September 3, 2009 at 8:44 a.m.
When we talk about drought stress we need to know what it looks like. Symptoms of drought injury to trees can be sudden or may take up to two years to be revealed. Drought injury symptoms affecting tree leaves including wilting, curling at the edges, and yellowing.
Many of the trees that show stress or are dying this summer most likely have been stressed for the last two summers. We have a cluster of maples that are mature and show beautiful colors come fall. This summer I noticed that the leaves were curling at the edges. Since I already had hoses going to my raspberries, I started to run the hose to those trees for one half hour everyday. As dry as it's been I don't think a soaker hose on a large tree for a few minutes is enough.
Deciduous leaves may develop scorch, brown outside edges or browning between the veins. Evergreen needles may turn yellow, red or purple. They may also turn brown at the tips of the needles and browning may progress through the needles toward the twig.
In continued drought, leaves may be smaller than normal, drop prematurely or remain attached to the tree even though brown. Often times, drought stress may not kill a tree outright, but set it up for more serious secondary insect and disease infestations in following years.
The last two summers the Chisago County Master Gardeners have been touring each others' gardens. While touring my garden I noticed ancient looking critters that had bored into one of my birch trees. The strange thing was, the insects had bored their back end deep into the trunk of the tree. Peggy Boike, one of our Master Gardeners, took a picture of them and discovered they were bronze birch borers. Peggy also found out that they attack stressed or dying birch. Sure enough, when the leaves came out this spring that birch tree was dead.
The last rain we received also had strong winds. The next morning all the rest of my birch trees had many small branches under them as though they had been pruned. I know that my birch trees are stressed because they (like the maples) have their roots close to the surface. Mulching around the maples and birch will help keep the surface roots cool and moist.
It is very important to keep yard trees watered going into winter, especially those that are stressed. In the past I have raked up the leaves in my yard, bagged them and used the bags for insulation around our old house. From now on however, I will leave some leaves under my birch and maples to help as a mulch base for next summer. There isn't anything I can do about the weather but there are some things I can do to help save my trees.