WINTER DAMAGE

This is a tough spring for landscape damage. I’ve received a number of queries from readers wondering why the tops of their trees and shrubs haven’t leafed out yet.

An old rule of thumb is that it will take a tree or shrub about as many years to overcome transplant shock as it was old when planted. For example, if you planted a 3-year-old tree it will take that tree about 3 years to catch up with itself and overcome transplant shock. In other words, that tree will not be "established" for three years and the damage you see can actually have been done in the nursery, or from transportation from nursery to the garden, or from transplanting. Such damage typically shows up as dieback of the tops of the branches.

If recent transplanting is not the case, then the problem could be from some damage incurred last season. Since this season has not yet begun, the damage would have to have been caused either last summer, last fall, or during the winter. Most of the damage we see in southwest Montana is desiccation damage, which usually shows up on the uppermost branches for a couple of reasons. A) The uppermost branches are most exposed to sun and wind and consequently get hit the hardest. B) Uppermost branches have large surface-to-volume ratios, meaning they have proportionately greater surface area for transpiration/water loss. C) The outside shoots, well exposed to the weather, are also the newest shoots, with less bark to protect them. D) The newest shoots are also the last to harden off in fall and a sudden drop in temperature will knock them out. This lack of hardening becomes even more problematic if the plants were watered or fertilized late in the season to keep them "green."

Roots are killed when soil temperatures hit about 15°F. Since we had a pretty good snowcover this year I doubt that outright kill is the cause of upper branches not leafing out. Last fall we had very warm weather and things grew late, then we got clobbered literally overnight with -20°F temperatures in mid-October. So the shoots may have gotten damaged at that time (most likely). Now, there was some storage of starch and nitrogen in the trunk and roots of the trees before that time. This is used for growth in the following spring. The nutrients stored are pushed up in spring to help the upper parts of the plant metabolize until the leaves can come out and do their job by sending photosynthates back down to the roots. If the leaves did not come out, obviously they cannot resupply the roots with nutrients. When the roots run out, the top of the plant dies. Also, the extreme cold may have damaged the phloem tissue in the trunk. If that is the case, even if the leaves do come out they cannot replace the root nutrients and the upper parts of the plant die.

All of this damage begins to become evident when the weather warms...right now, but it was induced last year sometime. I have seen many apple trees girdled by voles in winter that bloom well and even leaf out normally, only to wilt literally overnight when the weather gets warm...the damage the voles did to the phloem and cambium was irreparable. In that case the roots could supply the top with water and some stored nutrients through the xylem to get things in gear, but the top could not replace the photosynthates, and eventually when the tree was put under heat stress, it died.

If the branches are dead, prune them out any time of year. But be sure they are dead. Sometimes damage is not fatal and the branches will leaf out slowly and late. My rule of thumb is to wait until the 4th of July (see photo for branch that leafed out after being pruned). If they haven't leafed out by then they are likely dead, so the wait would not change things; if not dead, it is so late in the season that they won't have enough growing time and be weak going into another winter, and if the fall or winter is tough again, they will likely go out next year.

This is typical scenario for this area. A plant may hang on for several years and folks say, “What do you mean I can't grow that here? I've had it growing for X number of years!” But then the winters and summers find a way to clobber us and the plants that gardeners thought were hardy, weren't—and I feel vindicated!

Our ‘Adelaide Hoodless’ shrub rose which is supposed to be hardy here and which had done very well for us for a half dozen years, got killed back to the snowline this year. Our yucca (hardy to zone 4 according to the books) has severe dieback, and all four of our hardy plums may not make it. It is for this reason another old rule of thumb of mine is that hardiness zone designations are only accurate to +/- one hardiness zone. Gardeners put way too much stock in their zone accuracy—after all, we're dealing with weather!

Bottom line: I think much of the damage you see was done by the deep sudden cold last October that was exacerbated by the unusually warm weather leading up to it.

—Dr. Bob Gough