The Garden Professors™

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.

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.

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.

Do you wish you had a crystal ball that could tell you what the climate will be next year when you plan your garden? So do many other gardeners (and climatologists). But while there is no magic answer, we do know that in many parts of the United States and other countries, year-to-year climate variability is strongly dominated by what is going on in the eastern tropical Pacific Ocean. This is through a phenomenon called “El Niño Southern Oscillation” or ENSO for short.

What is ENSO and how does it affect climate?

ENSO has three phases—a cold phase with unusually cold water in the equatorial Eastern Pacific Ocean (EPO) called “La Niña”, a warm phase with unusually warm water in the EPO, and the neutral phase that occurs between the two extreme phases. The ocean see-saws back and forth between the two opposite phases on a semi-regular pattern that usually lasts between two and five years from one El Niño to the next. Sometimes you can have two La Niña years (or even three) back-to-back (the end of 2021 is expected to be a second La Niña in a row), but you almost never have two consecutive years of El Niño.

In many parts of the world, the phase of the ENSO is highly correlated with the climate. Scientists can use that relationship to predict what the climate might be like in the coming months. That is helpful for gardeners who need to know what to expect both next season and next year for planning purposes. Not all parts of the world have a climate that is well correlated with ENSO, however, and so folks in those areas will have to depend on other methods to look ahead to next growing season. Winter has the best correlation between ENSO phase and climate, while summer is much less predictable. And every El Niño and La Niña is distinct, leading to variations from the statistical pattern we expect.

How does the temperature of the tropical Pacific Ocean affect climate in other parts of the world?

You might think that unusually warm or cold water in the equatorial Pacific Ocean would not have much impact in other parts of the world because of the distances involved, but it does. Since the atmosphere flows like a river, putting unusually warm water (El Niño) into the EPO acts like putting a rock into a stream. The flow of water (or air) shifts around the rock, changing the pattern of atmospheric winds that blow weather systems around. When we are in a warm El Niño phase, the storm track shifts south and covers the southern US, leaving the northern US warmer and drier than usual. When I lived in Wisconsin, we noted that lake ice cover in El Niño winters did not last as long as other years, which made ice fishermen like my dad unhappy. La Niña shifts the storm track in the opposite direction. Because of that, La Niña winters are colder and wetter than average in the northern US since the storm track shifts north into the Ohio River Valley and sometimes even farther. This leads to cold, damp winters in the northern US. Similar correlations, called teleconnections, are seen statistically in climate records at many places on earth.

If we know what the phase of ENSO is likely to be, that tells us what climate conditions are expected in areas where there is a teleconnection between the EPO and that region. While every El Niño and La Niña is unique, statistically they do provide guidance on what to expect in that region, and most years they are correct, although once in a while a wildcard like a Sudden Stratospheric Warming will occur and give us an occasional busted forecast, as it did in February 2021.

What do we expect this year?

Right now, we are in neutral conditions following last winter’s La Niña, but we are headed back towards another La Niña in the next couple of months (almost an 80% chance in the November through January period). That phase should last for most of the winter but is expected to return to neutral by spring.  After that, it is too far out to make a believable prediction. The Global ENSO Temperature and Precipitation Linear Regressions website provides global correlations between the ENSO phase and what kind of temperature and precipitation anomalies to expect. In it, each three-month period shows the relationship between the temperature anomaly of the EPO and other parts of the world (regression) and how strong that relationship is (correlation).

In the map below for December-February (DJF) temperature, it shows that if the EPO is unusually warm (+) in an El Niño, then the northern part of the US will also be unusually warm (+) while the southern states are cooler than normal (-). The storm track over the southern US in an El Niño year brings rain and clouds to that region, keeping conditions wet and cool due to lack of sunshine. A La Niña year is just the opposite. The strong correlation in both southern and northern states shows that it happens most of the time, but in areas with little correlation, you can’t use ENSO reliably to predict seasonal conditions. If you have a hard time interpreting these maps, the website has a tab that explains it in more detail.

The bottom line

For this coming winter, I expect warmer and drier conditions than usual in the southern tier of US states as the storm track shifts north. That means more overwintering of insect pests and diseases; an early start to the growing season is also likely. The northern US is expected to see colder and wetter conditions than usual, which means a later start to the 2022 growing season but less chance of drought next year, although fungal diseases could be bad if the damp conditions continue into spring and summer. Western Europe could see warmer conditions than usual but the correlation is weak so that is not a strong forecast. Australia is likely to be colder than normal, with a fairly high probability because the correlations are high, at least near the coasts. This should last until spring, when the La Niña ends, and we swing back into neutral conditions when other climate factors become more important. In the Southeast, the summer after a La Niña ends is also a hot and dry summer due to the lack of recharging rain over the winter, so I think we have the potential for drought in the Southeast next summer.

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.

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.

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.

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.

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.

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.

References:

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

With drought conditions or lower than average precipitation becoming more widespread across the country, it’s time to revisit the principles of xeriscape gardening. Let’s take a look at the “classic” principles and we’ll update them, Garden Professor style.
Note: If you’re growing food crops to supply your table not all of these principles will apply. Some will, e.g mulching, and some won’t. This blog post is focused on ornamental landscaping.

SO WHERE DID IT ALL BEGIN?

As an “official” landscaping technique xeriscaping seems to have begun in the early 80’s. Denver Water, the largest and oldest public water utility in Denver, Colorado, coined the term xeriscape in 1981 by combining “landscape” with the Greek prefix xero-, meaning ‘dry’. The utility then began to formally define the main principles of xeriscaping for members of the Denver community interested in modifying gardening practices to save water. The results were the Seven Principles of Xeriscaping, listed below.

THE SEVEN PRINCIPLES OF XERISCAPING
1. Sound landscape planning and design.
2. Limitation of turf/lawn to appropriate, functional areas.
3. Use of water efficient plants.
4. Efficient irrigation.
5. Soil amendments.
6. Use of mulches.
7. Appropriate landscape maintenance.

Let’s review them and apply some up-to-date gardening information.

1. “Sound landscape planning and design” – the ideal starting point for all gardens, “Right Plant, Right Place.” This principle earns a GP thumbs-up.

2. “Limitation of turf/lawn to appropriate, functional areas” – turf has a place in the landscape but perhaps not everywhere or in every landscape. “Right Plant, Right Place” (hmm, that sounds familiar). Another GP thumbs-up.

3. “Use of water efficient plants” – it may be stating the obvious but you want water efficient plants that work in your grow zone or micro-climate. Do some homework and choose plants that will be happy in your region. We’ll give this one a GP “OK” with a few points lost for being vague.

4. “Efficient irrigation” – this one has always been a puzzler. Perhaps it was included for folks who can’t break the habit of watering their gardens. The goal of xeriscaping is to have a landscape that does well on the average precipitation of an area. Granted in times of drought some plants may need a good drink now and then and new plants may need help getting established. But for the most part watering should be at a minimum and at the correct time, seasonally as well as weekly or monthly. Don’t forget to include any natural slope and drainage in your efficient irrigation plan. And “efficient” includes a correctly working automated system if you use one. This gets a GP “OK” as well.

5. “Soil amendments” – We now know that amending the soil is not a recommended practice. It interferes with drainage, causes soil subsidence and is not conducive to root growth. Plants need to be planted in native soil, whatever it may be. This one gets a big “F” for Fail and shall be removed from our list.

6. “Use of mulches” – if you’ve been following the Garden Professors blog you’ll know this is a winner. You also know that, ideally, we recommend using arborist chips but we also know that not everyone has access to them. Mulch choice also depends on the landscape site, plant choice and, in many instances, local codes. An organic mulch (but not bark) is usually the best bet, but there are times when an inorganic rock mulch is desirable. Do your homework and choose the best mulch for your situation. Mulch!
This xeriscape principal gets the GP Seal of Approval.

7. “Appropriate landscape maintenance” – too often xeriscapes are advertised as “maintenance free”; this is false. Like all landscapes and gardens xeriscapes are an artificial environment and require maintenance to thrive. Established xeriscapes will, hopefully, need less maintenance but they do need care. This can include dealing with weeds, regular inspection and maintenance of an irrigation system, and regular plant husbandry items such as pruning and clean up. This gets a GP thumbs-up.

So, based on the above discussion, here are The Garden Professor’s Principals of Xeriscape, Revised Version

THE SIX PRINCIPLES OF XERISCAPING
1. Sound landscape planning and design.
2. Limitation of turf/lawn to appropriate, functional areas.
3. Use of water efficient plants.
4. Efficient irrigation.
5. Use of mulches.
6. Appropriate landscape maintenance.

Looking over these principles we see no reason why they can’t be applied in every region and in every landscape. Learning to garden with what you have and where you are is the hallmark of a wise gardener.
Garden smarter, not harder.

In the past week, two new major climate reports have been released. One is the latest (6^th) report from the Intergovernmental Panel on Climate Change (IPCC) and the other is the State of the Climate 2020 report. Of the two, the IPCC report has garnered a lot more press, but both are compilations of work by hundreds of scientists looking at recent weather and climate patterns and how they are affecting us here on earth. The IPCC report also provides projections of what the future climate might be like, using a number of assumptions about how the earth behaves, which can be difficult, and how humans respond, which is arguably even tougher to determine. In this post, there is no way that I can cover both sets of reports in meaningful detail and I won’t address how we need to address the rapidly changing climate here, but I do want to try to pull out some things that you can use as gardeners now. {Note, the pictures are ones I have taken myself on recent trips to use as eye candy!}

What do the new reports tell us?

The State of the Climate 2020 report, published jointly by NOAA and the American Meteorological Society, focuses on global climate events that happened in 2020. You can read some of the notable findings from the report at my blog. The report also discusses many of the “big” climate events of 2020 and puts them into historical context, including how frequently these extreme events occur and how the changing climate is making them more likely.

The United Nations’ IPCC 6^th Assessment Report presents similar information but also makes more explicit the cause of the warming, which scientists have known for well over 100 years has been caused primarily by human emissions of greenhouse gases in the atmosphere that trap heat near the earth’s surface. The IPCC report makes it clear that the rapid pace of the warming will cause severe changes to the earth’s climate that will be difficult for humans and ecosystems to adapt to.

What do the conclusions of these reports mean for gardeners?

Here are some of the changes that we will have to adapt to in the future:

• Rising temperatures across the globe—Temperatures are rising across nearly all the globe, both on land and in the oceans. Warmer temperatures mean warmer winters, hotter summers, and longer growing seasons. They also mean more increases in both evaporation from water surfaces and more evapotranspiration from plants, resulting in increases in water stress. That means you may need to water more often or use other techniques like mulch to preserve soil moisture. You may also need to switch to more heat-tolerant species as the USDA plant hardiness zones shift north (in the Northern Hemisphere). It may become harder to work in the middle of the day when it is the hottest.
• Rising temperature leads to rising humidity levels, at least where there is a source of water vapor nearby. The higher humidity is contributing to higher night-time temperatures, which puts stress on animals living outdoors (pets, livestock, and wildlife) and also stresses some plant species. It can also lead to more clouds, which reduce direct sunlight and cool the air but also reduce solar radiation available for plants, slowing their development. You may have to manage your gardens for more diseases that are related to the high humidity levels.
• Some areas like the northern US may see more rain, while others like the Southwest become increasingly dry. Year-to-year variability in precipitation is also likely to increase, with both more floods and more droughts. In both cases, water management of your gardens will become increasingly important, with the heavy rain events causing more erosion and the potential for loss of plants and trees from too much water and not enough air in the soil, and the longer dry spells making gardens more dependent on either drought-tolerant species or more frequent irrigation. You may have to put in rain gardens to help slow the movement of water through your gardens in heavy rain.
• With the rising temperatures, frost and snow will become less likely but will still occur (there will still be winter!). This will allow you plant earlier than in previous decades but will still make the plants vulnerable to late-season frosts.
• Increases in carbon dioxide may provide some fertilization of some plants, but only if there is enough water available for growth. Since some weedy species are more efficient at using carbon dioxide than other plants, you may need to deal with more weeds and invasive species in the future than you do now.
• Strong storms like hurricanes and derechos may occur more often and be more damaging than the ones we are already seeing now. The research in this area is less definitive than that for rising temperatures because there are many different factors that go into storm development, but scientists generally agree that the number of hurricanes seems to be climbing upward and that the seasons are getting longer. In addition, the storms appear to be moving slower, and that is likely to lead to more rain from the storms over a specific area and more likelihood of rapid storm development. If you live in an area that is prone to strong thunderstorms or tropical cyclones, you may see them more often and the season may start earlier in the year. Rains and winds are likely to increase, leading to more tree damage and flattened plants.

Will we be able to see these changes over the next few years?

Year-to-year variations in climate will continue to plague gardeners, since whatever happened last year is unlikely to occur again this year. The climate naturally varies over time and space as well as exhibits these long-term changes. That means it can be hard to see the creeping trends in temperature and precipitation in the noise of yearly climate swings. If you are only worried about next year’s garden, what is happening in 50 years may not be of much interest. But if you care about your children’s gardens and their future on a warmer earth, than it is something these two reports make clear we have to think about and do something about.

Personal note: This week I was also invited to participate as an author on another upcoming large climate report, this one the 5^th National Climate Assessment (NCA) that focuses on changing climate in the United States. I will be one of a number of authors contributing to the chapter on the Southeast US. If you are interested in what the content of that report includes, you can view the 4^th National Climate Assessment, released in November 2018. There are chapters for each section of the country, but also chapters that deal with economic sectors like water and agriculture. The 5^th NCA will update the information in the previous version as well as add additional information based on scientific studies completed since then.

References:

The State of the Climate report in a peer-reviewed series published annually as a special supplement to the Bulletin of the American Meteorological Society. The journal makes the full report openly available online, here. NCEI’s high-level overview report is also available online, here.

Sixth Assessment Report, Climate Change 2021: The Physical Science Basis is now out.  The report addresses the most up-to-date physical understanding of the climate system and climate change, bringing together the latest advances in climate science, and combining multiple lines of evidence from paleoclimate, observations, process understanding, and global and regional climate simulations. Get more information including links to the press release and some videos here.

In this blog I continue to examine maladies caused by environmental conditions in the absence of a disease agent or insect.

Salinity
Salt in soils or water is simply the presence of too many soluble ions in the soil-water solution. This tends to happen in dry climates where evaporation rates exceed precipitation rates. In these climates salts accumulate in soil when surface waters pick up minerals from soil that is high in precipitated salts. In wetter climates water leaches salts from soil so surface waters (rivers and lakes) have fewer dissolved salts. Also, in dry climates irrigation is often a must and irrigation sources usually have high amounts of dissolved salts. In high salt environments plants must use energy to increase their own salt balance at the root interface to make uptake of fresh water through their membranes possible. This energy is thus not available for growth. Salt affected plants are often smaller, even stunted depending on salinity levels and are more susceptible to root pathogens as their roots are more likely to be “leaky” giving pathogens chemical signals of their susceptibility.  Salt damaged leaves often show “edge” necrosis or burning on the oldest leaves.

Salt affected soils should not be allowed to dry out as roots will be damaged. Leaching to dissolve salts and move them below the root zone is one approach to prevent further symptoms.

Soil compaction
Soil compaction is the increase in soil bulk density beyond a point where roots function and grow. Bulk density is a measure of soil compactness and is calculated as the weight of dry soil per volume. Optimal and harmful bulk densities vary by soil texture. Sands have higher bulk densities than loams which are higher than clays in their growing range. Normal bulk density for a sand will be a compacted value for a clay. Values above 1.1, 1.4 and 1.6 g/cm3 can be restrictive for roots growing clays, loams and sands respectively. Compacted soils of any texture restrict plant growth. Stunting, poor growth and nutrient deficiencies due to loss of root function are common.

Extremes of light
Light is necessary for photosynthesis but it is also a radiation source that can include damaging light energy when it reaches tissues that are not accustomed to it. This happens frequently on over-pruned or damaged trees, where the canopy is suddenly reduced and stem tissues receive intense sunlight. On thin or green barked trees this can cause sun scald. Apples are particularly sensitive and will develop large cankers on upper branch surfaces if too much light is allowed into the canopy during summer. Canopy loss compounds light injury because the tree is not cooling itself as efficiently with fewer leaves. Infrared energy (heat) builds up on branch surfaces and can kill underlying stem cambium layers.

Low light levels also harm plant productivity. All trees tend to lose interior branches as normal growth increases canopy density and light levels decrease in the innermost canopy. Inner branches store less and less energy and essentially die due to light starvation. The same thing can happen to entire trees if they are overgrown by vines, other trees or shaded by buildings. While canopy thinning will preserve inner branches, it is not absolutely necessary as branch dieback is a natural process in most trees.

Effects of Herbicides  Sometimes herbicides cause damage to non target plants.  This happens when herbicides are applied unknowingly, such as residues in composts, drift from off-site applications, or by choosing the wrong herbicide to use in a garden setting.  Herbicides affect plants in different ways: some only affect tissues they contact, others are systemic, and some affect seeds as pre-emergent herbicides and have activity in soil over time.  Diagnosing herbicide damage often requires sleuthing and inquiry of what has happened in the past and what materials your neighbors may be applying.  As with any pesticide, herbicides should be applied according to label instructions. 

References

Costello, L., Perry, EJ, Matheny, NP, Henry, MJ, and PM Geisel. 2003. Abiotic Disorders of landscape plants a diagnostic guide. ANR publication 3420 University of California, Communication Services, Oakland CA.

Manion, P. 1981. Tree Disease Concepts. Prentice-Hall Inc., 399pp.