Companion plants, they are not what you think!

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.

Celtis reticulata growing in full sun has no apical control and sprawls as a giant bush yet in nature when it germinates in shade, it develops a straight trunk with fewer branches.

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.

This Indian paintbrush is a hemiparasite. It can be free living or associate with other plants and use their photosynthate.

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.

In this image agaves grow at the base of an Alligator juniper, very companionable. But is there a benefit for either plant?
Agaves like to grow next to rocks. My son’s theory is that both rocks and trees protect the agave from being eaten by javelina. Or perhaps there’s just more moisture under the stones?

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

Landscape Fabric Strikes Again–Oh the Horror!

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.

Landscape fabric in my test plots prevented rooting of coast live oak in the undersoil

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.

One of the oaks in the fabric affected block. Even coarse arborists chips can’t help this tree when landscape fabric is in the way. The yellow coloration of the leaves is chlorosis. Likely because the roots do not have enough oxygen to acidify their rhizosphere.

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.

Image from Shahzad et al., 2019. The chart indicates how fabrics and plastic are impermeable to carbon dioxide.

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.

As mulches break down “tatty” landscape fabric shows through giving a trash-like look to any landscape they are used in

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

Big Blog on the Block

There’s a new1 blog on the social media block—The Big Blog of Gardening (BBoG). Already it’s a heavy hitter in the gardening social media world. The question is: How may foul balls are hit?

My wife came to me recently saying “Hey! Did you know that your friend Linda Chalker-Scott changed her institution?”
“What?” I said.
“Yeah, she moved to University of Washington. It says right here on this MSNBC article.”
“It’s from the Big Blog of Gardening? What’s that?”
Turns out that the BBoG is hooked into national media and gets consistent play on the home pages of those who go to MSNBC. This is because the BBoG is “now part of the Microsoft Start Program” that places content on the MSN homepage whenever a user logs in.

The originator of the BBoG is not a scientist and in the “About” section of his web page states that he started gardening as a child (like many other of us that had school gardening programs around the country). I also started gardening as a child, volunteered at Descanso Gardens in La Canada, but in my own case I followed up my childhood experiences with a dual major in Botany and Horticulture, an MS in plant pathology, a PhD in plant pathology, and a 30-year career with Cooperative Extension advising and researching in landscape horticulture. These are the typical qualifications for the blog writers at the Garden Professors web pages. Unlike writers for the BBoG, we are the folks who actually conduct research on horticulture and gardening subjects that other people quote and cite.

For us scientists, one of the pitfalls of the BBoG is it’s not a science-based blog. In a blog on pruning, the title proposes to inform how to prune any landscape plant. When you read that article, it just directs you to a link to Amazon.com to purchase a publication of the American Horticultural Society (which is not a science-based organization even though it sounds very much like the American Society for Horticultural Science – the oldest horticultural science society in the US). Rather than cite current research or address the blog title’s topic, the article leads to a product you have to buy to get your information. BBoG posts are full of of links to products available online or to “paid for links” for non-scientific and misleading garden books or other resources.

Broad strokes are used in titles of the blog. For example: “Staking a tree is almost never the right thing to do”. In some ideal world where nurseries grow trees w/o stakes and landscapes don’t require protection from damaging elements this may be true, but this is not the world we live in. Trees grown in nurseries are often staked to facilitate production and shipping. Staking can be used as a protection process on trees that may suffer impact from moving vehicles (these stakes have no attachment to the tree but serve as protective bollards). Titles in some other posts are merely attention getting or serve to promote products – not to reflect accurate horticultural science.

The BBoG often cites Dr. Linda Chalker-Scott, quotes her published work, provides links to her white papers, but doesn’t actually email or otherwise contact the original author. The problem is that often there are peer-reviewed sources by the same author containing this newer information (e.g., WSU Extension publications). BBoG often seems to miss the actual scientific or peer reviewed work but focuses on popular sources such as Fine Gardening or, even worse, Consumer Reports. Consumer Reports is not a legitimate resource for science-based information. (link to Jeff Gilman’s blog post)

Beware the comments at the end of articles on the BBoG as there can be pseudo-scientific information there (using gypsum to create drainage in soils) that goes unrefuted by the article authors. It is important that site administrators approve comments before they are listed, or at least address the misconceptions in a response.

When BBoG stories hit the mainstream media (like MSN) the blog owner does not always mention the original sources of their stories, or the scientists who developed the information: they take credit and reap the rewards of increased eyeballs on their posts and clicks on their advertising links. Wouldn’t it be nice if members of the media could dig a little deeper and find the science-based gardening sites and give them some well-deserved publicity?

Water Woes

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

Good news — Bad news

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

The combination of drought and heat caused sunburn to these privet leaves

Mediterranean Climate

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

In a Mediterranean climate little rain occurs in the summer months

Stress

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

Adding stress bars (red) shows how stress builds over time if there is no rainfall or irrigation

Save your plants in the fall from deadly stress

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

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

Monsoons in Portal, AZ, July 2022.

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

Saving Your Trees From Drought!

Yikes!

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

Drought brings change in landscapes. Here European birch have died and provide opportunities for planting more drought tolerant species

Don’t assume your trees will die of drought!

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

Some trees such as this Indian rosewood (Dalbergia sissoo) are adapted to hot climates and endure long dry periods without damage.

Don’t plant new trees in the Summer!

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

Don’t fertilize trees during drought!

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

Fertilizers increase drought stress for trees if water is not available for the application.

Don’t prune trees during drought!

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

Pruning, especially over-thinning, stimulates new shoots to grow–something you do not want to encourage during drought periods

Do not install artificial turfgrass

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

Artificial turfgrass does not use water but it also gets hot. On the day this image was taken it was 50 degrees hotter than irrigated turfgrass and 30 degrees hotter than brown dry turfgrass

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

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

Do apply Arborist Chip Mulches

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

A common theme in these blogs: arborist chips straight from the chipper have real benefits for trees trying to survive drought

Do “top up” existing mulched areas around trees

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

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

Due to climate change, coast live oak (a native) is less adapted to inland valleys of Southern California than Eucalyptus camaldulensis (an exotic).

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

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

High temperature injury to cherry leaves is not a symptom of drought but heat intolerance. However, as trees dry down they have less ability to endure high temperatures.

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

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

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

Be hopeful

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

Garden Diagnostics

A garden plant with symptoms of an insect infestation

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

Looking for patterns in your garden can inform disease issues. here all the boxwood are yellow and all the redwoods are brown. See first paragraph! Yikes!

Patterns

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

Symptoms are plant responses to a pathogen or abiotic condition

Symptoms

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

Signs are the actual pathogen that is causing the symptoms in the affected plant

Signs

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

Powdery mildew spores (white) are signs and the broom-like symptoms are typical of the disease that forms in coast live oak.

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

Early symptoms of Ficus canker in Indian Laurel Fig
advanced symptoms of Ficus canker
Signs of Ficus canker disease. The black dots forming on the end of a cankered branch are fruiting bodies that hold the spores of the fungus causing the disease.

Time

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

Insects

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

Frass shown here is a sign of boring insects inside this tree.
Many insects cause their host to grow galls such as this oak apple gall caused by a small wasp
This leaf spot on Lantana camara was long thought to be a fungal disease but is actually caused by a ‘blotch miner’ insect in the genus Liriomyza.

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

Mycorrhizae! Myco what??

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

Mycorrhizal plant (right) vs non-inoculated plant (left). With permission from Linderman, 2005.

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

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

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

Ectomycorrhizae can be seen in soil threading in and around roots often covering them in white mycelium.

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

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

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

Amanita muscari is an EM fungus that grows on tree roots

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

A chip drop of fresh arborist chippings. Coarse woody mulch supplies additional carbon (energy) to soil fungi that benefit our landscape plants. [For a free, peer-reviewed publication on using arborist wood chips, please click here]

References

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

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

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

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

The Gardens of Chernobyl 30 years after the disaster

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

An image from the 2012 visit of the sarcophagus surrounding reactor IV of the Chernobyl nuclear powerplant.

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

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

A view toward the CNPP (on the horizon) from the roof of a 16 floor apartment building in Prypiat, Ukraine, note the verdant encroached forest.

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

A forest grows on the end zone of the Pirpyat Stadium.

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

Trees began the demolition of this structure in 2016, a school building I had walked through in 2015.

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

Horse chestnuts in Kiev, Ukraine have been crown raised, have large pruning wounds, decay, and branch faults such as too many branches from one point and co-dominant leaders.
Horse chestnut trees along Lenin Boulevard in Pripyat have fewer branch faults and literally no pruning wounds after thirty years on their own.

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

An extant rose plant in Pripyat, Ukraine.

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

A public park in the capitol city of Ukraine in Spring of 2018. Not a lot of maintenance but when in bloom full of beauty and impact.
Some color swirling through mowed weeds or “turfgrass” in a Kiev park.

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

References:

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

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

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

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

House Plant Basics

I love plants! I love gardens! I love nature! So why not bring it all inside the house? Who can resist those beautiful Calatheas (Prayer Plants) they sell at Home Depot? House plants afford us the opportunity to garden indoors when it’s hostile outside and they beautify a room like nothing else. There is an incredible selection of tropical, subtropical and succulent plants that we can cultivate indoors. Unfortunately house plants fade…waaa (sad). They lose leaves, they endure pests, they wilt, eventually they die. I am sure this is not the intended outcome when we purchased the beautiful plant in its six inch pot. However, since there are more to be had we can dispose of the tired ones and just buy more fresh ones. For lots of people, house plants are like cut flowers. Throw them away when they fade, especially true for orchids. This does not need to be the case if you understand the basic needs of container culture.

This Begonia is at the peak of its production cycle, the growing is done. To maintain this plant it will take nutrients, perhaps a different container and new soil.

Wise Choices Make Good House Plants

Not every plant you see in the nursery or box store is right for your location. Many tropical plants are grown in greenhouses under high humidity and carefully filtered water. For some plants like the previously mentioned prayer plants, this is a mimic of their natural growing conditions, moist, warm and humid. Many homes may have the warm part but not the humid. Heating systems often dry the air and increase the amount of water drawn from indoor plants. This can be damaging. Unfortunately, placing your plants on a tray of gravel filled with water will not cut it. Adding a humidifier in your plant room may be a solution if you are growing humidity loving plants, e.g., ferns, many orchids, Pipers, Philodendrons, Peperomias, Begonias etc. Another way around this is to cultivate humidity loving plants in terrariums (see blog on terrariums). If enough tolerant plants are grown in a room they will also increase the humidity in the room through transpiration. So add more plants! Always a good idea right?

Do some research on the house plant you want to purchase before you bring it home to find out where it grows in nature. This will give you hints on its needs. Also know when not to buy a plant, some are just too touchy without controlled environments. If you are new to house plants start off with the easy ones (Pothos, Syngonium, African violets and some Begonias). As you gain confidence move to more difficult or interesting types.

This Balfour Aralia is in trouble. Research shows that it prefers temperatures not less than 60F. It was in an unheated plant room that was seeing low temperatures of 45F. It also needs to be moved into new media and out of the nursery container.

Realize How Plants Are Produced

When you purchase a new plant it is already an old plant, production wise. The grower had a target size or bloom requirement and once the plant has achieved that it is sent out to the retail market. Often a plant has consumed its nutrient charge in the medium it may have filled that medium with roots and the medium may have begun the process of breaking down.

It is good to get plants as fresh from the grower as possible because once they enter the box store or even nursery or plant shoppe, they begin to be affected by the stress of being outside their cultivated environment. Retailers may skip irrigating or keep plants too wet, the air may be dry and the light levels not adequate to sustain growth. The longer plants are in poor growing conditions, the less able you will be to keep them going. Even when your plants look good they may be suffering already and need several things from you such as a soaking irrigation with pure water, fertilization, light, and new media.

First Things First: Repot

Remove your new toy from its container and inspect the roots. They should look white and healthy and should be throughout the medium. If they do not look this way take the plant back. Do not introduced diseased plants to your plant collection. Assuming everything looks good find a suitable container that has drain holes (very important: never use containers w/o drainage holes) larger than the one your plant came home in. Add container medium that you know grows plants well for you or make your own medium from known ingredients. Make sure your medium has a nutrient charge like a slow release coated fertilizer or add small amounts whenever you water your plants.

Next: Irrigate

Growers cultivate amazing house plants because they carefully control the purity of the water they irrigate with by using reverse osmosis filtration. Then they add back the nutrients the plants will need. Harmful salts like sodium, chloride, fluoride, and others are excluded or kept very low in concentration. After your plant is repotted saturate the soil with either distilled water or reverse osmosis purified water. Take careful note of how heavy the container is after irrigation and after all excess water has left the container. This is the weight of a just watered plant. As the plant dries out it will get lighter, simply pick up the plant to tell how dry it is. This is an art that works with smaller containers as well. You don’t have to get your fingers dirty to assess growing medium moisture. On large containers you can tip them to tell how heavy they are. You can also use moisture meters but they are often very inaccurate and will give variable results depending on the level of fertilizer salts in the medium. A last resort is to stick your finger in the pot. Very picky plants like the prayer plant are sensitive to salts in water and excess fertilizer, they will do best with distilled water with a very small amount of soluble fertilizer added back (1/8 of the recommended amount). In some areas tap water is very low salinity and grows good plants, in others it needs reverse osmosis filtration or buy it from the store.

Lifting a container after watering gives you an idea how much a well hydrated plant weighs. As it dries it will get lighter and you will know when to water it again.

Finally: Location

Now that your new plant is settled in its container, its time to place it in a location where it will get adequate light. Some plants are from dark areas of the forest floor (ferns, begonia etc) and do best with north window light. Others require strong window light (Ficus, many bromeliads, and some succulents). It is also possible to grow plants under lights such as LED’s with great success. This is where your research into plant origins will be helpful. Try to irrigate with high quality water and dilute fertilizer when the plant’s media begins to lighten.

The ficus and coleus are in bright light in the garden room. The Syngonium is off and on closer inspection an infestation of scale was found.. It will be treated and moved.

A Note About Pests

As your collection grows there is no doubt that pests may become a problem. They come in on new acquisitions or seem to appear on their own without explanation. Many times new plants are quarantined before joining others. In any case it is best to watch for the common culprits of scale, mealy bugs, and occasionally aphids. If pests are detected there are many remedies available at nurseries. Make sure you identify your pest (Cooperative Extension offices can help) and that the pest is on the label of any product you chose to control it with. Take your plant outside to treat it and let the spray dry before bringing it back in. Quarantine treated plants away from others until you are sure the pest is controlled. Always follow pesticide label instructions precisely. Monitoring for pests can catch an infestation before it gets bad.

Simply paying attention to your plants, occasionally slipping off the container to look at the roots, re-potting when needed and keeping the fertility levels up will ensure good growth and performance. At some point you can move on to propagation of your favorites.

Something must eventually be done. A plant can’t live on wax alone.

Reference

Faber, B., J. Downer and L.Yates. 1993. Inexpensive, hand-held moisture meters. HortTechnology 3:195-196

Pruning Mature Shade Trees

Large trees bear the burden of their insults over the years. From injury to pruning wounds these assaults add up. Keep in mind mature trees need all their leaves to sustain their energy balance. This tree needs little or no pruning at this time.

Very large and old trees don’t often attain size and splendor free of defect and disease. A mature tree may have accumulated some damage, injury or disease along its path to grandeur. It can contain hazardous branch attachments that have not failed yet, large sections of dead wood or it may bear the burden of an assemblage of cables and other devices installed over the years to sustain it. Very old mature trees have endured perhaps multiple human lifetimes of exposure to storms, droughts, pests and, perhaps worst, early pruning practices. Trees gain age and majesty when they don’t fall down or fail multiple large branches. To do this trees defend their wood biochemically from invading fungi. Trees also cover over decayed wood with new wood that is fairly impervious to the old infection. High vigor trees with adequate resources (water, soil, sunlight) should be able to contain decay that would shorten their lives otherwise.

Chart from Shigo (2003). P is % of either living (L) or dead (D) wood pruned. Y is a young tree, M is a mid age tree and OM is an over-mature tree. A is age of the tree. So as trees age we should remove less living and more dead tissues.

Dr. Alex Shigo (2003) devised a chart showing the amount of pruning over a tree’s life time. Young trees have all living cells and most of the wood is sapwood, therefore the trimmings in young trees are of living tissues. As trees grow older and shade themselves heartwood develops in the canopy. It’s not living, doesn’t conduct water and minerals but does provide support to branches and strength to the canopy. Heartwood is also more resistant to decay than sapwood. The pruning of a mature tree removes a combination of living and dead cells. An over mature or ancient tree has much more dead cell tissue and only deadwood should be removed with an attempt made to preserve all stems with leaves.

This tree was severely headed back a few years ago and did not have enough stored energy to regrow new scaffold branches. The tree is now dead.
This tree was “topped” by an unprofessional trimmer who left 4-5 foot bark peels behind on the remaining leaders. All the shoots are epicormic (an epicormic shoot grows from an epicormic bud which lies underneath the bark of a trunk, stem or branch of a plant) and will become poorly attached branches which may fail if not kept reduced by further pruning. This sort of pruning creates a need for more pruning to correct the bad branch faults.

Large trees have large branches and the temptation is to remove large branches to achieve pruning objectives. This would get more done and we would then have to prune less, right? But this is a flawed strategy and the results are the opposite of what the tree owners are trying to achieve. As trees get large, pruning cuts should be smaller and occur frequently. Guidance can be given to branch growth by many small cuts rather than large ones. This approach directs growth without causing rampant regrowth of dormant or latent buds. Latent buds have a purpose, they rescue trees from death should there be a catastrophe. They are preformed along large branches and will sprout when exposed to sunlight or when the terminal buds are removed. Large topping or heading cuts typically activate latent buds and the tree subsequently grows back too many branches in one place. These require selection and careful removal over time and can involve quite a bit of repair pruning to fix the tangle cause by massive heading cuts. Aging trees develop an extensive heartwood that is easily exposed to decay from large pruning wounds so large cuts should be avoided.

Large trees develop decay and cavities. This oak pictured above also has active Ganoderma spp. fruiting bodies indicating that decay is active in the tree (white spot in the mid trunk just to the right of the sign). Presence of decay does not mandate pruning or removal but rather monitoring and assessment for risk for the tree in its environment.

Larger trees develop more sapwood which needs energy for respiration relative to the amount of leaves in the canopy. The first duty of the leaves is to provide energy to keep all the cells in the young phloem and sapwood metabolically active. A typical tree only utilizes a few growth rings of sapwood for water transport and energy storage. As these rings age they narrow and there is less tissue from which to store and retrieve energy. Despite the subtle balance between canopy and energy utilization trees continue to grow throughout their lives at more or less the same rate. As trees age the amount of non-functional wood increases, this may be heartwood or deadwood in the canopy. At the end of their lives trees have nearly no living sapwood and the energy of the tree is mostly depleted. Over pruning causes further energy depletion. Removing leaves and buds that give rise to leaves from old trees reduces the amount of energy stored. This forces the tree to utilize sapwood reserves to recover from the pruning losses of the photosynthetic part of the tree. Severe pruning in old trees ultimately depletes them of stored sugars and leaves them without energy to make defensive compounds. This can exacerbate conditions by allowing disease or decay organisms to enter the sapwood.

This Liquidambar was headed and never grew back sufficiently. It suffered with bacterial leaf scorch and yet is still alive. Note the acorn wood pecker habitat on the right leader. The tree is not likely to fail and is providing habitat for small owls. While it is not dignified or healthy it still has value in the landscape.

What if your large tree was never structurally pruned and has branch faults or you have concerns for failure? You should always consult a certified arborist with training in Tree Risk Assessment. The health of the tree is important but the safety of those living around it is more important. Sometimes targeted pruning can be used to shift the balance of growth in a tree and strengthen branch attachments. This can take years of careful pruning with small or modest cuts. When large branches must be removed they should be left as long as feasible and the cut should be circular not an oval shape to decrease exposed surface area. Large pruning wounds should be limited to the fewest number required to achieve the pruning goal. Pruning paints and wound dressings are not recommended.

Large trees accumulate deadwood during natural senescence of inner branches or due to injury, disease or insect attack. Common practice has suggested removing deadwood from old trees but as arborists become more sensitive to urban ecology issues we realize that deadwood is prime habitat for cavity nesting birds and other life. Deadwood serves a purpose and, if it does not pose a threat to those living under or around the tree, it should be retained especially if evidence of utilization is present.

Caring for large trees is tedious because there is a lot at stake. Old trees are valuable and provide many benefits to their owners. Always seek expert opinions and be willing to pay for them, preferably from the arborist who does not do the pruning. Then act on those opinions with care and consideration and always closely supervise any pruning being done to your trees.
Use the following pointers from Gillman for mature tree pruning strategies:
-Evaluate your tree for health, strong branch structure and evidence of decay
-Remove dead branches as needed but retain some for wildlife if needed
-Perform structural pruning when feasible
-Shorten or remove branches with cracks swellings or deformities
-Remove parasites such as mistletoe or canker infested branches
-Retain as much live tissue as possible while achieving pruning objectives
-Keep cuts small (2inches) or moderate (4″) if the tree is a good decay compartmentalizer

References:
Gillman, E.F. 2012. An Illustrated Guide to Pruning 3rd ed.
Delman Cengage Learning, Clifton Park, NY.
Manion, P.D. 1991. Tree Disease Concepts. 2nd ed. Prentice-Hall, Englewoood Cliffs, N.J.
Shigo, A.L. 2003. Modern Arboriculture A systems approach to the care of trees and their associates. Shigo and Trees Associates Durham, NH. 3rd ed.