New leaves on rhododendrons or azaleas sometimes unfurl bright yellow rather than green. The yellow is made all the more striking in contrast with the leaf veins, which remain green. Although many find this variegation attractive, it is the symptom of a problem that can kill a plant.
You might say that the plant has "tired blood," because the symptoms are of iron deficiency. Plants don't have blood, of course, but iron is needed for chlorophyll, which makes plants green (and actually has a similar structure to blood hemoglobin).
Before dosing the soil with iron pills or rusty nails, know that there is probably enough iron in the soil. Deficiencies usually occur when a plant cannot absorb iron because the soil is too wet or not acidic enough, or has too much clay in it.
Plants in the heath family -- rhododendron, azalea, mountain laurel, and blueberry -- need soils that are especially acidic and well-aerated, and thus are prone to iron deficiency.
Pin oak, gardenia, and citrus also are commonly afflicted. Problems frequently occur on "foundation" plants near homes because of alkaline-building rubble buried near the foundation.
Correcting soil conditions takes time, perhaps more than an ailing plant has to offer. In such cases, give the plant iron sulfate or iron chelate. Sprinkle the powder on the soil, then water it in thoroughly. A shrub or small tree needs a quarter of an ounce to two ounces, depending on plant size; a potted plant needs about a half-teaspoon. For a plant severely deficient in iron, spray iron solution, mixed according to the package directions, directly on the leaves. Within days the leaves will turn dark green.
Avoid iron deficiency by checking soil conditions before planting. If the soil is too acidic, correct the problem with sulfur. The amount to use depends on the change in acidity needed and the soil type. Maintain soil acidity by using natural fertilizers such as soybean meal or chemical fertilizers whose nitrogen is in the form of ammonium.
Organic matter can help prevent iron deficiency. Mix some decomposed organic matter, such as peat moss or leaf mold, into the planting hole for a rhododendron, azalea, or other acid-loving shrub. Organic materials acidify the soil, loosen it to prevent waterlogging, and are sources of naturally occurring chelates.
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