Out with the old, . . .

There are two types of horticultural sunburn. Sun scald is what happens to formerly shaded bark if it suddenly becomes too exposed. (As I mentioned in my gardening column for this week, which posted last Monday here, sun scald that occurs during winter in colder climates is caused more by frost than by sunlight.) Scorch is what happens to overly exposed sensitive foliage.

The picture above is an example of scorch on a significant scale. The bigleaf maple was formerly shaded by a big Douglas fir that fell last May, leaving the maple both severely disfigured and very exposed. Such exposure would not have been a problem it the foliage had always been so exposed. The problem was that it developed in shade, so could not adapt to the new exposure.

The exposed foliage survived for a while, but eventually succumbed to warmth, sunlight and aridity (minimal humidity). Deterioration accelerated as the weather became warmer in just the last several days. Because the foliage scorched rather than succumbed to cooling autumn weather as it would have done a few months later, it remains attached to the stems that it grew on.

That is actually an unsightly advantage for the bark of the stems that are shaded below. If the bark suddenly became exposed too, it would be susceptible to sun scald. This tree knows what it is doing. Deteriorated foliage that does not get dislodged by later winter weather will be dislodged as new foliage develops next spring. The new foliage will be adapted to the new exposure.

The picture below shows how new grow that is adapted to the new exposure develops adventitiously from the exposed trunk. It does more than just exploit the increase of sunlight. Ideally, it shades suddenly exposed bark to protect it from sun scald.

. . . and in with the new.

Sun Scald Happens Here Too

Sun scald ruins otherwise good stems.

Those who enjoy gardening where winter weather is harsh likely know what sun scald is. It happens late in winter, if exposed bark warms enough to prematurely resume vascular activity during the day. Vascularly active tissue then succumbs to hard frost at night. Wintry sunlight is not sufficiently intense to scald bark; but the damage suggests otherwise. Glare from snow enhances exposure.

Of course, without hard frost or snow, this sort of sun scald is not a concern here. However, there is another sort of sun scald that happens during the warmth of summer. It truly is scald, caused by exposure to sunlight that is sufficiently intense to literally cook vascular tissue just below thin bark. Although induced by opposite extremes of seasonal weather, the damage is remarkably similar.

Since even deciduous trees are foliated during summer, most bark is safe from summertime sun scald. Bark becomes more exposed and susceptible if deprived of some of what shades it. That can easily happen if aggressive pruning diminishes the foliar canopy above. Removal of a nearby tree also eliminates significant shade. Painting an adjacent wall a light color can enhance glare.

White paint applied to the trunks of susceptible orchard trees reflects most of the damaging sunlight, but is too unsightly for landscape trees. Stubble of small twiggy stems can shade the trunks of some young trees until their canopies are broad enough to provide shade. Sun scald typically develops on southwestern and upper exposures that are more exposed to the most intense sunlight.

Maple, oak, ash, birch, flowering cherry, flowering crabapple, English walnut and almost all deciduous fruit trees are innately susceptible to sun scald. The interior stems of privet, holly and English laurel are more resilient to sun scald if exposed by major pruning during late winter rather than during summer. Although, with only a few exceptions, any thin bark can be susceptible to sun scald.

Foliage of many plants can be damaged by enhanced exposure too, but that is known as ‘scorch’, and is another topic.

No Cherry On Top

P90811Weeping flowering cherry is another type of tree that almost never gets appreciated like it should. Like so many Japanese maples, they get planted into situation where so-called ‘gardeners’ shear them into nondescript globs of worthless foliage that only get in the way. Some get shorn so regularly that they are deprived of bloom. Their form and bloom are their two main assets.

The climate here is not easy on them either. Although comfortably mild, the climate is also arid. This aridity enhances the potential for sun scald of exposed bark. Because upper limbs bend over to hang back downward, their bark is more exposed than that of upright flowering cherries. Consequently, upper limbs are often scalded and ruined, disfiguring the remaining canopy.

Pruning can be complicated. Removal of scalded upper growth exposes inner growth that is more sensitive to scald. It is sometimes necessary to leave damaged upper growth until it gets replaced from below by newer growth. Regular pruning to remove as much of the superfluous lower growth as possible should stimulate more vigorous growth among the stems that remain.

This is really the best technique for preventing scald among upper growth. It may sound silly, but pruning from below to concentrate growth into fewer stems that extend from or through the top of the canopy keeps the top of the canopy healthy and resilient to scald. It also elevates the pendulous canopy that needs to be pruned very regularly for vertical clearance anyway.

It is not easy to see in this picture, but this small weeping cherry tree is developing two distinct canopies. The original upper canopy was damaged by scald and is now disfigured. It is mostly to the upper left of center of the picture. A more symmetrical inner canopy that is developing where superfluous inner growth was pruned out last winter is evident below and to the right.

If there were not a walkway so close to the tree, and I were not concerned with maintaining vertical clearance, I could prune the upper canopy back over a few years, and subordinate it to the healthier lower canopy as it grows through it. Instead, I will prune out most of the inner canopy, leaving only a few vigorous stems that can replace what is missing in the upper canopy.

There is no need to be concerned with it now. The lower stems are not too obtrusive yet. They can bloom next spring, and get pruned out next summer before they really do get obtrusive.

The Wrong Time For Pruning

80801thumbNot many plants are sensitive to mere heat alone. Actually, many plants prefer warm weather. The difficulty that some plants have with heat locally is that it typically accompanies aridity, and often accompanies afternoon breezes. As appealing as breezes and minimal humidity are to us while the weather is warm, they promote and accelerate desiccation of exposed sensitive foliage.

Pruning, which obviously becomes necessary while warm weather promotes growth, can make plants more sensitive to damage caused by warm, sunny, arid and perhaps breezy weather. It exposes formerly sheltered stems and inner foliage, which are more sensitive than outer foliage is, to more sunlight and drying breezes. Exposed foliage can either desiccate or roast, or both!

A bit of unsightly but relatively minor foliar damage on the extremities of the outer canopy might be only superficial, but major damage can be dangerous. Superficial damage often gets replaced by fresh new growth before it deteriorates enough to expose more foliage and stems below. However, recovery from major damage can be delayed by the distress associated with the damage.

Japanese maple, aralia, philodendron, rhododendron and all sorts of ferns can easily get damaged by increased exposure. Low ferns are not likely to become too exposed by any loss of their own foliage, but often become more exposed by the pruning of plants above them. Like frost damage, foliar scorch might need to be left to shelter remaining foliage until new growth develops.

The bark of many plants, although not susceptible to desiccation, is very sensitive to sun-scald if too exposed. Young and smooth bark is the most sensitive, particularly if it had always been shaded. Scald kills bark and the vascular tissue below. As it decays, it exposes interior wood to more decay that is likely to compromise the structural integrity of the affected stems and trunks.

Pruning during relatively cool weather and while there are a few relatively cool days in the forecast allows foliage a bit of time to adapt to a new exposure before the weather gets dangerous. Through summer, pruning should not be so aggressive that too much sensitive foliage or bark are exposed, even if it is necessary to leave a bit of unwanted sloppy growth to partly shade bark. Aggressive pruning of exposed and sensitive plants should be delayed until autumn, when sunlight is not so intense, and weather is cooler and wetter.

Summer Weather Can Scorch Foliage

80801thumbSevere summer weather is something that we think that we do not need to contend with. It only rarely gets as unbearably hot here as it does elsewhere, and when it does, it usually gets breezy by evening, and somewhat cooler overnight. Aridity, or the lack of humidity, is another advantage, at least for us. The plants in our gardens are affected by warm weather very differently than we are.

Plants will tolerate significantly more warmth than we will, but only in conjunction with humidity. In our climate, we get one or the other, but not often both. In fact, humid warmth is so rare here, that when it happens, it causes spontaneous limb failure in trees that are not accustomed to it. Spontaneous limb failure occurs as vascular activity accelerated by warmth increases foliar weight, but humidity inhibits evapotranspiration (evaporation of moisture from foliar surfaces) that would decrease the weight.

The aridity and breezes that make warmth more comfortable for us accelerate evapotranspiration, which increases the need for moisture. Plants that lack adequate moisture wilt, and the foliage of some can get dehydrated or scorched. Wilted plants recover if watered soon enough. Dehydrated foliage is crispy and can not recover. Severe dehydration kills buds, stems and entire plants.

Scorch is quite different from dehydration. It happens as overly exposed foliage literally gets cooked by sunlight. It is similar to sun scald on formerly shaded bark that gets cooked by sunlight after being exposed by pruning or other means of removal of adjacent vegetation. Scorch is more likely on inner foliage that had been recently exposed by pruning, or foliage near reflective surfaces.

Foliage can not recover from scorch. Damage is permanent, and should not even be pruned away. Just like foliage damaged by frost, outer foliage damaged by scorch shelters the inner foliage. Removal of damaged foliage exposes foliage behind it to subsequent damage. Besides, scorch typically damages only parts of individual leaves, so that undamaged parts continue to function.