Causes and Cures
When you see a tree that has yellow leaves with green veins, sometimes even dead terminals, what is your first thought? It’s likely to be due to iron chlorosis. There can be several other causal agents, such as waterlogged soils or inadequate organic matter, but the common limiting factor is micronutrient related.
Plants need various amounts of 16 essential elements to grow well. In the most common form, iron chlorosis develops due to a limit on the capacity to extract nutrients that are necessary for healthy growth from the soil. One or more elements may be limited.
Most commonly, the controlling factor is the pH 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 rather than being a part of the soil solution that is utilized by plants. In most situations, it’s not a lack of iron in the soil that is the problem, it’s that iron is unavailable for uptake by 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, 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, sweetgum, sycamore, silver maple, river birch, magnolia and spirea are plants that commonly develop iron chlorosis.
The good news is that there are plenty of tree species that can be utilized instead of the problematic ones. The sustainable, or common sense, approach toward dealing with the issue is to identify the cause of the poor growth, experiment with a couple of treatment methods that are appropriate for the site, 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.
In many situations, the symptoms of chlorosis can be alleviated by treating the soil with certain nutrients. Two methods can be used: surface applications and the placement of vertical nutrient columns. Several researchers, including Carl Whitcomb, professor emeritus of Oklahoma State University, have reported that placing concentrated nutrient columns throughout the rootzone of a chlorotic 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. Generally, dirt and/or sod are not recommended to cover the fertilizer column. To be effective, many columns are required. Various formulas exist to calculate the number required to alleviate symptoms; however, 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.
On the plus side, if the treatments are successful, they tend to be effective for five to 10 years. They are noninvasive to the tree and cause no damage to the bark, cambium 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 may be objectionable to some clients. Third, it is time-consuming to dig and fill the columns.
Surface treatments involve broadcast application of elemental sulfur following aeration of the turf 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-inch-diameter trees require 4 to 6 pounds. Large specimens should be treated with 7 to 8 pounds. The 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 the surface application are overall lower cost of materials required, faster completion of the treatments and the avoidance of voids in the turf. On the negative side, one to two years are required for results and more applications are required than with the fertilizer column approach.
There are two main advantages of injection treatments for chlorosis. First, the speed of effectiveness. Most treatments change yellow leaves into green ones in two to three weeks. If a tree is severely chlorotic, getting iron into the nutrient flow during the current growing season is crucial to success. Because soil treatments are not likely to provide rapid uptake of iron, it may be necessary to inject. Secondly, if a tree is located where much of the surface surrounding the tree is covered with concrete—such as near a driveway and street, or in a parking lot—soil treatments are not really feasible.
In general, liquid injections are much more effective than dry powders or capsules. Liquids are readily absorbed, while dry products must be dissolved by the flow of water in the cambium and sapwood.
When choosing from the various available injection methods, Arboricultural Consultant Phil Pierce, Springfield, Neb., indicates that “less wounding of the tree is best; so, smaller holes, shallower holes, fewer holes, less frequent injury, injecting as low to the ground as possible and as soon as the tree is leafed out in spring are best.” Ferric ammonium citrate is the product of choice.
Because yearly injections are not recommended, treating the soil as well as injecting the tree is a good approach for severely chlorotic specimens.
Another option for severely chlorotic trees is foliar treatment. If symptoms are noticed early in the growing season, it may be effective in getting iron into the nutrient flow of the tree. Unfortunately, foliar treatments affect only the leaves that exist at the time of spraying, and are not effective for future growing seasons. However, if the tree has been injected previously 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. For help with this, contact your local botanic garden or university extension office for assistance.
The author is a horticulturist, certified arborist and frequent contributor to Tree Services located in Omaha, Neb.