Jorielle's Blog

http://www.hort.purdue.edu/
ext/HO-073.pdf 

Sadly, most potted Poinsettias end up in the garbage after their bloom period is done. It doesn’t have to be that way if you have the time and patience to do what's necessary to make your Poinsettia rebloom.

When the leaves have fallen (usually by late March or early April), cut the Poinsettia back to about 8" in height. Continue a regular watering program, and fertilize your plant with a good, balanced. By the end of May, you should see vigorous new growth. Transplant the Poinsettia into a larger pot (no more than four inches larger than the original pot).

Continue regular watering during the growth period, and fertilize every two to three weeks. Once all chance of frost has passed and night temperatures average 55° F or above, you can put the Poinsettia outside (being sure to bring it back in if there's any chance the temperature will fall below 50° F)! If pruning is necessary to keep the plant bushy and compact, be sure pruning is done by September 1.

Then comes the hard part; complete light control. From October 1, the Poinsettia will need to be kept in complete darkness for 14 continuous hours each night. You’ll have to cover the plant with a black polythene bag or a large box, as any stray light, such as that of a streetlight or lamp, can stop the re-flowering process.

In the daytime, from October through December, the plant will need 6 - 8 hours of bright sunlight daily, with night temperatures between 60 - 70° F. Temperatures outside of this range can also delay flowering.

Following this regime faithfully for eight to ten weeks should give you a flowering Poinsettia for the Christmas season; the actual bloom time will depend on the particular variety.