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


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!


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!


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


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


…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.


…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.


—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]


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.


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.


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

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.

Pruning established trees

This month I continue the series on pruning with a look at pruning established landscape trees.  These are trees in the prime of life, growing well, starting to shade the garden beneath them and expanding their canopies. Various reasons can prompt the call for tree care professionals.

What reasons would we have to prune a healthy vigorous mid-aged tree? For those we have to examine what may have happened in the past. The fact that a tree is growing well does not always mean it was “selected” well. After a few years time, that cute little nursery tree is flexing its branches and spreading out and, more worryingly, upward. One of the prime reasons for pruning is to reduce the size or expansion of a tree canopy. There may be impending interference with power transmission or other utility lines. The tree may be blocking a view, it may just be frightening in its shear mass or size and pose a psychological threat to its owner. Size reduction is a frequent object of tree pruning operations.

The tree is too large for my comfort

If you find that you want to reduce size of a tree you should ask yourself is it possible and is it sustainable? Ultimately, do I have the right tree for this spot?
Size reduction pruning is a battle with tree genetics. The tree wants to achieve a designated height and the tree owner wants to limit that height. Terminal or leaders can be pruned back to a lateral branch to reduce the length of stems and branches. This kind of “thinning” is effective as long as the branch that a leader is trimmed to is large enough to resume the hormonal role for that portion of the tree. Arborists have a 1/3 rule that suggests the branch you trim back to should be at least 1/3 the diameter of the stem it is attached to. That is, you don’t trim back to a twig otherwise it is a heading cut and re-sprouting will be abundant and require more pruning. The problem with the 1/3 rule is that trees don’t respond consistently to pruning on this basis. Some will re-sprout at the cut, others will not. In any event if you embark on size reduction pruning you will likely be maintaining that strategy over the life of the tree and this may not be sustainable. Removal and replanting with a smaller tree species that can be cultivated with little or no pruning would be a more sustainable approach.

Deadwood, I see Deadwood

Deadwood, especially large stems provides habitat for woodpeckers and other cavity nesting birds.

Deadwood is not the end of a tree or necessarily a reason to call the tree trimmer. Deadwood can serve as habitat for cavity nesting birds and is an important part of tree ecology. But dead branches that hang down and have broken from the canopy pose hazards and should be carefully removed. A dead leader can be pruned back and retained especially if woodpeckers have excavated nests that other cavity nesting birds, such as owls, will subsequently use. Of course if the deadwood is part of the active pathology of an ongoing tree disease, its removal may be warranted to prevent disease progress. This diagnosis would involve consultation with your state extension specialist or agent and or qualified arborist with training in tree diseases.

I have a tree with uncorrected branch faults

Codominant stems are likely to fail. The bulge and crack indicate the presence of included bark.
Codominant stems often develop included bark (dark area) and later fail as the stems push each other apart.

Perhaps after careful study you realize your tree has co-dominant stems or too many branches coming from one place on the main stem. Both are important branch faults that precede branch failures. Co-dominant stems look like a “slingshot for giants”, two big stems both the same size. Often they are accompanied by included bark that separates the stems. The problem is often that when the stems are large, pruning one will leave a big wound that can let in decay and decrease the life of the tree. If a codominant is 6 inches or larger, it may be best to not remove it. Or remove the entire tree thus fixing the issue. If you want to keep the tree you should choose one stem to thin and prune rather aggressively (not topped) and leave the other stem unpruned. Over time the thinned out stem will grow slower and the unpruned one faster and the codominance will decline. This strategy will require year over year pruning to achieve your goal of shifting the codominants.

All branches from one point. Note large branch that failed at the attachment. This tree is too large to sustain. It could have crown reduction to prevent more failures and reducing the end weights of other branches. But, given the situation, it would be best to completely remove it.
The same tree as above. Note large branches from the same point on the main stem. Instead of crown reduction, “Lion’s tailing” pruning was conducted as the canopy was cleaned on the inside. This will force more weight to the ends of branches and stress at their attachments, hastening failures.

If the problem is multiple branches coming from one point the correction is far more difficult. Removing branches from a cluster again provides decay entry and potentially can weaken the attachments of the other branches. Large trees (especially if a branch has already failed) with multiple attachments at the same point are likely to have a failure. The only correction for some cases is tree removal. If there is no or low risk to people or property the tree can remain as such and or pole braces installed to prevent failure. Again, failure likelihood depends greatly on the species. Elms, Oaks, Acacia, Carob, etc, all are at risk of rapid failure.

What if my tree is storm or wind damaged and needs repair?

Storm damaged trees require pruning to repair broken branches and may require several years of pruning cycles to recover a stable crown architecture.

The occurrence of severe weather events is on the rise. Intense winds, increased rain, high temperatures are all commonplace now. Trees that withstood the elements in the past can now be damaged in these storms. In every case a professional opinion on the salvage of storm damaged trees is important. Expectations for restoring an acceptable crown that is safe for the site and those that use it is highly variable. A qualified consulting arborist can guide you in working with storm damaged trees or providing an opinion on removal vs restoration. It is best to contract an arborist who has no interest in a trimming/removal operation since they do not stand to gain from an easy and profitable removal job.

I realize my tree was harmed by previous tree trimming and I want to make it better

This pruning job did little to correct branch faults (too many branches from one point), left stubs and injuries on the main stem. It will take additional pruning over a period of years to correct this damage. Because the tree is young it can still be trained effectively.

Much like the other scenarios, growth patterns and targeted branch work can fix or resolve past insults to trees from inferior pruning . However, don’t expect to resolve all the issues in a single pruning. Often it takes years of careful work to restore the canopy of an abused tree.

I just want to have my tree thinned

Thinning trees is no longer considered a legitimate pruning objective. The recent trend of thinning Canary Island Pines in Southern California is unnecessary and destructive.

If your goal is to get more light into the garden and the tree is a variety that responds well to thinning, this is great. If you don’t have a reason then this is not a legitimate pruning need. Trees don’t need to be thinned and thinning is actually detrimental to some oak species, depriving them of the inner canopy of leaves that they rely upon. Thinning slows the overall growth of trees and reduced stored carbohydrates. Extreme thinning on a regular basis can predispose trees to some fungal pathogens.

At the end of the day….

Mid size, mid life trees require no pruning. People may have reasons to prune when the tree is too large for the site, has been damaged or needs further training to develop its architecture. Trees don’t require pruning if they grow according to their genetic program. Pruning the maturing tree really seeks to undo damage or correct issues that arise in its culture. Branch faults and tree branching structure may not be apparent to every gardener so seeking a professional opinion is always worth the money if you want to be sure of your tree’s pruning needs.

Pruning newly planted trees

As the climate warms the value of trees for cooling the environment around buildings, especially in cities, drives tree planting programs. Planting trees is just the first step in growing a tree in a sustainable landscape. Successful plantings require evaluation and guidance of the new tree’s current and future branch architecture. In almost every case, nursery grown trees will require some structural pruning so that a shade tree can develop strong and effective branch attachments that will support the canopy for the coming decades without failure. In this blog I cover maintenance of the newly planted tree including how to structurally prune young trees so that they develop strong and sustainable canopies.

As mentioned in earlier pruning blogs, trees do not require pruning. This is predicated on the assumption that trees are allowed to grow in the way they are genetically programmed to grow without damage. Unfortunately many container nurseries prune trees with a heading cut to the central leader in order to create branches that can further be pruned to make a “lollipop” canopy that mimics the form of a large tree. Consumers have become accustomed to this “in-pot” miniature version of a shade tree and nurseries are accustomed to producing them. Low branches are removed to enhance the tree lollipop shape. Nurseries often stake trees tightly to provide a way to keep them from being blown over in wind events and since all the temporary branches are removed from the low trunk they are top heavy and require rigid staking usually with a stake taped to the trunk. Tightly staked trees grow taller than unstaked trees and their trunks may lack caliper or taper (increase in trunk diameter lower on the stem). This requires that when these trees are planted out that they continue to be staked, otherwise they would fall over. This creates another burden in getting the newly planted landscape tree to survive—helping trees stand on their own.

This newly planted coast live oak complete with gator bag for water retains the nursery stake which should have been removed and has two other stakes because it does not have enough taper to stand on its own. There are no temporary branches low down and it has been “lolipopped” during nursery production. Branch faults such as “all branches from the same point” will certainly develop if it is not structurally pruned.
Crape myrtle is notorious for lacking taper when tightly staked during nursery production. this tree retains the nursery tape and stake and has the classic lolipop shape that will require structural pruning to correct.

Nursery pruning creates two kinds of branch faults that if left in the tree canopy will lead to failure later. These result from heading the main leader of the young tree. When buds grow from the pruned tree, they often produce too many branches from the same place or two branches or new leaders that are the same size. We call these faults: too many branches from one point and codominant stems respectively. If the nursery tree retains these branches and they are allowed to mature in the landscape tree, one or more branches may break loose. Almost all structural pruning seeks to correct these faults at some point in the life of a nursery-grown landscape tree. The approaches are different depending on how long the branch fault is left in the tree after planting. Branch faults of newly planted trees are best corrected in the first year–they are easy to correct in the first few years and problematic after that. This is because when poorly attached branches grow well and attain greater size over time, they will pose a problem upon removal as pruning will leave behind a substantial wound which provides an entry point for wood decay. Structural pruning is best done in the nursery or if in the landscape, in the first year after planting.

This young oak retains the nursery stake even after several years post planting. The lolipop shape is indicative of inherent branch faults that have not been corrected

There are several goals of early pruning (1-3 years post planting):
-Retain temporary branches on the stem to assist trunk growth (but keep them pruned)
-Remove competing leaders (remove a co-dominant stem)
– Thin clusters of branches (fix the all branches from one point fault)
-Leave the first permanent branch unpruned
-Subordinate all other branches to “temporary” status by heading them back
– Leave unpruned branches along the stem that will take a permanent place in the crown of the tree.
-Leave enough space between permanent branches to support their sustained growth over the life of a tree
-Permanent branches should be spaced vertically and helically around the main or central leader

Most trees will do all of this without any pruning if they are unpruned from the seedling stage. They will shade out their temporary branches and permanent large branches will form strong attachments and uniform spacings. Heading cuts on young trees destroy their form and this should be avoided. In the next blog I will cover pruning young to mature trees.

Pruning Basics

As we head into Fall garden routines and leaves start to turn color, the smell and feel of the Fall weather is in the air. Winter is just around the corner and with those horticultural routines comes the urge to prune stuff . Both fruit producing and shade producing trees often get a hair cut during fall and winter months, herbaceous perennials are often cut back in the fall after bloom and before their winter rest so it seems a good time to blog about pruning before you get the urge!   After years of pruning demonstrations for Master Gardeners and the public I have noted a common thread in how gardeners think about pruning. Pruning is a mysterious process. How we take that tangled mess of a plant (tree) and fix it? What do we prune? And the less frequently asked question: What do we not prune? To add confusion, some plants such as roses seem to have their own pruning “culture”.  In this blog post I will cover the basic principles that apply to pruning all plants and then expand into specifics in upcoming blogs.

“Lion tailing” is a form of pruning that removes branches from the interior of a tree leaving tufts of foliage at the ends of branches. This kind of pruning is destructive to oak trees as it lets too much light permeate the crown of the tree

Plants don’t want to be pruned!
The first point is that no plant wants to be pruned. Gardeners prune plants because they think it is necessary for horticultural, aesthetic or safety purposes. Gardeners should temper their pruning by understanding plant responses that result from pruning. Generally plants don’t respond much when dead portions are pruned away. In some cases removing large dead portions of a plant will allow more light to enter and some response can occur, e.g., damage to portions of a plant not used to such sunlight intensity. So even a dead plant part may be doing something that you don’t understand. Also dead wood or dead plant parts may be part of some other organism’s home. Owls and other birds nest in cavities, some kinds of bumble bees will reproduce in old flower stalks of desert plants such as Nolina, etc. So dead plant parts are not always useless. If you are pretty sure that nobody else is using dead material go ahead and remove it if bothers your garden aesthetic.

This Ancient sycamore is falling apart with dead wood as younger stems continue to grow. The deadwood provides habitat for animals and in this location poses no risk to people so there is little reason to remove it.

There are two physiological responses to pruning

The two principles of pruning can be used to train plants and in the case of trees, produce a strong architecture that will not easily fail (drop branches). To achieve pruning goals two kinds of pruning cuts are used: the heading cut and the thinning cut. Heading cuts are often made in the middle of stems and do not have a branch that can take over the terminal role of the removed portion. Heading cuts are often used to reduce size or volume of plants. Thinning cuts remove branches at their origin. If thinning cuts are not too large and don’t allow excessive light into a canopy the plant will not respond by invigorating buds. Thinning cuts are used to maintain the natural form of a plant but can be overdone. Over-thinning results in plants that have so much light now entering that buds are invigorated and new shoot form in overwhelming and unnatural locations just as when heading cuts are made. Excessive use of thinning cuts can also produce trees that are “lion-tailed” where all the leaves occur at the end of Pom Pom branches. Remember from a tree or shrub point of view they don’t need or want to be pruned.

A thinning cut removes a branch at its attachment.
A heading cut removes a branch or stem without a side branch to assume its dominant role in the plant

Back to plant responses. There are two responses that most plants have to pruning. When living portions are pruned the remaining portions are then invigorated. This implies that dormant or “latent” buds will grow that would not ordinarily grow so plants will produce flowers or foliage in new places. In this way we can re-direct the growth of plants to achieve pruning goals we may have. This is how we can pleach a tree to grow flat along a wall or produce topiary shapes with shrubs. These kinds of pruning that dramatically alter form of a plant will require successive and significant pruning to maintain the altered form or shape. Not all plants can tolerate this and even those that do can be subject to sunburn or other processes that cause them injury. The second common response to pruning is that the more a plant is pruned, the less it will grow—pruning is a growth reducing practice. Even though buds are invigorated through pruning they can’t make up for the lost leaves and buds taken away without utilizing stored energy. The overall effect of having leaves removed is to slow the growth of the entire plant. Pruning when used as high art results in Bonsai plants that are really stunted individuals with highly stylized forms.

These plane trees along lake Como in Italy have been pollarded to dwarf them. Removing branches each year stunts the tree and limits its growth in a sustainable way. Pollarding is a style of pruning that requires continued removal of branches each year.

Pruning devigorates plants

Since pruning removes leaves and buds (which make more leaves) it is a devigorating process. You are taking away a plant’s ability to harvest light energy and convert carbon dioxide and water to sugar. All this happens in leaves. The fewer leaves a plant has the less sugar it can accumulate and then the less work it can do in terms of growing. On old or slow growing plants pruning removes energy needed for growth and also the energy needed to make secondary metabolites or chemicals which fight insect and pathogen attacks. This is why old trees pruned hard often died not soon after or become susceptible to pathogens they may have been able to fight before the pruning happened. Whenever you prune something think about how you are taking away photosynthate and what it might mean to the plant.

Fruit Trees and Roses

We have to prune fruit trees to make them fruitful? NO. Fruit trees produce lots of fruit when they are not pruned. The goal of pruning fruit trees is to modify trees so fruit is:
• in an easy to pick location,
• so there is less of it
• and so the fruit that forms is of higher quality.
An unpruned tree will make the most fruit but it may not be the quality or size you desire or where you want it in terms of picking height.

The same goes for roses. There are many pruning schemes for roses, but the most flowers will be found on the least pruned shrubs. Flower size is mostly determined by genetics. Shrubs that are severely pruned will have fewer flowers than their unpruned counterparts.

Roses have may pruning paradigms but the basic rules of pruning apply the more you prune it the less it will grow. The less you prune it the more flowers you will have.

Pruning and Disease

Pruning to remove diseased parts is often cited as a common garden practice. With some diseases like cankers and blights it is a good idea to prune out infected portions before they make spores or other inoculum to further infect the rest of the plant. In most cases it is important to prune well beyond the diseased portion so all of an infection is removed. Some diseases are “systemic” such as wilt diseases and while pruning will remove a dying portion it will not rid the plant of the infection. It is always best to identify the cause of disease even before pruning it from the plant.  As we will learn in an upcoming blog I rarely recommend sterilizing your pruning equipment with disinfectants.  A stiff brush and water is all that is needed when removing most diseased plant parts.

Pruning is a useful tool for gardeners. To get the most from the practice it should be conducted with knowledge of the effects it will have on the plant that is being pruned. This is quite variable and in some cases pruning is really contraindicated. While some plants like herbaceous perennials will be pruned to the ground either by the gardener or by frost, others maintain above ground architecture and pruning choices make permanent impact to many woody plants. In the next blog I will write about pruning young trees to create strong structure.

This Maten tree (Maytenus boaria) has a canker disease. A good reason to prune out branches, but in this case pruning may have been delayed too long as the tree will be quite disfigured after removing all the affected branches.


Downer, J., Uchida, J.Y., Elliot, M., and D.R. Hodel. 2009. Lethal palm diseases common in the United States. HortTechnology: 19:710-716.

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

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