Extension Educator, Horticulture
There seem to be two camps when it comes to what to do with poinsettias after Christmas. One camp's motto is "I'm so glad Christmas has passed; now I can let the poinsettia die." The other camp cheers, "This poinsettia is so pretty I must keep it to flower again." Getting poinsettias to reflower next year is possible, but it takes diligence. So for all of you poinsettia keeper campers, here is the process.
After Christmas, grow the poinsettia as a houseplant. Keep it evenly moist and in fairly bright light.
In February or early March cut back each of the old flowering stems to 4 to 6 inches in height to promote new growth.
In May repot into a slightly larger pot. Water well and place in a sunny window. When all danger of frost has passed and night temperatures are above 60°F, the plant can be placed outdoors in a shady location. Sink the pot in a protected outdoor flowerbed. Some morning sun is ok.
Periodically turn the poinsettia pot to prevent rooting through the bottom hole and to keep the shape more uniform.
To have a short plant with many flowers, pinch out the top one-fourth inch of the growing shoots to encourage branching. Do this in three to four week intervals, according to the speed of growth. Two or three large fully expanded leaves should be left below the pinch; this serves as a guide for knowing when the shoots are ready for pinching. Continue this practice until mid- August, when the plant should have a satisfactory shape and number of shoots.
Keep the plant growing actively all summer by regular watering and fertilizing every two weeks with a complete soluble fertilizer (20-20-20).
Before night temperatures fall below 55-60°F at night, lift the pot and drench the leaves and soil with water to help remove any pests. Bring the poinsettia indoors to a sunny location. Keep moist but reduce fertilization.
With poinsettias, as well as Christmas cactus and kalanchoe, flowering is "photoperiodically" induced. This means that flowers begin to form when the days are a certain length, or, more accurately, when the nights are long enough. The poinsettia is a short-day or long-night plant. Without long nights, poinsettias will continue to produce leaves but will not flower.
Very short periods of lighting at night may be enough to prevent or interfere with flowering. Even light from a streetlight can stop flowering. If the plant is to be grown in a room that is lighted nightly, cover it completely at dusk (5 p.m.) every day with a heavy paper bag, a piece of opaque black cloth, other light-tight cover or place in a dark closet. However they must receive light during the day.
Flower initiation begins in late September and early October. Dark periods longer than 12 hours are necessary for flower set.
Because flower initiation depends upon the length of the dark period, your poinsettia must be kept completely dark from 5 p.m. to 8 a.m. In order to get them in flower at Christmas, this treatment should be from the end of September until December 15.
Once you can see the flowers developing and the bracts show color, it is not as important to continue giving the dark period, though it is advisable to continue until the bracts are almost fully expanded.
Temperatures should be no less than 55°F at night, but not more than 70°F.
High night temperatures, coupled with low-light intensity, low nutrition, dry soil or improper photoperiod may delay flowering.
If all this seems like a lot of work, then it's time to change camps and leave poinsettias to the professionals. For more poinsettia info, check out www.urbanext.uiuc.edu/poinsettia