AMES, Iowa – River birch trees, a popular tree for river banks and wet parts of the garden, are hardy in USDA zones 4 through 9. They are more heat tolerant than most of their birch relatives and prefer full sun. Horticulturists with Iowa State University Extension and Outreach describe a few common issues river birch trees experience and steps for treating them. To have additional questions answered, contact Hortline at firstname.lastname@example.org or 515-294-3108.
Japanese beetles are eating the leaves on a river birch. What should I do?
Japanese beetles feed on the foliage, flowers and fruit of more than 300 different plants. However, birch trees are one of their favorites. Linden, elm, horse chestnut, cherry, plum and pussy willow are other tree species favored by Japanese beetles. Japanese beetles eat the leaf tissue between the veins, resulting in leaves with a lacy or skeletonized appearance. Severely damaged leaves turn brown and fall from trees.
It is not necessary to control Japanese beetles on well-established, healthy trees (including birches) as they are able to tolerate defoliation. Defoliation is most harmful to recently planted trees (those planted in the last two or three years) and trees in poor health.
Small, recently planted trees can be protected by spraying the trees with Sevin (zeta-cypermethrin) or Eight (permethrin). Because of the short residual effect of the insecticides, repeated spray applications will be necessary. Organic options include beetleGONE (Bacillus thuringiensis galleriae), which controls Japanese beetles, and Surround (kaolin clay), which makes the foliage less palatable to insects. Also, be sure to water small trees on a regular basis during dry periods. Defoliation and drought stress may be too much for young trees to endure.
Some of the leaves in my river birch are turning yellow and falling to the ground. Why?
Hot, dry weather may be responsible for the leaf drop on the river birch (Betula nigra). River birches shed some of their leaves in hot, dry weather. Leaves in the interior of the river birch turn yellow and fall to the ground. Healthy, well-established trees should not be seriously harmed by hot, dry weather. The hot, dry weather poses a more serious risk to trees in poor health and those planted in the last two to three years. Recently planted trees should be watered on a regular basis in hot, dry weather.
Why are the leaves on my river birch yellow-green?
In Iowa, the foliage of the river birch is often a sickly yellow-green. The yellow-green foliage is due to a deficiency of iron. The problem is referred to as iron chlorosis. A close examination of chlorotic leaves reveals that while most of the leaf is yellow-green, the tissue around the major veins is a darker green. Most soils in Iowa contain sufficient amounts of iron. However, in alkaline soils (those with a pH above 7.0), the river birch is unable to absorb adequate amounts of iron. Chlorotic river birches are rather common in Iowa as many soils in the state are slightly alkaline.
Correcting an iron chlorosis problem is difficult. Lowering the soil pH to 6.0 to 6.5 would allow roots of the river birch to more readily absorb iron from the soil. Unfortunately, lowering the soil pH over a large area is extremely difficult, if not impossible. Applications of iron-containing compounds, particularly chelated iron, may help trees that are mildly chlorotic. When using iron chelates, carefully follow label directions. Oftentimes, the injection of an iron-containing compound into the trunks of trees is the most effective treatment for chlorotic trees. Recovery is often quick and treatments are usually effective for two to three years. Trunk injections should be made by a professional arborist for best results.
Photo: New birch leaves by candy1812/stock.adobe.com.