Answer: Poinsettia leaves change color because they are 'short-day' plants. In order for the leaves to change color, the plants need to be kept in total darkness for 12-14 hours every night beginning in September. The best way to provide the necessary darkness is to place your plant in a closet or cover it with a bucket or carton to block out all light, even from a streetlight or lamp. It sounds finicky, which is why many people purchase new plants every year. But it can be fun to see your old plant friend change color as the holidays approach.
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