Throughout the County, iron chlorosis is appearing in trees and landscapes. It can especially be seen in the silver maples (shown in this photo) and pin oaks. Iron chlorosis symptoms appear as yellowing of leaves while the leaf veins remain a dark green color. Eventually the leaves become so chlorotic that leaf tissue begins to turn brown/black and die. Severely affected leaves often drop from the tree and new leaves emerge in 7-14 days. Alkaline soils (pH above 7) reduce the availability of iron and manganese; two important nutrients found in tree leaves. It’s not necessarily that these nutrients are deficient in the soil. The high pH just makes these nutrients less soluble and unavailable to the trees. Adding iron to the trees or the landscape around the trees won’t correct the problem as the pH ultimately needs to be corrected.
Early in my Extension career, I inherited the job of injecting trees with a solution of iron sulfate. There would often be a waiting list of people to help, particularly in Sutton, when they’d see the jugs hanging from trees. As a plant pathologist, I was concerned about how much damage we were doing to the trees with repeated injections every five years or so…if secondary pathogens would affect these areas we were drilling in the trees. I was also concerned about providing a free service that took a great deal of time and if we were competing with industry by doing so. Eventually we stopped providing the service but we do rent out the equipment and can provide the recommendations on how much iron sulfate to add based on tree diameter if you are interested. But you may be interested in another less invasive method instead.
I learned of another potential method to consider from John Fech, Extension Educator in Douglass/Sarpy Counties, and we tried it at the Sutton Cemetery for two years and at the Clay County Courthouse last year with decent results. The first step is to double aerate the area from the trunk to the dripline of the trees showing iron chlorosis symptoms. The dripline is the outer leaves of tree. Do so several days after irrigation/rainfall to avoid tearing up the grass in your lawn. Often people aerate their lawn in the spring and/or fall, so if you have a problem with iron chlorosis, you could include the following treatment into your routine.
The second step is to broadcast spread granular sulfur on the soil surface. Then broadcast spread iron sulfate on the soil surface. What was recommended to us by Omaha Master Gardeners was using 3 lbs of sulfur + 3 lbs of iron sulfate for “small” or newly established trees; 4 lbs of sulfur + 4 lbs of iron sulfate for “medium” sized trees; and 5 lbs of sulfur + 5 lbs of iron sulfate for “large” established trees. I realize that’s not very scientific based on sizes so do the best estimation of tree size that you have. We used 5 lbs for everything at the cemetery and courthouse. Here Mike, Clay County Courthouse Custodian, is spreading the product close to the trunk first.
He continued applying around the tree in a circle gradually moving out towards the drip line. The final step is to then water the lawn.
We have numerous silver maples at the courthouse. This is a photo of one of the trees and a close up of these leaves was shown in the first photo. You can see the yellow cast to this tree in this photo and how severely affected many of the leaves were from the first photo.
This is the same tree a year later from the opposite side. The photo doesn’t do it justice as it truly is greener than last year and the leaves are hailed damaged. It didn’t really change during the season last year but all the courthouse trees are greener this year. They could use another application though as they still show symptoms of iron chlorosis. This method may take several years before the soil pH changes enough for symptoms to disappear.
You most likely will not see any changes the first year, although the grass under the tree may appear greener. The goal is to change the soil pH to make the iron more available to the tree in the future. This may be a process that needs repeated for several years in a row and while it doesn’t show instantaneous results, it will most likely aid in the longevity of your trees by reducing the symptoms without additional harm to the tree through injections.