Poinsettias (Euphorbia pulcherrima) are desirable for their colorful bracts, or modified leaves, ranging in hue from creamy white to brilliant red. Some are pink and others are variegated. Poinsettias are often grown as potted plants and given as gifts during the holiday season. Although attractive, they are not the easiest plants to grow and often suffer from dropped or yellowing leaves. This problem is usually caused by one of a number of environmental factors.
Poinsettias need exposure to bright but indirect light each day. Place them on a windowsill that gets at least six hours of sunlight each day, but not direct sunlight, as this will fade the color of the bracts. Leaves that do not get enough sunlight may turn yellow and curl up.
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Another reason to avoid direct sunlight is that it may become too hot for the plant. Temperatures that are too high (above 70 degrees Fahrenheit) will cause the leaves to turn yellow and drop off, according to Clemson University. The same holds true for cold temperatures. Never expose your poinsettia to temperatures below 50 F, or let the bracts touch a cold windowpane.
Poinsettias thrive in moist soil – but not too moist. Water your poinsettia only when the top layer of soil feels dry to the touch. Allow the water to drain freely from the plant. Too much water will cause the lower leaves to droop, turn yellow and fall off the plant. Ironically, too little water will also cause the leaves to yellow and drop, so never let the soil dry out entirely.
Disease and Pests
Diseases can also cause the leaves of a poinsettia to turn yellow or brown and curl up. Many of these are fungal diseases that are caused by over watering. Root rot in particular can destroy the roots of the plant, causing the plant to wilt and the leaves to turn yellow, shrivel and fall off. Insect pests such as whitefly and scale are common to indoor poinsettias and will suck the juices from the leaves, causing them to dry up.