Gardening: Many trees showing signs of drought stress

After two hot, dry summers, we knew this would happen. Trees are starting to show drought stress. Several people have come into the WSU Master Gardener Plant Clinic in the past couple of weeks with trees that are dropping leaves and needles from the middle of the tree at a high rate, branch tips that are dying back and bug infestations. All these are defense mechanisms or the result of stress.

When a tree is deprived of water for a length of time it reacts by dumping leaves and needles that aren’t critical for collecting sunlight to make food for the tree. First the tree will shed leaves or needles from the interior of the tree as a way of conserving its resources. This is quite common even in a normal summer when our summer heat hits and the days shorten. We’ve all seen dry leaves coming down in mid-August. This year as a result of lingering dryness, the trees are shedding a little early.

When a tree is seriously stressed as happened last summer, branches may die back at the tips or whole branches die back. I watched a couple of silver maples at the old Painted Hills Golf Course completely wilt last summer and thought they had died. Nope. They came back this year but in a much reduced state with a lot of dead tips. Trees are tough.

Bug infestations increase when trees get stressed. Research has shown that when they are stressed, the tree emits a chemical signal that will draw in insects. As a result, this year we are beginning to see some infestations of spider mite, woolly adelgid and spruce bud scale on conifers. Spider mites will create fine webs in dense conifers and suck juice from the plant, turning the needles a tawny color. Woolly adelgid and spruce bud scale also suck plant juices but do it from under a protective coat. The adelgid creates a woolly covering that protects it from predators and insecticidal sprays, while the spruce bud scale has a waxy shell that does the same thing.

Another reason that the bugs may be more abundant is that when trees are in a state of drought, the sugars in the sap become more concentrated, which means the smorgasbord just got tastier for the bugs.

There aren’t any quick fixes to this. The key is to make sure you get water deep enough to reach their roots. I have said many times in this space, do not expect sprinklers set to water a lawn to also give your trees enough water. Lawns are 6 inches deep. Tree roots are 12 to 18 inches down. They need a deep soaking that will get water to this level. If you can’t put your trees on their own sprinkler line, then run a soaker hose around the drip line of the tree and leave it on for several hours every couple of weeks. Your trees will thank you in the long run.

Pat Munts is co-author, with Susan Mulvihill, of the “Northwest Gardener’s Handbook.” Munts can be reached at pat@inlandnwgardening.com.