If you saved last year’s poinsettia plant, and you want it to flower in time for the holidays, now is the time for action. Poinsettias need about 8 weeks of special handling to force them to re-bloom.
Poinsettias are sensitive to photoperiod — the length of the day. Actually, it’s the number of hours of darkness that is most important. Poinsettias flower during short days, with long periods of darkness each night. In the home environment, even a dim lamp is enough to delay the initiation of flower buds.
To get your poinsettia to re-flower, place the plant in complete darkness for 15 hours each day, for instance, between 5 p.m. and 8 a.m. the next morning. A dark closet that is not frequently used is ideal. Any interruption of the dark period, including merely opening the closet door, can result in delayed flowering.
During the day, the plant should be given a sunny location. Water as needed to allow a slight drying of the soil between applications. A fertilizer that is formulated for blooming houseplants is also helpful; follow the label recommendations for rates and frequency.
To have your plant flowering in time for the holidays, you’ll need to begin the dark treatments around the beginning of October. The treatments can be discontinued when the plants begin showing good color, usually by early December.