Sunburn

P00705-1
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.

P00705-2
. . . and in with the new.

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.