Why Chlorosis treatment can be complicated

Spring is here and trees will soon be full of leaves again. As your trees develop their leaves, you may want to watch for signs of chlorosis. You may notice the leaves looking more yellow than usual, often while the leaf veins remain green. This is a sign that your tree is in distress.

Why do we worry about chlorosis? The yellowing of the leaves is a sign that the tree is struggling to produce chlorophyll. This means that the tree is not photosynthesizing as well as it should be. Without proper photosynthesis, the tree cannot provide itself with the sugars it needs to sustain basic life functions. Sever cases of chlorosis can lead to branch dieback and weaken the tree enough to leave it susceptible to other pests and diseases.

Chlorosis is defined as the reduction or loss of normal green coloration of leaves of plants, typically caused by iron deficiency in lime-rich soils, or by disease or lack of light. This definition does give suggestions for the cause of yellowing, but not every case of chlorosis is due to iron deficiency or high pH. In trees, when we see chlorosis, the only sure thing we know is that a micronutrient (usually iron to manganese) is not reaching the leaves. From there, determining which nutrient the tree is deficient in, and why, can be at times complicated.

The type of nutrient deficiency can depend on the species of tree. Trees that are often deficient in iron include Pin Oak, River Birch, and Catalpa. Trees that are commonly deficient in manganese include Silver Maple, Red Maple, Dogwood, Flowering Cherries, and Crabapples. When we see Eastern White Pines showing chlorotic symptoms, it can be caused by either iron or manganese deficiency, and often testing needs to be done to determine which nutrient is needed.

Often times, chlorosis due to iron deficiency is not necessarily due to a lack of iron in the soil, but rather the iron being unavailable to the plant roots due to the soil being alkaline. In southern Wisconsin, we often have alkaline soils because of our limestone bedrock. When the pH level of soil is too high, iron and manganese becomes unavailable to plant roots. In this situation, simply adding iron or manganese to the soil will not solve the issue. Ideally, the solution would be to lower the pH level of the soil, but this approach has its limitations. You may see recommendations to add sulfur to the soil to lower the pH level. This can help in the topmost few inches of the soil for a limited amount of time. Repeated applications can build up in the soil and lead to roots burning. There is no permanent solution to lowering the pH level of soil deeper than the top few inches. The majority of tree roots exist in the top 18 inches of soil, so they go deeper than the roots of lawn and garden plants, thus making it harder to make lasting change in the pH levels of soil for trees. Talk to your Certified Arborist about ways to improve soil health overall.

Keep in mind that not all cases of chlorosis are due to issues with the soil. There can be other reasons why nutrients aren’t reaching a tree’s leaves. Chlorosis can also be caused by issues in a tree’s vascular system or roots. Many issues ranging from fungal infections, insect infestations, large wounds, root rot, or limited root space can lead to a tree struggling to update nutrients. Girdgling roots can also be a common contributor to chlorosis in Maples. This is why arborists may take multiple different approaches to treating chlorosis in trees.

First, direct injections of missing micronutrients into the tree’s vascular systems is the most effective way to improve chlorotic conditions in the short term. Injections last for two to three years in the tree, depending on the species and dosage. Injections are available for both iron and manganese. They can provide immediate relief for struggling trees, but these injections are intended to be only one part of a broader management strategy. If the source of the nutrient deficiency is a soil issue, your arborist may recommend soil injections to improve the nutrient content or alter the pH level. If a soil test returns results that don’t explain the chlorosis, though, your arborist may recommend other treatments. Growth hormone regulators can help trees to further develop fine fibrous roots, which can improve a tree’s ability to uptake nutrients. They also provide a side benefit of causing thicker leaf cuticles and helping the tree produce more chlorophyll. If your tree is suffering from an insect infestation that is damaging the tree’s vascular system, they may recommend an insecticide to stop the infestation. If your arborist suspects girdling roots or compacted soils to be the culprit, they may recommend an air excavation of the root system. This allows arborists to de-compact soil and add organic matter to improve soil health. Some trees that experience standing water may by struggling with root rot pathogens, which may be treated with a soil drench, and your arborist may recommend other ways to improve drainage.

Each tree’s situation may be unique, so don’t be surprised if your first attempt at solving chlorosis doesn’t immediately work. If you see signs of chlorosis in your trees, contact Tree Health Management for a professional assessment and recommendations.