Pin oak trees can be a beautiful asset to the landscape. Their pyramidal form, pendulous lower branches and reddish or bronze fall color are striking. Unfortunately, most pin oaks planted in the Midwest are plagued by a yellowing of the leaves known as chlorosis. Other landscape plants are also susceptible to chlorosis, including rhododendrons, river birch, holly and sweet gum.
Chlorosis gets its name from the lack of chlorophyll, the pigment responsible for healthy plants’ green color. When chlorophyll is not present, the resulting color is usually yellow. The major cause of chlorosis in landscape plants is a deficiency of either iron or manganese. Both are considered to be plant micronutrients, meaning they are needed in small quantities by plants.
Iron and manganese deficiencies usually are not caused by an actual lack of these nutrients in the soil, but by soil that is too alkaline. As soil pH becomes more alkaline, iron and manganese are chemically tied to the soil, making them unavailable for plant uptake.
Iron deficiency causes interveinal chlorosis — a yellowing of the tissue between the veins while the veins remain green. This striking contrast becomes apparent on the youngest foliage first. In extreme cases, the tissue may turn brown and plants may be stunted.
Manganese deficiency symptoms are similar to those of iron. Silver and red maples are especially sensitive to manganese deficiency. However, if manganese-deficient leaves are treated with iron, they become even more chlorotic.
Iron and manganese chlorosis can be corrected in several ways. For a long-lasting solution, make the soil more acidic to free up the existing nutrients. Small areas can be made more acidic by applying acidic organic matter, such as peat moss, to the soil. Larger areas are more feasibly treated with elemental sulfur, iron sulfate or aluminum sulfate to the soil. The amount needed depends on the size of the area, the current soil pH and soil type. These materials are relatively slow acting, and the soil will have a tendency to return to alkaline, so it can be a never-ending battle.
To bypass the problem of soil alkalinity, iron or manganese can be applied directly to the plant. The nutrients can be sprayed on the foliage, but such treatments generally give only temporary relief. And, of course. you’ll need sprayer equipment that can reach the entire plant.
Nutrients can be injected directly into the trunk of the tree. Injections are very effective, however they are expensive and create wounds that can provide entry for insect and disease organisms.
Adding nutrients to the soil near the plant is yet another option. Use specially formulated nutrients, known as chelates, to avoid the problem with soil alkalinity. These materials can be expensive and slow to work.
The best solution is to choose plants that are adapted to your location. Avoid chlorosis-prone plants if your soil is alkaline.