Texas Spur readers who saved their poinsettias from last Christmas should start now to initiate the flowering process for this season. Botanically, the showy structures of poinsettias are modified leaves, called bracts, while the flower of a poinsettia is the small yellow blossom in the very center of the plant. The small indistinct flower is surrounded by the showy colorful, usually red, bracts.
The process of initiating a poinsettia to rebloom requires several weeks. Starting a regime now gives time to have colorful poinsettias by Christmas.
Flowering in poinsettias is photoperiodic. Flower initiation in poinsettias is dependent upon exposure of the plant to a critical day length. Poinsettias are known in the floriculture trade as “short-day” plants, which means they initiate flowers when exposed to a short day length. However, it is actually the length of dark that is critical in forcing poinsettias to rebloom.
Without long nights, poinsettias will continue to produce leaves and will grow but will never flower. You must make certain it receives no light from any source.
Depending upon the variety, poinsettias require approximately 60 days under a short day / long night regime to express full color. A strict regime of 14 consecutive hours of total darkness must be followed until color begins to show.
If a plant is grown in a room that is lighted at night, the dark period can be accomplished by placing the plant in a secure room or dark closet or covered with a box or black plastic from which all light is excluded for 14 hours. Note that even a few seconds of an incandescent light bulb or flashlight will interrupt the transition to flower initiation, requiring the plant to reset its flowering response. Following the dark period, bring the plant into full sun for six to eight hours. This regime must be followed every day.
Once developing flowers are visible, and when the floral bracts start to show color, it is not critical to continue giving the dark period, but it is advisable to continue until the bracts are almost fully expanded.
Maintain soil moisture but between waterings allow the surface of the soil to dry out. Water thoroughly without wetting the leaves; never let the plant to sit in water. Fertilize every two to four weeks with dilute complete formulation such as 20-20-20. High night temperatures, low-light intensity, and dry soil or may delay flowering.
Ellen Peffley Harp, a retired professor of horticulture at Texas Tech University, writes about gardening for several Texas newspapers.
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