Sunburn

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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.

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. . . and in with the new.

Sun Scald Happens Here Too

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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.