16 December 2012
Poinsettias are tender, tropical plants, and as such, do not do well at our house as a rule. But since my in-laws gave us one this year, I'm determined to take care of it the best that I can. So I set out to learn more. During the winter, poinsettias need about six hours of bright, indirect light. They tolerate temperatures between 50 and 70 degrees, but they should avoid drafts if at all possible. They like moderately moist soil, but they can't tolerate standing water - that can happen if you water your poinsettia while it's sitting in the decorative foil it came in. Take it out to water and make sure all of the water has drained before putting it back in the foil. While it is at its most colorful, showing off its red (or another color) bracts - modified leaves, not flowers - there is no need to fertilize.
It is now spring, and if you still have your poinsettia, you will notice that the colorful bracts have died back. Prune the plant back to eight inches, and once nighttime temperatures stay above 50 degrees, take the plant outside - remember, it need bright, indirect sunlight. At this point you can begin fertilizing every 2-3 weeks with a complete fertilizer, i.e. 10-10-10, and in June transplant into a pot about 2-3 inches larger than the one it is in, making sure to add lots of organic matter like peat moss, leaf mold or compost. If you want to prune it again, you can, but do it before September 1st. When nighttime temperatures dip to 55-60 degrees, it's time to bring it back in. Now all of that was easy. Now it's time to get it to rebloom.
So, you've really committed to this plant, huh? If you thought it needed lots of attention before, just wait. Poinsettias, like other Euphorbias are what's called "short day" plants. Some plants need shorter days - actually longer nights - to initiate the flowering process. With houseplants being in the environment they are in, they don't experience the natural change in sunlight, so they have to be given it artificially. Begininning the first week of October, poinsettias need 14 hours of continuous darkness each night for 10-12 weeks. That means moving the plant to a dark closet, or putting a box over them from roughly 5pm to 7am every day. During the day, they will need 6-8 hours of bright indirect light. If you have done all of this, your poinsettia should begin to bloom - form colorful bracts - in November or December, depending upon the cultivar.
Wish me luck with my efforts with Poinsettias, and if you want more info on this or other "short day" plants visit HGIC 1561 Poinsettia or Wikipedia's article on photoperiodism.