My Soil is Crap

My Soil is Crap! Or is it?

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

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

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

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

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

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

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

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

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

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

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

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

The contrarian rosarian–debunking rose mythology

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

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

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

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

Myth II Roses Require lots of irrigation

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

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

Myth III Roses require rose specific fertilizers

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

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

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

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

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

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

Myth VI Grafted roses are better than non-grafted roses

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

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

Reference:

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

Why Fresh is Best—when it comes to mulch?

Fresh wood chips!

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

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

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

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

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

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

Pathogens buried in fresh yardwaste do not survive for very long

Literature

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

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

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

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

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

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

Pruning Paints Debunked

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

Wound dressings did not protect this apple branch from decay fungi

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

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

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

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

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

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

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

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

Literature:

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

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

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

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

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

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

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

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

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

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

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

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

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

Fall is for fungal fruit

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

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

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

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

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

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

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

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

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

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

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

Extremes

Extremes

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

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

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

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

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

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

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

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

A time to deadhead!

Summer is here in the west in a big way. We are just coming off of one of the largest heat waves ever recorded, and while temperatures are down they are not done. Its hot. Depending on where you live your gardens may have suffered. In the East Hurricanes are starting and extreme rains are occurring. I have images of bent over palm trees in Florida. No matter the season, plants respond with their own growth stages providing they are not blow away or burnt up by raging wildfires. Here in Arizona we have had moderately hot weather in my location but the garden is surviving with irrigation. My Iris plants remind me that it is long past time to deadhead and remove spent flower stalks. Deadheading is second nature to most gardeners and other than making the garden look better, you may not realize why you have or have not adopted this common garden practice.

Deadheading involves removing the spent flowers or inflorescences have withered.  Sometimes pruning back to a lower leaf or adventitious bud in the case of roses, or completely removing flower stalks in the case of German Iris is required.   The immediate result is a neater looking garden and an emphasis on remaining blooms. When the dead flowers are gone the remaining flowers look better the garden is refreshed. Depending on the plant there can be other benefits if deadheading is done consistently and is well timed.

This portion of the flower garden needed deaheading weeks ago. Even though I won’t advantage the plants by deadheading it will look better if I do.

We grow many kinds of plants in our gardens and deadheading has varied physiological impact depending on the subject being pruned. Properly timed, deadheading can extend the bloom of some plants for example Calendula.  However, Calendula produces lots of flowers and removing spent flowers can become an enduring task if you have a lot of Calendulas.  Deadheading some garden plants seems pointless such as impatiens which just regenerates flowers on its own. Deadheading soon after a flower passes prevents the plant from investing energy in seed development. If the plant has a long enough bloom cycle, so that energy can be put into other flowers then trimming back the flowering stem stems that are destined to fruit production often releases other buds to grow more flowers. Since photosynthate (sugars) flows in plants on a source-sink model, taking away the “sink” or developing fruit allows  energy to be used for growth elsewhere in the plant. The trick is to remove spent flowers soon because seed begins to form immediately after flowering and the plant will rapidly allocate its energy to reproduction once the flowers are pollinated.

with dead heads removed this corner of the garden looks a bit better

Not all garden plants respond to deadheading–the number of flowers some plants present is genetically regulated and dead flower removal does not promote more flowering (many bulbs produce only one set of flowers). Other garden plants will re-bloom if given a chance, and with deadheading (no matter what the flowering habit) the garden will look better without the dead flowers. Some bulbs can be deadheaded to prevent seed formation so that the energy is put back into the bulb or bulblets for next year’s display. Many roses will re-bloom after deadheading. This is not a wild-type characteristic of roses but a quality that has been selected for after years of plant breeding.

Deadheading can also be an excellent method of excluding diseases. Botrytis on rose blossoms and petal blight on Camellia are both controlled to some degree by removing infected blooms as soon as they are observed and disposing of them away from the garden.

This bag contains Penstemon with ripe seed heads ready for harvest and seeds for planting somewhere else in the garden.

Sometimes deadheading results in seed collection. Left too long, some plants go to seed but have not yet released their seed. If you want to save seed for propagation, strategic deadheading will allow you to collect seed while redirecting the plants growth patterns for more vegetation or more flower shoots. It is also helpful with our more ruderal garden friends to remove flower stalks to prevent their reproduction and taking over of smaller garden spaces that endure frequent cultivation or soil disturbance. Some plants are desirable but their progeny are a bother….

Terrariums

Glass bowls make excellent closed terrariums. This one has been planted for about three years. Episcia cupreata. and Begonia luzonensis dominate this planting.

Terrariums are are contained environments that allow culture of plants. They take many sizes, shapes and dimensions and can be sealed or open. At the least terrariums are just plants in a bottle, in their highest form they are cultivated landscapes in miniature. Closed terraria create a unique environment and opportunity for plant growth. The transparent walls of the container allow for both heat and light to enter the terrarium while maintaining high relative humidity and preventing system water loss. Sealed containers combine retained moisture and heat which allows for the creation of a small scale water cycle. This happens because moisture from both the soil and plants evaporates in the elevated temperatures inside the terrarium. Water vapor then condenses on the container walls and eventually drips back onto plants and soil below. A sealed terrarium is ideal for growing some kinds of plants due to the constant supply of water, thereby preventing them from becoming dry. Lowland jungle plants from warm climates will do well. Some cloud forest plants, orchids and bromeliads will not fare well in sealed environments because they require more air movement and/or cooler temperatures. Terrarium culture can allow growth of plants difficult to cultivate even in greenhouses. Terrariums can be displayed to great effect and are an easy method of indoor gardening. Success with a terrarium garden requires an understanding of the container, light, media, and the plants themselves.

My favorite terrarium fern Lemmaphyllum microphyllum (center). On the very bottom is the vining Peperomia prostrata and at the top is a runner of Ficus minima ‘Quercifolia’.

A Word about the Plants
A contained environment is not for all plants. When in a sealed environment, certain plants such as cacti or succulents will grow poorly or in a manner not suited to their habit (lanky or etiolated growth). Problems arise when plants not suited to a small contained environment are used. Plants such as Syngonium, Diffenbachia, and the larger Peperomia spp. look good when planted initially, but will soon outgrow their space–they are not suitable for closed terrariums. The classical “florist” terrarium planted with very young houseplants looks good at first but is completely unsustainable for months or years. A well designed terrarium should grow for multiple years before a complete tear down and replant is necessary. Thus it is necessary to select truly miniature and high-humidity-loving plants for closed terrarium culture. Ferns, sellaginellas, gesneriads, begonias and some peperomias are suited for these conditions. Obtaining truly miniature and humidity loving plants is difficult. Online vendors are the most accessible sources, but also other hobbyists or plant societies can be sources at their annual sales. Nurseries carry some of these plants but the vast array and diversity of rare plants are found on Ebay and Etsy. Many nurseries list plants under the ‘terrarium plants’ search words that are not really suitable, so take care to look for truly small or miniature plants. Perhaps start with the list I have provided at the end of this article for some of the tried and true plants that will work well. Terrarium gardens are not sustainable if you make bad plant choices, you will eventually end up removing plants that outgrow their containers.

The Container
Once you have your plants, you are ready to start. Or you can start before getting your plants and set up your terrarium now to plant later, or in stages, as you acquire new specimens to add into your contained garden. The first consideration is a suitable container. The larger the container the easier it will be to plant, grow and maintain your garden. Larger containers will also allow for a greater diversity of plant types. Fish aquariums may not be the most attractive, but are the most practical in many ways. Because they are rectangular they allow for placement of a light on the lid and they are easy to cover and place on square surfaces such as tables or window sills. Glass containers are preferred over plastic because they maintain transparency better over time. While bottles are attractive, if you can not get your hand inside they can be very difficult to plant and maintain.

Small containers are not optimal but if the plants are small, they can work well. This sundew has been in this container almost for a year

The Media
Lowland, humid jungle plants grow in decomposing organic matter. For our purposes peatmoss is the best medium. It can be amended with fine horticultural perlite (20-30%) or sand. Sand will make a heavier mix, and, if you are doing a large terrarium, mix weight is important. If not, sand is ideal. Also, since terrariums are contained, they may become disease gardens if you are not careful. Therefore I recommend sanitizing your media in a microwave until the media temperature exceeds 160F. Keep the bag closed until the media cools. A turkey roasting or other microwave safe bag works well. Media can be sanitized in a conventional oven–it just takes longer. Media should be moist but not wet when microwaved. Distilled water can be added later to moisten the media after planting. Commercial mixes can be used for terrarium media but care should be taken. Search the blog for my article on potting soils.

Since terraria are sealed environments, you need a reservoir for the water and a filter. Create the reservoir with coarse horticultural perlite (#3) up to an inch thick (the bottom most layer) depending on size of the container –the bigger container, the thicker the layer. Cover the perlite with activated charcoal. Fish aquarium charcoal or horticultural charcoal from the nursery is fine, but NOT charcoal briquettes. The charcoal layer just covers the perlite. Now add soil. Slope the soil from thin in the front to thicker in the back. You can also add wood, sticks, and rocks to make interesting landscapes. They should all be sanitized in the dishwasher or boiled or microwaved until sterile. After placement of soil, rocks and sticks are ready to plant. Place larger growing plants in the center and rear and small vines up front.
Your container should be sealed either with “cling tight” plastic wrap or glass. I prefer glass for most applications.

Light
While terraria can grow in window light, especially north light, it is not optimal for most plants and they will grow slowly. You can’t place terraria in direct sunlight or the plants will “cook” because closed terraria can’t dissipate heat that rapidly. The old standard for light sources is fluorescent tube fixtures, but they have been supplanted by Light Emitting Diode (LED) technology. Grow-light LED fixtures are expensive, but provide some performance differences. Terrariums are not crops and we don’t want them to grow too fast so find an affordable light source that works for you. LED sources are nice because they are not bulky and do not add large amounts of heat. A bit less light or less optimal wavelengths of light are ok because we want to sustain plant growth for a long time, not grow the plants to the edge of the container real fast and have to prune or start over. The Costco brand shoplight LED fixture is perfect, but it is four feet long. Smaller LED fixtures would be appropriate for smaller containers. The Costco fixture is perfect for a 60 gallon fish tank. White light works well and looks best. Red and blue LED fixtures change the way we see the plants and are not best for viewing. Light should come from above so plants will appear to be growing normally. If the terrarium is placed near a window it will need to be rotated to keep plant growth even.

Moisture
Moisture is critical in terraria. The growing medium should hold a shape when squeezed but not be saturated when you plant. After the terrarium is planted, you can “water it in” with a dilute -1/4-strength fertilizer solution mixed into distilled water. Watering amounts will vary by container size. Water should penetrate soil to the depth of roots and some should enter the reservoir. No more watering is necessary again until some time later when plants have grown considerably—usually months later. I usually water the glass to clean it from the initial planting with a turkey baster. At some point in the future, months not weeks, the soil may dry as growing plants use up water. When this occurs, water again with another dilute fertilizer solution. Do not over water your terrarium or bad things will happen. Also resist misting or spritzing as this will cause leaves to rot and is not necessary in a sealed environment.

Pruning, Replanting and Maintenance
Some of your chosen plants may outgrow their space. Some like Ficus minima ‘quercifolia’ will just overgrow everything, the same can happen with common Sellaginella sold in nurseries such as S. brownii. You should plan on pruning back the plants and making cuttings or planting other terrariums with the prunings. Cut begonias below a node or along the rhizome. Rhizomatous ferns can be clipped or dug and planted elsewhere. If you have to remove a really big plant it will leave a hole. New sterilized mix should be added to fill the hole along with the new plant occupant. Removal of flowers, mushrooms (should they form) and dying leaves is important. They will cause rots on plants they fall on. Sticks are usually always a problem since it is very difficult to kill mushroom fungi living in them. Mushrooms are mostly non-toxic to plants, but they drop spores and these lead to rot on sensitive begonias and ferns. Clip back Begonia, Episcia Sellaginella, Peperomia or Ficus to prevent them from overgrowing other plants.

Recommended Plant List
If you can find them, here are some recommended plants for terrariums.
Begonias
B. prismatocarpa
B. prismataocarpa variegata
B. versacolor
B. ‘Raja’
B. ficicola
B. exotica
Ferns
Edanoya spp.
Humata parvula
Lemmaphyllum microphyllum
Microgramma spp.
Pecluma pectinata
Tectaria spp.
Quercifelix zelanica
Others
Peperomia prostrata
Sininngia pusila and all its variants
Episcia spp. (there are many, I like the pink ones)
Saintpaulia (african violets-only miniatures)
Sellaginella erythropus
Sellaginella spp. (there are many kinds, S. brownii is most common)
Ficus minima ‘quercifolia’

Summertime pruning

Summer is a time of bounty in the home garden. During June, July and August the majority of small fruits ripen on home orchard trees. Plant health care is important to consider in advance of summer bounty. Careful dormant season pruning, dormant sprays, mulching and care helped to produce a nice harvest. As the fruit comes off the tree, some summertime options are available. This is a time when some limited summer pruning can be done to manage the physiology and growth of many fruit trees. Even some citrus will benefit from careful summer pruning.

Summertime brings a harvest bounty for many home gardeners, and with it an opportunity to modify tree growth with pruning

One obvious reason to prune in summer is to repair broken and remove dead branches that may have occurred from excess fruit weight or other injuries. Breakage is common in peach, plum and apples if fruit loads are not thinned earlier in the season. Cut the broken branch from the stem it attaches to with an angled cut that leaves the branch collar intact. Do not cut branches flush with the stem they were attached to. Many years ago the myth of flush cuts for shade trees was found to permanently damage trees, but flush cuts are often still practiced on fruit bearing trees. Flush cuts allow decay organisms to enter trees leading to heart rot and other kinds of wood decay.

Sporophores or fungal fruiting bodies indicate the presence of wood decay in trees. Usually be the time sporophores are showing wood decay is extensive in the tree.

Another myth that persists in home fruit orchards is painting wounds with a ‘sealant’ or ‘protestant’. There is no reason to paint cuts. They do not limit the progress of decay or prevent decay from forming behind the paint. Pruning paints do not promote “healing” or callus formation to close the wound. There is some thinking that pruning paints may even accelerate the process of decay formation. So throw away the black tar, it has no practical purpose in support of pruning.

Pruning paints are a relic of past horticulture traditions. They have no place in modern arboriculture or pomology

While pruning paints are no longer used, paint has other functions that can be helpful. If a large branch was removed from a tree, sometimes the remaining branches may require protection from sunlight. Apples and other thin-barked trees (citrus, cherries, etc.) are very susceptible to sunburn. If branches that were previously shaded are suddenly exposed to high light levels, the bark can be destroyed leading to sunburn cankers and entry of disease-causing fungi such as Botryosphaeria spp. If repair pruning exposes a large gap in the canopy, it is appropriate to apply white wash or diluted white latex paint to exposed branches in order to protect them from bright sunlight. The most severe damage occurs on southern and west facing branches. Sunburn is one of the leading causes of abiotic damage and a predisposing factor for disease such as stem and branch cankers in apples.

Fireblight is another common disease on pears and apples and develops after bloom. Pruning out fireblight affected twigs helps to arrest disease progress. Finally, bacterial canker can be devastating to Prunus (plum, cherry, peach, nectarine and almond) in parts of the country with warm summer rains. Immediate removal of bacterial canker affected branches is necessary to prevent permanent damage to the tree. Tools used to remove cankered branches should be sanitized by flame (torch) or with disinfectants. Canker diseases are active in the warm summer growing season. Cankers can be caused by bacteria or fungi and should be dealt with as soon as symptoms are noticed. The earliest symptom of an active canker is slowed growth relative to other branches on the tree. Slowed growth results in smaller leaves and fruit and fewer leaves. Affected branches seem more open and just look “weaker” than their healthy counterparts. Slowed growth is often followed by wilt, leaf drop and eventually necrosis or death of the branch. It is best to remove diseased branches early before the organism spreads to the main stem. Since symptoms occur when leaves are on, summer pruning is the best approach to remove cankered branches. Regardless of where or when damage occurs, using correct pruning practices should be adhered to.

Cankers kill branches in fruit trees, they can be caused by either bacteria or fungi. Here Botryosphaeria dothidea has killed a branch in this apple.

Healthy growth on the tree above but thin, weak, small leaves on the tree below indicate a developing branch canker.

Pruning is used most widely on fruit trees to dwarf them so that fruit is produced at a height convenient for harvesting. Pruning creates two universal responses that apply to all woody plants:

I. Pruning is growth retarding. The part of a tree pruned will grow less than the unpruned part. Or, a pruned tree will grow less than an unpruned tree.

II. Pruning is a bud invigorating process. A pruned tree or branch will have more of its buds released to grow compared to the unpruned branch or tree where many buds remain in a dormant state.

The more a tree is pruned, the less its roots and stems will grow. Even though the more a tree is pruned the more latent or axillary buds will be released to grow, it will not be able to make up for the lost leaf potential of the unpruned tree. The pruned tree has reduced photosynthetic capacity, makes less energy and will grow less overall. The thing that is not very clear is how the timing of pruning affects the basic processes. In his review, Chandler makes clear that pruning in the dormant season will retard the growth of apples less than if they are pruned in the summer. Summer pruning also significantly reduces the growth of roots compared to dormant season pruning. Removing leaves in mid-summer or after all shoot growth has stopped (summer rest period), removes photosynthetic capacity and reduces stored energy in the tree, thus retarding growth overall. While buds may be invigorated and new summer growth may occur, this rarely makes up for the tissue lost and still results in growth reduction.

Summer pruning does not result in more fruitfulness the following year, and in apples does not increase the number of spurs formed for fruit formation. Summer pruning can open the canopy and allow branches to form lower down that are useful for easy harvest. The effect of summer pruning on next year’s fruit quality is uncertain. Summer pruning can accelerate the ultimate scaffolding or canopy shape for the mature tree.

Pruning citrus after harvest, during the warm season can affect fruit size in the following year. This may be due to fruit thinning as some citrus have green fruit formed by summer that ripen in winter or spring. Summer pruning removes fruit and remaining fruit can grow larger.

Summer pruning of fruit trees before fruit harvest increases light penetration into the tree and can increase color development of the fruit. Pruning must be done cautiously to avoid excess light penetration and sunburn to scaffold branches and resultant canker diseases. Summer reduction pruning is most often accomplished by pruning the ends of branches back to other branches or twigs. Removing about one half the current season’s wood (on a given branch) will achieve objectives usually without causing excessive light penetration into the canopy. Not every branch need be pruned but an even approach, removing branches consistently around the tree, will maintain form. No more than 15-20% of the canopy should be removed by summer pruning. On some vigorous growing trees such as Persian mulberry, pomegranate, or some peaches, heavier pruning doses can be used. Pears, apples, plums and cherries require less pruning and cuts should be made to preserve spurs and other fruit bearing wood. Some varieties of cherries can become ‘over spurred’ and thinning cuts to remove excess spur wood can sometimes be helpful to limit production and increase fruit quality in the next season.

References:

Chandler, W. H. 1923. Results of some experiments in pruning fruit trees. Cornell University Agriculture Experiment Station bulletin 415.

Ingels, C. and P. Geisel. 2014. Fruit and Nut Tree Pruning Guidelines for Arborists. University of California Agriculture and Natural Resources publication 8502. http://anrcatalog.ucanr.edu

Saure, M.C. 1987. Summer pruning effect in apple—a review. Scientia Horticulture 30: 253-282