Causes of Yellow Plant Leaves

Nutrients, environment are common causes for yellow leaves

Bill Sevier:  Ask a Master Gardener

Saturday, May 21, 2016

Q: My azaleas have some yellowing of their leaves and did not bloom well. Any suggestions? R. T., Tulsa

A: There are several factors that can cause yellow leaves and poor performance in all plants. Although various insects and diseases may produce this, far and away the most common causes fall into the nutrient and environmental category.

When planting anything, it is best to know what type of growing conditions the plant prefers and should be coupled with a recent soil test. If there are any mismatches, they should be corrected before planting.

Azaleas must have acid soil (low pH) to absorb iron and other nutrients. If the soil is not acidic enough, the plant cannot absorb iron and iron chlorosis develops, as seen in the photograph. With iron deficiency, the leaves are yellow except for the veins, which remain green. This is called “interveinal chlorosis.”

Another nutritional deficiency that produces yellow leaves is lack of nitrogen. Among the several nutrients needed by all plants, nitrogen is used in the largest amounts and is the major component of most commercial fertilizers.

Nitrogen deficiency causes the leaves on the lower part of the plant to turn yellow before the upper leaves are involved; while iron chlorosis tends to affect all leaves similarly. A soil test will identify nitrogen deficiency and most any type of organic or commercial fertilizer will correct it.

Environmental changes involving temperature, light and water are common causes of yellow leaves in all types of plants, including houseplants. Houseplants often develop and shed yellow leaves when being moved from indoors to outside or vice versa. The yellow leaves are an adaptation to change in light and temperature. It has no significant effect on the plants health.

This time of year too much water in the soil commonly causes yellowing of leaves in shrubs, ornamentals and in the vegetable garden. This is due to water forcing out oxygen from the soil which causes the roots to suffocate. If the roots cannot function, the plant cannot obtain adequate amounts of not only oxygen, but water and nutrients. This produces yellow leaves and poor health of the plant. Over watering houseplants produces a similar condition and is probably the number one cause of yellowing of houseplant leaves.

The other causes of yellow leaves such as disease and insect pests, are often evident from the appearance of the leaves. However, root diseases such as root rot, acts exactly like overly wet soil and causes the roots lose their absorbing ability.

It sometime is confusing when evaluating a yellow leaf with fungal spots as to which came first, the yellowness due to an environmental condition and the fungal infection being an opportunist, or the fungus being the primary problem.

The Tulsa Master Gardeners can help with all these problems. Call 918-746-3701 or come by with photos or samples to the OSU Extension Office at 4116 E. 15th St. at the fairgrounds, gate 6.

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For more information or to ask a question about gardening, contact the Master Gardeners at 918-746-3701 from 9 a.m.-4 p.m.

Garden tips

§  Clean out water garden and prepare for season. Divide and re-pot water garden plants. Begin feeding fish when water temperatures are higher than 50 degrees.

§  Plant warm-season vegetable crops such as watermelon, cantaloupe, cucumber, eggplant, okra, sweet potatoes, etc., now.

§  Late May is the best time to control borers in the orchard. Contact OSU Tulsa Master Gardeners for fruit tree spray recommendations.

§  After all the recent rains, you should let soils drain from soggy to a moist condition. Spading or tilling wet soil will cause collapse of small air spaces, producing long-term damage.