The Cause of Yellowing Tree Leaves -
Iron chlorosis affects many trees, as the leaves turn a yellowish color while the leaf veins remain dark green. Chlorosis is caused by the plant not being able to acquire the iron it needs.
Iron is necessary for the production of chlorophyll. The lack of iron results in the loss of the green color. In severe cases, leaf color changes from yellow to white to brown. If left untreated, twigs or even the tree can die.
Trees susceptible to iron chlorosis
Not all plants are susceptible to iron chlorosis. Chlorosis is mostly likely to be seen in:
- pin oak,
- silver maple,
- bald cypress,
- crabapple, and
- sweet gum.
Symptoms may appear over the entire tree, on one side, or be limited to a branch or two.
Soils in Kansas usually have adequate amounts of mineral iron. The problem arises when the soil pH rises above 7.0. Iron found naturally in the soil changes from a soluble form to an insoluble form that many plants have difficulty drawing out.
Treatment for iron chlorosis
There are several methods to help a plant affected by iron chlorosis overcome this problem. It will take a lifetime of treatments as there is no one-time fix to the high pH soils that create the problem. Young trees affected by the problem should be removed and replaced.
- A foliar application of iron sulfate or iron chelate solution may be applied when the tree is fully leafed out. This will give a quick green up of the leaves but will not last. Thus it is not highly recommended for a tree with a chronic chlorosis problem.
- Soil treatments with a mixture of equal parts sulfur and iron sulfate are another method of treatment. This granular mixture should be placed in holes drilled around under the canopy of the tree. Space the holes two feet deep and about fifteen feet apart beginning three feet from the trunk. Make additional circular patterns around the tree until the mixture is applied. The treatment amount is equal parts of sulfur and iron sulfate at the rate of one pound each for trees under 4 inches in diameter, and two pounds for trees over 4 inches in diameter. This treatment may last two to three years before reapplication is needed.
- Trunk injection is often the preferred method of treatment if done correctly. There are do-it-yourself kits on the market that are applied by drilling holes into the trunk and releasing a cartridge filled with an iron solution into the water transport system. Unless treatment is done in the spring, results may not appear until next year as the iron is absorbed. Another down side to some home treatments is that the holes are quite large and can result in long term health issues for the tree as the treatments must be applied every few years.
Commercial arborists can also do trunk injections. These treatments may be the most effective for the long term, often lasting up to five years. Equipment used results in much smaller holes being drilled into the tree to reduce injury. These systems place the solution in the vascular system of the plant for uptake. They work with the gravity flow and pressure internally in the tree.
No matter what system is used, this is a tree ailment that must be corrected for long term health of the tree. It is also recommended in many cases to avoid the trees listed in the second paragraph if your soil has a high pH or a history of iron chlorosis.
For more information to combat iron chlorosis, download the publication Iron Chlorosis in Trees, from the Kansas State Forest Service.