October 10, 2015

1 - During the fall poinsettia plants need daytime sunlight to photosynthesize and long dark nights to induce flowering if they are to look good for the holidays.

Yard and Garden October 10, 2015

Q.

I was able to keep my poinsettia alive all year and now I need to try to make it bloom again. I know I have to put it in the dark to stimulate flowering, so I put it in the closet every night. I have been doing that for a month now and it is a lot of trouble bringing it back out every morning. Is it really necessary to bring it out every morning? Why?

A.

Yes, the poinsettia needs light every day during the flower induction period. You need take it out of the closet to properly care for the plant - to water it and to watch for insect pests. However, the most important reason is that the plant needs food to survive. The food it needs is the carbohydrate that it manufactures from sunlight when it photosynthesizes. If it remained in the closet for several months, the leaves would lose their green color and perhaps fall from the plant. The bracts, the colorful leaves just below the true flowers, would not develop good color for the holidays if the plant remained in the closet.

It is a lot of trouble carrying the plant to the closet every evening and back into a well-lit location every morning so that it can receive sunlight and photosynthesize. Perhaps it would be easier to cover the plant with opaque black fabric or black plastic each night. If your plant is small enough, you might be able to cover it with a cardboard box. The key is that no light gets through to the plant during the night if there are lights on in the room, or if car lights or street lights shine on the plant during the night. Even a brief exposure to car lights could delay or prevent development of colorful bracts and flowers. Cardboard boxes often have places where light can penetrate and you may need to cover that with duct tape or some other opaque tape. Remove covering, especially black plastic, early enough to prevent heat from accumulating when the sun shines on it. However, the plant should experience at least 14 hours of uninterrupted darkness during the period of floral induction.

Poinsettias are known as "short-day" plants. This name was given before scientific studies determined that it was not really the length of the day, but the length of the uninterrupted night that induced flowering in short-day, long-night plants. Interruption as brief as the flash of a photographic strobe light or as dim as a flashlight at any time during the night period can inhibit or delay flowering in these plants. So, your practice of providing darkness from September until the holidays is a good thing. For those who are not as interested in the challenge of carrying a poinsettia plant through the winter, spring, summer, and into the fall to induce flowering again, purchasing a new plant is a lot easier, but nearly as much fun. Best wishes for a beautiful poinsettia this winter.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

Links:

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