When you see a tree that has yellow leaves with green veins, sometimes even dead terminals, it’s likely to be due to iron chlorosis. There are several other possible causes, such as waterlogged soils or inadequate organic matter, but the common factor is micronutrient related.
Plants need various amounts of 16 essential elements to grow healthy. In the most common form, iron chlorosis develops due to a limit on the capacity to extract nutrients from the soil that are necessary for healthy growth.
Most commonly, the controlling factor is the pH (the measure of acidity) of the soil. Iron is a nutrient that is available at the lower end of the pH scale, in the 4 to 6 range. In mid-Atlantic, Midwest and many other states, the natural pH of landscape soils is 7,8 or 9. In these higher pH soils, iron is held tightly to the soil colloids (mixture) rather than being a part of the soil solution that is utilized by plants. In most situations, it is not a lack of iron in the soil that is the problem, it’s that the iron is unavailable for uptake by the plant roots. Manganese is another element that can be lacking due to high pH soils.
The second most common factor is simply an inadequate amount of necessary nutrients in the existing landscape soil. A basic soil test will be helpful in determining the pH level, which elements are lacking and how much needs to be added to return it to a suitable growing medium.
Tree species most commonly affected
Many woody plant species are routinely affected. Pin oak, sweet-gum, sycamore, silver maple, river birch, magnolia and spirea are plants that commonly develop iron chlorosis.
To solve the deficiently we must identify the cause of the poor growth, experiment with a couple of treatments, and if they fail, replace the plants with ones that are well adapted to the high pH or low nutrient levels of the landscape soil.
Two methods can be used: surface applications and the placement of vertical nutrient columns.
Surface treatments involve broadcast application of elemental sulfur following aeration of the area surrounding the tree. The amount of sulfur to apply is dictated by the size of the tree. Small trees, 3 to 6 inches in diameter, call for 3 to 4 pounds, while medium sized, 7 to 10 inches, require 4 to 6 pounds. Large trees should be treated with 7 to 8 pounds. Applications should be made evenly across the rootzone of the tree and repeated every spring and fall until the symptoms are alleviated.
The advantages of using surface applications are overall lower cost of materials required, faster application of the treatments and the avoidance of holes in the landscape. However, 1 to 2 years are required for results and more applications are required than with the fertilizer column approach.
Placing concentrated nutrient columns throughout the rootzone of a chlorosis tree will cause a reduction or elimination of symptoms. The basic treatment protocol is to drill holes, 6 inches deep and 3 to 4 inches wide and fill them (in this order) with 8 ounces of Micromax Micronutrient fertilizer, 8 ounces of elemental sulfur and enough slow release nitrogen fertilizer to finish the column even with the surface grade. Concentric circles of holes that are approximately 3 to 4 feet apart are involved, If the tree is severely affected, several circles are called for with at least one at the drip line, one inside the drip line and one outside the drip line.
If the treatments are successful, they tend to be effective for 5 to 10 years and are noninvasive to the tree and do not cause damage to the bark, cambium layer or sapwood.
There are three disadvantages of the fertilizer column methods. First, on average, it takes about two years for positive results. Second, due to the high salt index of most fertilizers and the avoidance of soil/sod over the top of the columns, visible voids in turf cover are created, which some may consider an eyesore. Third, it is time-consuming to dig and fill the columns.
There are two main advantages of injection treatments for chlorosis. First, the speed of effectiveness. Most treatments change yellow leaves into green ones in 2 to 3 weeks. If a tree is severely chlorotic, getting iron into the nutrient flow during the current growing season is crucial to success. If rapid uptake is required, it may be necessary to inject. Secondly, if much of the area surrounding the base of the tree is covered by concrete or another barrier, soil treatments are not possible. Liquid injections are generally more effective than dry powders or capsules.
When choosing injection methods, less wounding of the tree is best; smaller holes shallower holes, fewer holes, less frequent injections and injecting as low to the ground as possible and as soon as the tree is leafed in spring are best. Ferric ammonium citrate is the product of choice.
Yearly injections are not recommended, so treating the soil as well as injecting the tree is a good approach for severe deficiencies.
If symptoms are noticed early in the growing season, this may be effective in getting iron into the nutrient flow of the tree. However, foliar treatments only affect the leaves that exist at the time of spraying and are not effective for future growing seasons. If the tree has already been injected and further treatment is necessary to keep the tree healthy until soil treatments can take effect, foliar application may be feasible.
The best control method
The best way to control iron chlorosis is to plant trees well adapted to the site conditions, however, this is not an option for already established landscapes. When designing a new landscape take into account the soil needs of the plants and the soil conditions of the landscape.