Cold Injury to Woody Ornamentals

We have been deluged with phone calls and samples concerning tree problems during the last three weeks. Symptoms vary widely, from leaf spots and blackened leaf margins on maple, to undersized and defoliating leaves on oak and ash, to complete failure to break bud in a few cases. Conifer problems include needle browning or yellowing, drying of emerging candles, and branch dieback. What's the explanation? The consensus of Extension specialists in plant pathology, horticulture, and forestry is that the vast majority of the problems are related to cold-temperature injury.

The leaf damage on maples was especially confusing. In some cases, the leaf spot symptoms were fairly similar to those caused by anthracnose and other leaf spot fungi. However, the widespread occurrence of fungal leaf spots this spring is highly unlikely, because the period between bud break and appearance of the symptoms was quite dry over most of the state. Without prolonged periods of wetness, such outbreaks do not occur. Instead, freeze injury appears to have caused the damage. Much of the state experienced temperatures well below the freezing mark on at least two nights in late April and early May, during the period after bud break but before leaves had fully unfurled. The unusual, spotty pattern of damage on leaf blades appears to have resulted from differences in exposure of the young, emerging leaf tissue during these freeze events.

But many of the tree problems we are seeing this spring probably originated with the extreme cold snap that occurred during early November 1991. On the coldest night, November 7-8, lows ranged from 4 above in the extreme southeast to 16 below in west central Iowa! What was exceptional about this cold was that it struck so early in the fall. Many trees had not fully entered into a dormant state yet, so they were not as prepared to withstand freeze injury as in late November or December. The most immediately visible result of this cold was browning of needles on conifers, particularly Scots pines. The November freeze also caused damage to buds and cambial tissue of hardwoods; however, this damage became visible only when growth resumed this spring. Buds that died in the November cold failed to open, and branches with cambial damage produced small leaves that soon fell off.

With such a vast range of symptoms and severity of injury, it's hard to make all-encompassing predictions about how the injured trees will fare in the future. In most cases, it seems likely that established trees will develop new shoots and leaves to replace those killed by freeze injury, and that they will regrow normal foliage in a matter of weeks. If the damage is confined to leaf spots, however, the spotty leaves may remain on the tree throughout the season. In situations where the cambium has been injured, recovery will be hindered if drought prevails this summer.

What can be done to help cold-injured trees? Not a great deal, in most cases. Watering newly planted trees weekly, and established trees every two weeks or so, during dry periods will help recovery, and mulching is also beneficial. Dead branches can be removed, but be sure they are actually dead (i.e., dry and brittle) before making any pruning decisions. . .

This article originally appeared in the June 3, 1992 issue, p. 90.


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