Submitted by Calvin Finch, Ph.D., Manager, Conservation Division, Water Resources & Conservation Department, SAWS, and Horticulturist
Iron chlorosis is a common ailment of plants in a soil with a basic pH. We call such soils alkaline. It is not that our soils lack iron, they have large quantities of the nutrient. The problem is that the iron is locked up in chemical compounds in the soil that make it unavailable to some plants.
Plants that evolved in an alkaline soil such as ceniza, Texas mountain laurel, and bluebonnets do not have trouble taking up iron. They have mechanisms for active collection of the nutrient, store it better, or just do not need as much. Plants such as roses, hollies, fruit trees, nandina, and St. Augustine grass that evolved in acid soils often have difficulty picking up enough iron, especially when the soil is cold (springtime), soggy, or dry. Some plants are very sensitive to the problem. Acid loving plants like azaleas, camelias, gardenias, and red-tip photinias usually do not survive long in alkaline soils.
The symptom of chlorosis is a light green color instead of dark green. In severe cases, the leaves turn white and the ends burn, leaves defoliate, and plant death can occur.
There are several options with the problem of iron chlorosis. The easiest thing is to avoid sensitive plants. Native plants rarely need an iron treatment. Organic material in the soil and decomposing mulch also help address the iron problem. For other plants, we can just tolerate a temporary light-green color and reduced growth rates.
There are two strategies to treat chlorosis. Iron can be offered in a form that can be used by the plant, or an attempt can be made to acidify the soil and release the iron that exists in the soil. The most effective strategy is to treat the plant with iron. Our soils are highly buffered, which means they are chemically potent. It is nearly impossible to affect any long-term change in acidity by normal methods such as applying sulfur or acidification products. Over time (usually minutes or, at most, days), the soil alkalinity overwhelms the acidifying agent.
Foliar iron treatments are effective for plants like St. Augustine grass and fruit trees that do not have waxy leaves. A product like iron sulphate (copperas) can be dissolved in a bucket of water. Add a cup per 5 gallons. Iron sulphate is not especially soluble, some of the material will not dissolve. Pour the liquid into your backpack sprayer and spray your grass as you walk the lawn. The remaining solid can be poured on the roots of chlorotic plants.
There are products called chelated iron (Sprint 138, Ferrimac, Ferriplus, etc.) that will treat iron chlorosis. Chelated iron products have their iron in a form that allows plants to use it, but it does not get locked up in the soil. Apply the chelated iron to the soil or dissolve it in water for a foliar application.Chelated iron products are expensive. You can make your own inexpensive chelated iron products with iron sulphate. Mix one cup in a bushel of compost and apply it to the mulch over the plants root system or directly to the soil. Homemade iron chelate works especially well for roses, perennials, and hollies.A common iron product, Ironite, was not effective in experiments that I conducted on public TV several years ago. A new product called Ironate, however, has received positive reviews as an iron treatment.