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Why are the leaves on my trees and shrubs yellowing?
Why are the leaves on my trees and shrubs yellowing?
Yellow leaves and green veins on trees and shrubs indicate a condition commonly called chlorosis. Although this is a general term to describe leaf yellowing, with certain plants like pin oak, sweetgum trees, rhododendrons and azaleas this condition is quite common and reflects a nutrient imbalance often caused by alkaline soil.
Soil alkalinity or acidity is measured on a pH scale ranging between zero and fourteen. A soil with a pH reading of 7 is considered neutral, a pH reading greater than 7 is alkaline, and a pH reading below seven is acid. This nutrient imbalance is more accurately called "alkaline soil induced chlorosis". Some nutrients, particularly iron and manganese, chemically change in alkaline soils to a form plants cannot absorb through their roots. Signs of this problem show up in the new leaves, which may be small and appear yellow with green veins. The yellowing can appear on a few branches or the entire plant, and usually becomes worse over time. In severe cases, leaves turn brown and branches die.
Other factors affect chlorosis development in plants. Lime increases soil pH. Plants growing near lime-leaching sources like concrete in sidewalks, driveways, building foundations, or marble or crushed limestone ground mulches, can develop chlorosis more readily. Poor horticultural practices such as liming alkaline soils or applying too much lime can cause chlorosis. Plants suffer from chlorosis more readily in poorly drained, poorly aerated soils, compacted soils, excessively wet soils, or soils affected by extended periods of drought.
Susceptibility to this problem varies by species, but in general plants that thrive in acid soils are most likely affected. Pin oaks are particularly susceptible. White oaks, some maples, rhododendrons, azaleas, river birch and holly are also susceptible.
The best cure for this problem is prevention. Have your soil's pH tested before planting, then select plants able to thrive in your soil type. If you must plant acid-loving plants in alkaline soils, amend the soil with sulfur and organic matter to increase acidity before planting. The addition of soil acidifying agents after planting may be necessary to keep the plant chlorosis free. Refer to Hortline message #103 for more about acidifying soils.
Currently, the most widely recommended treatment for alkaline induced soil chlorosis is focused on lowering the soil pH. This can be a lengthy process taking several years. For many years chlorosis cures focused on treating plants with iron as iron deficiency was thought to be the primary cause. While true iron becomes deficient in alkaline soil, treating plants only with iron is an inadequate solution. If the chlorosis is caused by low levels of manganese, the addition of iron will not cure the chlorosis. Alkaline soils also make phosphorus and potassium more available, which can affect the chlorosis problem in certain plants. Because of these complex nutrient imbalances, adding specific nutrients to the soil like iron is no longer generally recommended. These treatments frequently fail and may, in fact, worsen the problem. Monitoring and correcting the soil pH and improving the planting site prior to planting are the best ways to avoid this soil nutrient imbalance. For established plants, working sulfur into the soil in the root zones, reducing compaction, improving drainage, and fertilization may lessen the problem. Stem or trunk implants with sequestered iron or iron citrate may temporarily alleviate chlorosis. A arborist should be consulted when considering this alternative.