Yellowing Shrub and Tree Leaves? Diagnosing and Treating Iron Chlorosis
By Nate Dalman, Horticulture Researcher II
Have you noticed unhealthy leaf coloring in either your shrubs or trees? If both new and old leaf growth is yellow and the veins are bright green, the culprit might be iron chlorosis. This disease is caused by a lack of iron uptake which leads to photosynthesis not occurring at the rate that it should (see picture, to the right which shows symptoms of iron chlorosis). This will ultimately lead to the death of the plant. But don’t worry! Iron chlorosis can be treated and the plant can make a full recovery.
The lack of iron uptake by the plant is not usually caused by depleted iron levels in the soil. Most often there is plenty of iron in the soil but in forms that the plant can’t use. Iron uptake is also limited in compacted or wet, poorly drained soils. Many ornamental shrub and tree species have difficulty with maintaining the proper iron uptake due to their fast growth and flower production.
Iron chlorosis can be difficult to treat because it takes several months to even years for the plant to respond. Several types of treatments exist including soil amendments, foliar applications, trunk injections and physical alterations to the soil such as aerating.
Foliar applications involve spraying an iron sulfate solution on the leaves where it is then absorbed. Results can be seen in just a few weeks but are typically short lived. Applications must be applied 2-3 times per growing season and take several years of applications before the disease is fully treated.
Soil amendments can be applied in several ways based on the product being used. They must be either buried or trenched in at the base of the plant or sprinkled around on the ground surface and then incorporated via watering. The best time to do this is late winter to early spring. This method typically takes several months for results to be seen but only one application is needed per year (pictured left, healthy leaves after two months of soil amendment treatment).
Finally, altering the soil to reduce compaction or increase drainage can be one of the most effective treatments. Aerating the ground around the plant will both reduce compaction and help the soil dry out quicker. This promotes microbial activity which then in return can help the plant uptake iron more efficiently. However, this treatment method takes months to over a year to notice results but those results are long.
Hopefully this information can help you treat any iron chlorosis that may appear in your home garden. Just remember, changing the iron uptake rate of a plant is a slow process but this disease can be cured.