early in the season will slow powdery mildew down on sensitive plants. Since many powdery mildews have broad host ranges they form on weeds an other plants and can move into protected plants later in the season. Frequent sprays will be required if conditions for the fungi are optimal and the spores are present. Bicarbonates do not have long-term protective effects since they work only when the solutions contact living fungus.

References

Zivand, O. and A. Hagiladi. 1993. Controlling powdery mildew in Euonymus with polymer coatings and bicarbonate solutions. HortSceince 28:134-126

Moyer, C., & Peres, N. A. (2008, December). Evaluation of biofungicides for control of powdery mildew of gerbera daisy. In Proceedings of the Florida State Horticultural Society 121: 389-394.

Holb, I.J. and S. Kunz. 2016. Integrated Control of Apple Scab and Powdery Mildew in an Organic Apple Orchard by Combining Potassium Carbonates with Wettable Sulfur, Pruning, and Cultivar Susceptibility. Plant Disease, 100(9), 1894-1905

El-Nogoumy, B. A., Salem, M. A., El-Kot, G. A., Hamden, S., Sehsah, M. D., Makhlouf, A. H., & Nehela, Y. 2022. Evaluation of the Impacts of Potassium Bicarbonate, Moringa oleifera Seed Extract, and Bacillus subtilis on Sugar Beet Powdery Mildew. Plants, 11(23), 3258.

Türkkan, M., Erper, İ., Eser, Ü., & Baltacı, A. 2018. Evaluation of inhibitory effect of some bicarbonate salts and fungicides against hazelnut powdery mildew. Gesunde Pflanzen, 70(1), 39-44.

We are again in the midst of excessive heat events in many parts of the United States. Records were broken for the highest temperatures ever recorded just a few days ago. This is also a time when the days are at their very longest, so high temperatures have large impacts on plants in landscapes.

High temperature can have immediate (acute) and continuing impacts (chronic) on plants. When temperatures get much over 90F photosynthesis becomes less efficient and in some plants may stop all together. As temperatures increase beyond 90F photosynthesis shuts down and transpiration may also stop to avoid breaking the chain of water molecules that plants must have to move water. When this happens heat builds up in the foliage leading to cell death and eventually symptoms (acute response). These may initially show as wilting, loss of color in the leaf and rapidly within days show as yellowing and then necrosis. This is usually seen in the center of the leaf first as the edges of leaves dissipate heat faster and more efficiently than around the mid vein area of leaves.

Chronic effects of heat are related to the poor efficiency of photosynthesis at high temperatures. When plants are hot and the photo systems that capture sunlight energy are impaired, or not working, the plant must still use energy in all its cells for respiration. Stored carbohydrates are not available for growth as cell maintenance (respiration) is the first demand for energy. When temperatures are high for long periods, stored carbohydrates in roots and stems are depleted. Since energy for growth is not available, slowed or stopped growth is the biggest chronic effect of hot days on most plants. This is why even hydrated plants just seem to stop growing in hot weather.

What can be done to mitigate high temperatures? First, never let plants dry out during high heat events. Evenly moist soil (but not saturated) will allow plants to absorb water and cool themselves as much as their physiology will allow. If soils are dry the damage of high heat events is “magnified” many fold and foliar damage will increase. Irrigate late in the day or early to avoid evaporation of applied water. Get your plants ready for high heat by irrigating before it hits. We usually have good weather prediction a few days ahead of high heat events.

Another way to mitigate high heat is to avoid plantings in “high albedo” environments. Albedo is the reflection of sunlight. Low albedo surroundings abosorb sunlight energy, high albedo environments reflect it. Plants exposed to reflected sunlight will be more readily damaged by sunlight during high heat events because they can not transpire enough water to cool their leaves. Reflective soils like decomposed granite, or some kinds of rock will damage young trees during heat events. Cover the soil with arborist wood chips which have a relatively low albedo. Young plantings can be protected by placing shade cloth over their canopies until the high heat subsides. If you don’t have shade cloth, a white sheet will do fine as it will reflect heat away from the canopy.

Ensure that the mulch or soil is moist before the heat of the day starts so humidity increases during the day. This will reduce the demand on transpiration and and the possibility of cavitation (the disruption of water chains in the plant and introduction of air which stops water movement), thus preventing a catastrophic heat death event.

A final word of precaution- Never fertilize during high heat events. Even when watered this changes the osmotic potential of water in soil making it harder for plants to pull water in. Adding fertilizer is like adding salt and this is a big NO during high heat events. Try to ensure that plants have all the mineral elements they need before heat becomes an issue.

You might think that during heat events its a wise idea to prune. This is not the case! Avoid pruning, especially thinning, as the removal of leaves will increase the impact of heat on the remaining canopy. Pruning and removing leaves will decrease the humidity around a plant and the remaining leaves will have to transpire more to cool the plant. This can be a disaster during a high heat event.

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