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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.

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