In the Garden:
Southwestern Deserts
September, 2003
Regional Report

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Iron chlorosis on citrus shows up as yellow leaves with green veins.

Yellowing Leaves

Yellow leaves on desert landscape plants are usually caused by iron or nitrogen deficiency. Iron deficiency is the more likely candidate at this time of year, particularly on non-natives, such as citrus, roses, queen palms, and hibiscus. Iron chlorosis appears on new, young leaves, which turn yellow while the veins remain green. If the condition is severe, the entire leaf may turn yellow.

Iron Chlorosis
Iron chlorosis occurs when plant roots can't absorb existing iron from the soil, either because the soil is too wet or the soil pH is too high (alkaline). That's why it sometimes appears during the summer monsoon season when rains are heavy or frequent and saturate the soil. Plant roots need oxygen to absorb iron from the soil. Too much rain soaks the soil, basically forcing out available oxygen molecules and filling their spaces with water. Another factor is that non-native plants aren't adapted to the alkaline soil, so they may struggle with nutrient deficiencies. Native plants are already adapted so they are less likely to suffer from iron chlorosis.

The Remedy for Chlorosis
Since wet soil can worsen the problem, ensure that you are watering plants efficiently. Check irrigation systems for leaks or errors in the automatic timer. Sometimes thunderstorms can knock out the programs. This happens to me, and I notice it when the system, which is near my bedroom window, turns on with sufficient vigor in the middle of the night to wake me up. If you are overwatering to the extent that soil stays constantly wet, reduce watering frequency. If rains are heavy, skip automatic irrigations. Often times, the problem will correct itself when irrigation problems are addressed and/or monsoons end. If chlorosis continues, supply iron in a chelated form, which is more readily available for uptake by the plant roots.

Nitrogen Deficiency
In this case, the entire leaf turns yellow, but it is the older, lower leaves that typically show the deficiency. Again, natives are less likely to suffer as they have adapted to the lower amounts of nitrogen found in desert soils. In addition, many desert plants are legumes, which are capable of {"fixing" their own nitrogen in a symbiotic relation with soil bacteria.

Correcting Nitrogen Deficiencies
One of the reasons I like native plants is that they seldom suffer from nutrient deficiencies. Non-natives will probably require an application of nitrogen fertilizer, but don't apply it during the heat of summer. It's too stressful for the plant and may cause fertilizer burn. The best time to apply nitrogen is late winter/early spring just as new growth begins. A light application at the end of summer/early fall also can help stressed non-natives.


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