Birch trees can be beautiful, but they are water-loving trees. Planting them in the desert stresses them, even when they are properly planted in a landscape and thoroughly watered in the spring and summer. It's the dry weather of fall and winter that is so hard on birch trees.
The paper birch native habitat is northern states from the Atlantic to the Pacific. The river birch is native to Massachusetts south to Florida and west to Kansas and Minnesota. These locations have winter moisture, primarily in the form of snow, so the ground stays damp. Nevada has many sunny, windy winter days that dry out the soil. We can go months without any precipitation. This is not good for birch trees.
How do you know if a birch tree is suffering? This summer, if the top limbs were starting to yellow and thin, or if any limbs were dying back from the top down, your birch may be in trouble. Look for little holes with one flat side, like the letter "D," on the trunk and branches. The "D"-shaped holes are the exit holes of the bronze birch borer. The larval or worm stage of this beetle feeds on the inner tissue of the tree, cutting off its supply of food and water.
Usually, insect management is done in the spring or summer, depending when the pest is evident. A few foliar and bark sprays are listed for bronze birch borer, but it is quite difficult to get into the tops of large trees with these for effective control.
However, there is now a chemical, imidichloprid, in some insecticides that are applied as soil drenches. These insecticides require no spraying. The chemical is easy to use and doesn't kill nontarget species, as the sprays would. It acts inside the tree to reduce the damage by the larval stage of the borer, and it needs to be applied now. It doesn't work very well if it is applied in the spring because it needs four months to be absorbed by the roots and moved throughout the tree.
It is simple to use. As with any insecticide, read the entire label prior to application and follow the directions. The basic idea is to measure the distance around the trunk and mix one ounce for every inch of circumference in a few gallons of water. Then, water in this mixture in around the base of the tree.
Remember to water birch trees at least once a month from now through the fall and winter, more if the weather is warm.
For additional information, see the "Bronze Birch Borer" publication online at www.unce.unr.edu/publications/FS02/FS0238.pdf or call for a copy.
For more information, e-mail firstname.lastname@example.org or call me at 887-2252. You can "Ask a Master Gardener" by e-mailing email@example.com or call your local University of Nevada Cooperative Extension office. Check out many useful horticulture publications at www.unce.unr.edu.
- JoAnne Skelly is the Carson City/Storey County Extension educator for University of Nevada Cooperative Extension.