Even though poinsettias are native to Central America, you should keep them in a relatively cool environment. They do best in temperatures from fifty to seventy degrees fahrenheit. Temperatures above seventy degrees will cause the leaves to turn yellow and fall off with chilling temperatures causing a similar reaction. Similarly, keep your poinsettia plant out of direct sunlight. Instead, place it where the plant can get at least six hours of indirect sunlight every day and remember to water it regularly.