How can I keep a poinsettia for next Christmas?
It is a somewhat tedious job to get the poinsettia to rebloom at the desired time. Poinsettia are called photoperiod (or short-day) plants and need complete darkness at night to develop coloured leaves (blooms). Cut the stems back to a height of about 8” (20 cm). Continue to water the plant and give it a good, balanced all-purpose fertilizer now if you haven’t done this since it stopped blooming.
The poinsettia should be kept outdoors once there is no further risk of frost, in a location that is partially shaded – not direct sun, but not too shady either. Keep it well watered during the summer and fertilize it lightly every 2 – 3 weeks with a balanced all-purpose fertilizer. It may need to be pruned during the summer to keep it bushy and compact – don’t prune after end of August.
Between Labour Day and mid-September, move the plant inside and continue watering and fertilizing the plant on a regular basis. The demanding part of the process now begins – you need to ensure that the plant has the necessary lighting conditions for the next 3 months. Without this it will not develop the coloured bracts (which are modified leaves) that we all love so much.
For 6-8 hours during the day, the poinsettia needs bright sunlight and temperatures between 60 – 70F. Every night, it requires complete darkness for 14 hours – not even light leaking in under a door. Some people achieve this by placing a large completely light-tight box/carton overthe plant each night. Alternatively, an unused closet with the gap at the bottom of the door blocked would work.
This regimen needs to be continued for 10 – 13 weeks (the length of time differs for different cultivars of poinsettia). Following this, the poinsettia should have developed its coloured leaves in time for next Christmas. Unfortunately, if the poinsettia gets any stray light during the 14-hour night or you forget to give it the completely dark, long night even once during the period, it will be set back and may not develop the coloured leaves.