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The Homeowners Column
Yellow leaves can indicate plant problems
State Master Gardener Coordinator
According to plants, green is "in". Gardeners, however, add plants known for everything but green. In the gardening world purple or yellow colored leaves are "in". Sometimes a change in leaf color can be an indication of nutrient or environmental problems. If your green plants are now yellow, chlorosis may be the issue with the tissue.
Abnormal yellowing of leaf tissue is called chlorosis. Leaves lack the essential green pigment chlorophyll. Possible causes include poor drainage, damaged roots, compacted roots, high soil pH, and nutrient deficiencies in the plant according to James Schuster University of Illinois Extension horticulture educator.
Nutrient deficiencies may occur due to insufficient amount in the soil or because the nutrients are unavailable due to high pH soil. Or nutrients may not be absorbed due to injured roots or poor root growth. Plant requirements also vary in the amount of 17 essential nutrients needed.
Herbaceous and woody plants are susceptible to chlorosis. It usually starts as lighter green tissue between darker green leaf veins. Not only does the plant look unattractive, but leaves or entire plant may be stunted and may fail to produce flowers and fruit. In addition, chlorotic leaves are more prone to scorching and leaf diseases. With severe chlorosis the leaves, affected branch, or entire plant may die.
The most common nutrient problem associated with chlorosis is lack of iron, but yellowing may also be caused by manganese, zinc, or nitrogen deficiencies.
According to Schuster one way to separate iron deficiency from other deficiencies is to determine what foliage turned yellow first. Iron deficiency starts on young terminal leaves and later works inward to the older leaves. However, deficiencies in manganese, zinc or nitrogen develop on inner or older leaves first and then progress outward.
Treatment for chlorosis varies depending on the plant and the cause.
Rule out or correct problems due to soil compaction, poor drainage, poor root growth, or root injury. Core aerification, tiling, mulching, or some other cultural practice may be needed.
Nutrient deficiencies can be treated in one of several ways.
Get a soil test to determine soil pH and nutrient levels. Iron becomes more insoluble and less available to plants as soil pH goes above 6.5 to 6.7 [7.0-neutral; below 7.0-acidic; above 7.0-alkaline]. Plants such as blueberries, rhododendrons, azaleas, and pin oaks have a high need for iron, hence their acidic soil requirement.
High amounts of other elements such as calcium, zinc, manganese, phosphorus, or copper in the soil can make iron unavailable to the plant. A shortage of potassium in the plant will also reduce iron availability. Insufficient iron in the soil may also be the cause. Based on a soil test, the pH can be lowered by adding sulfur or the appropriate nutrients can be applied.
Foliar applications of water-soluble or chelated nutrients can temporarily correct the problem. However it only helps the leaves present during application. Several treatments per growing season may be necessary to keep the foliage green.
For trees with moderate to severe chlorosis another method is trunk application. It is quick and may last several years. However, it may take 30 days for the tree to respond. There are two ways to apply nutrients via the trunk. Both methods involve drilling holes in the trunk--the number of holes is based on trunk diameter.
With the first type of application, containers with tubes are then inserted into the holes. The tree's movement of moisture draws the nutrients into the trunk. After the containers are empty, they are removed and holes are plugged. With the other method plastic capsules are hammered into the drilled holes. In both cases, hire a certified arborist.