|My hibiscus tree leaves are turning yellow as well as my gardenia shrub. I do not know where to start.|
|Although hibiscus are sold in the low desert, as a tropical plant they are not ideally suited to our conditions, so our arid environment can take a toll on them. I assume that you purchased a variety that is bred to do better in the desert. You didn't say how long you've had the plant or how you're caring for it, so it's difficult to pinpoint the problem. I've included some info on potential problems as well as care info.
Yellow leaves can be caused by many things including lack of nitrogen, insufficient light, water-logged soil (plant roots need oxygen to thrive), dry soil, or iron deficiency. If the older bottom leaves are yellow, but new growth is green, it's usually a lack of nitrogen. If new leaves are yellow, with green veins, it's usually a lack of iron. Soil should be kept moderately moist (but not wet). Finally, transplant shock can contribute to yellowing. If new growth shows up as green, that might be the problem. Try to isolate each of these possibilities one at a time to determine the problem.
Iron chlorosis is a problem in the desert for non-native plants. Chlorosis is recognized by new leaves that are yellow, while the veins remain green. If the condition is severe, the entire leaf may be yellow. Although iron is present in the soil, it is not always in a form that non-native plants can use. (Native plants seldom suffer from iron chlorosis.) Overly wet soils are depleted of oxygen. (As water fills in the minute spaces between soil particles, air moves out.) Plant roots need oxygen to absorb iron in the soil. To help prevent chlorosis, always water slowly, deeply and infrequently.
Soil with a high pH (alkalinity) also inhibits iron absorption. If you are using correct irrigation methods and symptoms are still present, apply iron chelates or ferrous sulphate to the soil. Both are readily absorbed by a plant?s roots.
For about two months after transplanting, keep the soil moist (not wet) around the entire rootball out to the plant's drip line (canopy), so that it can establish a strong root system. Hibiscus are frost-tender, so monitor weather forecasts and protect if frost is predicted. Fertilize in February at the start of the growing season. Hibiscus also prefer acid soil conditions and our desert soils are alkaline, so you might want to use Miracid. Read instructions and follow carefully. Water thoroughly before and after applying any fertilizer to reduce chance of burn. Summer is considered a dormant period for many of the non-native plants that grow here. It's best not to "force" them to produce at this time with fertilizer. It stresses the plant and may cause fertilizer burn. Provide protection from the hot afternoon sun and strong winds. Water slowly and deeply, and let the water leach salts past the root zone. "Sprinkling" with water lightly and frequently allows salts to accumulate in the soil, burning roots. Salt burn shows up as yellowing and browning along the edges of the leaves. Fertilize again just as weather cools in the fall. This information applies to gardenias as well. They are non-native, and even less adapted to desert conditions. If you continue to have problems, you might consider switching to desert-adapted plants, which suffer far fewer problems. Ask your city water conservation department (480-644-3306) for a free copy of Landscape Plants for the Arizona Desert. It contains photos and details on over 200 fabulous, colorful plants that are relatively low-maintenance, especially in comparison to gardenias! I hope this info helps.