If the leaves of your trees or shrubs are turning pale green, yellow, or white, but have much darker green veins, they may be experiencing iron chlorosis. Iron is a necessary element for the development of chlorophyll. Chlorophyll is what gives leaves their green color and is the source for the plant’s food and energy. When iron is insufficient for normal growth, yellowing leaves may occur. These symptoms may appear over the entire tree, on one side only, or be limited to an individual branch. Iron chlorosis is common in pin oak, white oak, silver maple, crabapple, white pine, magnolia, holly, sweet gum, dogwood, azalea and rhododendron.
Iron chlorosis may occur as the result of several causes including:
1. High pH: Iron is more soluble and available to plants at a soil pH range of 5.0 to 6.5. When pH is higher than 6.5, due to the application of excessive amounts of lime or phosphates to the soil, other elements interfere with the absorption of iron, even though it may be present in the soil. This condition worsens when large amounts of copper, manganese, or zinc is present or there is high soil moisture. Ammonium sulfate or sulfur may have to be applied to lower the pH levels.
2. Over-watering or poor drainage: Trees need 1 inch of water per week. Adjust your watering times so that you water less frequently but for a longer period of time to encourage deep rooting. Poor drainage may also be a problem in urban plantings where original topsoil has been replaced with subsoil. You may need to correct this problem by
installing tile lines near valuable trees.
3. An actual deficiency of iron: If the tree or shrub is not exhibiting symptoms related to high pH or water, then there may be an actual iron deficiency. There are three ways to provide iron to the plants: foliar application, soil treatment or trunk injection.
Foliar application uses a spray of iron sulfate or iron chelate solution. It provides quick results but is short-lived and requires repeat applications. Adding iron to the soil has a much slower response, but can prevent iron deficiency for 3 to 4 years. The rate of iron
sulfate varies by tree size. Contact your Extension agent for specific application instructions. Trunk injections have been found to be effective under specific conditions although results may vary with tree species, severity of chlorosis, time of year, soil moisture and individual tree response. Homeowner trunk injection systems are available, however commercial arborist have access to even more systems and the experience of application.
Submitted by Kelly Jackson, Agent for Horticulture, Christian Co. Cooperative Extension Service