Chlorosis is a Common Problem
by Don Janssen, Extension Educator
Pin oak, birch, and other plants may exhibit yellowish leaves by early summer. A closer examination of these leaves may show while most of each leaf is yellowish-green, the veins are still green. This condition is known as chlorosis.
Chlorosis is a common problem with some plants growing on the Great Plains. These plants may exhibit chlorotic leaves by early summer. By late summer, the leaves may have brownish patches of dead tissue. In extreme cases, the leaves may die, shoot growth becomes reduced, and eventually the plant dies.
Alkaline soils (pH above 7) reduce the availability of iron and manganese; two important nutrients found in tree leaves. These nutrients may be abundant in the soil, yet the alkalinity makes them insoluble and unavailable to trees. Merely adding iron to the soil will not improve the condition of the tree. The problem can be further compounded by low oxygen conditions which are common in poorly drained soils.
The best way to avoid chlorosis problems with pin oaks and river birch is to not plant them in alkaline soils, particularly poorly drained ones. If you already have one of these trees in the landscape, there are some possible treatments.
Lowering the soil pH is performed by core aerating the soil under the drip line of the tree then broadcasting granular sulfur on the soil surface and watering it in. This method may provide a reduction in soil pH for several years, but the benefits of the treatment will probably not be seen until the year following application.
Chelated iron can also be applied to correct chlorosis. Iron chelates are organic complexes of iron that remain soluble even in alkaline soils.
Iron chelates can be spread on the soil surface or a similar amount can be placed in a series of holes around the tree. The holes should be 1 1/2 to 2 inches in diameter and should be augered 2 feet apart beginning about 5 feet from the trunk and expanding out at least a distance equal to two thirds the height of the tree. The holes should be about 6-8 inches deep. Backfill the holes with compost or soil.
Chelated iron is available in both liquid and granular forms and is sometimes incorporated in complete fertilizers. Follow the application rates on the package.
Trees and shrubs can absorb many nutrients, including iron and manganese, through their leaves. Foliar treatment only affects the leaves present at the time of spraying. The treatments are good only for the current season and will need to be repeated at the beginning of each growing season.
Iron or manganese compounds can be injected or implanted into the trunk of a chlorotic tree. Consider these treatments only after other measures have failed. Repeated drilling of holes in trees can result in long-term injury or decay.
University of Nebraska-Lincoln Extension in Lancaster County is your on-line yard and garden educational resource. The information on this Web site is valid for residents of southeastern Nebraska. It may or may not apply in your area. If you live outside southeastern Nebraska, visit your local Extension office