Trees that exhibit yellow leaves with prominent green veins are often suffering from a micronutrient deficiency called iron chlorosis. Symptoms may range from a mild yellowing of the leaf tissue between the veins to severe yellowing. This can lead to the death of leaves, dieback of branches, lack of vigor, and possibly death of the entire plant.
Initially, the problem may affect only a few branches, though it is common for the whole tree to take on a yellow-green hue. Trees may not show symptoms every year if only mildly affected. Trees with distinct symptoms will continue to deteriorate in following seasons.
Iron chlorosis is often a problem of pin oaks, though other tree and shrub species can be affected. Chlorosis is typically associated with one of three site conditions--a soil with a pH above 7 (an alkaline soil), a heavy soil containing a high percentage of clay, or a site where there has been extensive soil fill or removal, compaction, or construction injury that has damaged the root system. Older, established oaks are especially subject to the latter type of injury.
There are three ways to treat iron chlorosis problem with iron-providing fertilizer: foliar sprays, soil incorporation, or trunk injection. The first two can be done by anyone. Trunk injection is best done by a trained arborist, because of the specialized equipment and procedures involved.
The foliar spray treatment uses an iron chelate and water mix sprayed directly onto the leaves of the affected tree or shrub. The results are usually noticed within a few days, but the problem is only temporarily corrected. A fertilizer that supplies iron, such as Greenol, Mir-Acid, or Acid-Gro should be used. Follow label instructions carefully.
Some type of spray equipment will be necessary for applying the fertilizer. Large trees may need to be treated by a tree company with powerful sprayers that can reach the upper portions of the tree. A hose-end sprayer may be adequate for smaller trees or shrubs.
The second method, soil incorporation, is slower to produce results, but the treatment lasts longer. Acidifying compounds such as elemental sulfur or iron sulfate are used to lower soil pH, making iron more available to the plant roots. Elemental sulfur may take several months to react with the soil whereas iron sulfate reacts within a few weeks. In either case, these compounds must be incorporated into the soil to lower the pH where roots are growing.
One method of iron sulfate application that has shown some success is to concentrate the material in holes around the tree. The first step is to determine the number of holes that need to be dug. A total of 2 to 3 lbs. of iron sulfate per inch of trunk diameter will be needed for each tree. Twelve ounces to one pound of iron sulfate is placed in each hole. The holes should be 12-18 inches deep and spaced two feet apart at the dripline of the tree. These holes should be at least three feet from the trunk of the tree. (See illustration.)
After the holes have been prepared, place the iron sulfate in each hole, water well, then refill the holes with soil. If greening is not observed within three weeks, repeat the procedure.
Elemental sulfur may also help to correct an iron chlorosis problem, however sulfur should not be used at the same time as iron sulfate. Sulfur should be spread around the base of a plant, then incorporated into the soil to a depth of at least six inches. Apply 5 to 6 lbs. per 100 sq. feet of soil surface. Check the soil pH after two months, then reapply the elemental sulfur if the pH is not in the desired range of 5.5-6.0. These treatments may correct the chlorosis problem for a number of years. If chlorosis reappears, further applications may be necessary.
Testing done on the University of Minnesota campus using these methods have shown mixed results. Trees and shrubs with a severe chlorosis problem may continue to exhibit signs of yellowing and poor growth despite repeated treatments.
In many cases iron chlorosis can be avoided by choosing trees that are appropriate for the existing conditions. Avoid planting oaks, especially pin oaks, and other acid-loving species on soils with a high pH. The following table* contains names of some trees able to withstand somewhat alkaline soil.
|Scientific Name||Common Name||Scientific Name||Common Name|
|Acer saccharinum||Silver Maple||Alnus glutinosa||Common Alder|
|Betula papyrifera||Paper Birch||Carpinus caroliniana||Northern Hornbeam|
|Catalpa speciosa||Northern Catalpa||Celtis occidentalis||Hackberry|
|Cornus sericea||Red Osier Dogwood||Juglans nigra||Black Walnut|
|Pinus banksiana||Jack Pine||Pinus nigra||Austrian Pine|
|Quercus bicolor||Swamp White Oak||Quercus macrocarpa||Bur Oak|
|Salix alba||White Willow||Tilia americana||American Linden|
|Tilia cordata||Little Leaf Linden|
* From Tree Fertilization: A guide for Fertilizing New and Established Trees in the Landscape, University of Minnesota Extension Service folder 07410. For a more comprehensive list of plants and their pH preferences, see University of Minnesota Extension Service bulletin 01731, Soil test interpretations and fertilizer management for lawns, turf, gardens and landscape plants.
If you are uncertain as to the type of soil you have, the University of Minnesota Soil Testing Lab can help. Visit their website at http://soiltest.coafes.umn.edu or call them at (612) 625-3101.
Sometimes, under very dry conditions, trees will exhibit iron chlorosis because there isn't enough moisture in the soil to carry dissolved mineral nutrients so roots may absorb them. Adding sulfur compounds will not help in that situation.