Every region of the country has certain trees that are known for specific problems. The causes are regional soil types, fungal diseases and insect infestations. One of the common maladies is iron chlorosis. In fact, in some areas, frustrated arborists have started calling species such as oak, birch and maple “golden oak,” “golden birch” or “golden maple” because of the large number of trees affected by iron chlorosis.
If iron chlorosis is suspected in a landscape, the good news is that the key symptoms are easy to spot. The bad news is that iron chlorosis is difficult to treat — more on that later.
The telltale symptoms of iron chlorosis are yellow leaves with green veins, especially on trees that are just starting to become affected. Trees that have been suffering from the malady for a few years show other indicators such as bare branches and shorter than normal twig growth in the most recent years.
Although it may not be the first thought when diagnosing tree disorders, keep in mind that iron chlorosis can also affect shrubs; in fact, if you see chlorosis symptoms on privet or spirea shrubs, look up, you just might see it on nearby trees as well.
As with many maladies, look-alike symptoms can be found in just about any landscape. For example, the fungus disease apple scab can also create yellow leaves and stunted growth. Further inspection usually shows classic round spots in a feathery pattern on lower leaf surfaces. Black spot of roses also produces symptoms similar to iron chlorosis.
A confusing situation can exist where both look-alike causal agents and chlorosis are found in the same landscape close by each other. It can be difficult to determine which cause is responsible for the unhealthy tree specimens.
All 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.
In some landscapes the cause of iron chlorosis is simply an inadequate amount of a necessary nutrient(s) 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.
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 the 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 it’s unavailable for uptake by plant roots. Manganese is another element that can be unavailable due to high pH soils.
There are three methods of treatment for iron chlorosis: to inject trees with nutrients, to treat the foliage and to apply various elements to the soil surrounding the tree(s). Each method has pros and cons associated with it. As arborists, tree workers and landscape managers, your job is to determine which will provide the best results on a case-by-case basis.
Injections — Many products are available for injection. Some are injected under pressure, some via gravity and others by simply squirting product into drilled holes. Because all injections involve a break in the bark to gain access to the cambium to facilitate uptake of the product, the potential for damage, such as reduced uptake of moisture and the onset of decay, is significant. To limit the likelihood of injury, consider injection methods that utilize the smallest holes possible.
From the positive standpoint, injections offer the opportunity to introduce the exact elements that are needed directly into the tree’s vascular system, usually resulting in quick relief from the symptoms of iron chlorosis. As well, injection can usually be performed in an hour or less, depending on the size of the tree. Injection offers a treatment opportunity for trees with limited soil surface area for the introduction of nutrients, which can be a real plus, which is commonly the case for street trees.
Foliage — In those instances where a high-value tree is severely chlorotic, foliar treatments may be justified. Such a scenario might involve a signature tree for a golf course or subdivision called “Oak Meadows” or “Swaying Oaks.” If the tree were to die from iron chlorosis on either of these sites, it would probably be necessary to rename the facility.
Unfortunately, foliar treatments will benefit the tree in terms of the current year’s crop of leaves only; no long-term value is realized other than keeping the tree alive in the hopes that either an injection or soil treatment will lessen the extent of the chlorosis. Foliar applications are most effective when symptoms are noticed early in the growing season, increasing the chances of getting iron into the nutrient flow of the tree.
Soil — The symptoms of chlorosis can also 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 to be realized. 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, and this 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 trees, 7 to 10 inches in diameter, require 4 to 6 pounds. Large specimens should be treated with 7 to 8 pounds. The applications should be made evenly across the root zone 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.
Disadvantages for this method include that one to two years are required for results and more applications are required than with the fertilizer column approach.
Fortunately, there are plenty of tree species that can be utilized instead of the problematic ones. The sustainable, or common sense approach towards 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, to replace the plants with ones that are well adapted to the high pH or low nutrient levels of the landscape soil. Overall, 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.