What’s one of the main causes for the yellowing of trees?

Iron chlorosis is a yellowing of plant leaves caused by an iron deficiency in the soil. This usually occurs when the soil has a pH above 7.0. Most soil in Idaho falls into this category.

There may be other causes of chlorosis including overwatering, insects, disease problems or herbicide misuse. If you have investigated and ruled out these other causes, the next step is to have your soil tested to see if the pH is above 7.0. If it is, you are most likely dealing with iron chlorosis.

Plants with iron chlorosis will develop leaves with bright green veins and yellow blades. In severe cases the veins may also turn yellow or even white. The margins of the leaf may scorch and turn brown. Chlorosis can appear on a few leaves, individual branches, part of the crown, or the entire tree. Not all plants in the landscape are affected.

Prevention

Treatment for iron chlorosis is more expensive and difficult than prevention. The best thing to do to prevent iron chlorosis is to know the pH of your soil. If it is a problem, select plants that are tolerant of high soil pH and less likely to be affected by low iron availability. Some of my favorites include ash, boxelder, catalpa, most oak trees, ginkgo, hackberry, linden basswood, and Norway and canyon maples. Avoid silver maple, red maple, river birch, pin oak and most berries.

Areas with low rainfall are more susceptible to having a high pH, so even if a plant is native to Idaho like quaking aspen, it may exhibit signs of iron chlorosis if it is planted in areas that have low rainfall. Just using a sprinkler to add more water does not solve the issue. Most of our water has a high pH, so the problems tend to get worse over the years.

Treatment options

Soil treatments in the fall or early spring to individual trees and shrubs or in small areas of the landscape are possible. The biggest issue is that when the pH of the soil is above 7.0, iron may be in the soil, but it is unavailable to the plant. To correct pH of the soil, use elemental sulfur. Iron sulfate and elemental sulfur can produce lasting results and are relatively inexpensive.

To treat soils with iron sulfate, make holes one to two inches in diameter and 12-18 inches deep. Space holes 18-24 inches apart around the drip line of the tree or shrub. Fill the holes with iron sulfate up to four inches from the soil surface. A soil probe or soil auger will allow you to make the holes without causing soil compaction. Over time, the concentrated sulfur in the holes will react to form acid, neutralize the soil and make the iron more available to the plant.

Some iron chelates can be used as a soil treatment; however, this only lasts one year and chelates are expensive. Read and follow the label for application instructions.

Foliar applications can be made directly on the leaves of affected plants if the temperature is below 85°F. If done above 85°F, the fertilizer can burn the leaves. Foliar applications do lead to spotty results and are difficult to apply to large trees. Iron chelates are available as foliar sprays, just remember to read and follow the label for application instructions.

Iron sulfate, elemental iron, and iron chelates can be purchased at larger garden supply stores or agricultural chemical dealers.