First of all, I commend you for maintaining you poinsettia past "prime." However, most horticulturist will tell you it's not worth the effort to carryover your poinsettias. BUT, it can be done and you're not too late, so if you like a challenge, here goes:

Poinsettias are short day plants. That means that they start the blooming process when the hours of darkness exceed the hours of daylight. In my area (NC) the magic day (when it is naturally dark longer than it is light) is around Sept 23. To rebloom your poinsettia, you need to expose it to natural daylengths from that point on. Excess light will delay blooming (and the turning of green to red).

As an aside, the poinsettia flower is actually the yellow centers, the red part are modified leaves called bracts.

It might sound easy, but remember, it will be getting darker earlier and earlier. If the plant is in the house, you probably won't want to turn the lights off at 5:30. Some folks keep the plant in a separate room than can receive adequate sunlight and will be dark with nightfall, others move the plant in and out of a closet daily. If in a warm area, where the plants are grown outside, streelights can affect the flower timing.

The light is the key element, but you should also fertilize it and "pinch" it to maintain a compact shape. Pinching removes the end of the branch and encourages more flowers. Just be sure to leave 4-6 leaves on every branch you pinch. (If you want to try next year, I believe the recommendation is to cut the branches back and let the plant send out new growth when the flowers fade.)
If they don't manage to color up by Christmas, don't give up, it will change eventually, but you might want to purchase some new ones for holiday decor. In commercial production, greenhouses are located away from artificial lights unless they are purposefully lit to offer fresh flowers late in the season.

How long did your poinsettia stay red? My grandmother would consistently keep hers going through Easter. Even if you decide the reblooming effort isn't worth it, they can be lovely green foliage plants through the summer, even planted in the garden (just know the frost will get them).
Good Luck!

Hold on! Do you live in a tropical area? Poinsettias can't be left outside to freeze...all that work will result in limp, black leaves. 62 degrees F is the suggested minimum temperature, but they'll usually withstand as low as 45. Lower than 45 is really pushing your luck.

Glad you've found a new gardening niche with the containers!

I live in zone 7 and my poinsettias are stil red and doing well from Christmas.  I would like to see if I can keep them until this coming Christmas.  Should I keep them in the shade or sun?  Plant in ground or container?

May 8, 2009 -- 10:14AM, Melissa wrote:

I live in zone 7 and my poinsettias are stil red and doing well from Christmas.  I would like to see if I can keep them until this coming Christmas.  Should I keep them in the shade or sun?  Plant in ground or container?

Melissa, I'd transplant your poinsettia into a larger pot (say a 12" pot from the 8" pot they are currently in, etc) and then sink the pot into the ground to grow over the summer.

By keeping it in the pot, it will allow you to dig up the plant in fall before the killing frost and more easily bring it indoors with minimal transplant shock, etc. You just pull up the pot, rather than hack and cut up all the rootball. There are issues of timing the undisturbed darkness in September/October so your leaves redden again but that's another discussion on another thread! ;P

By sinking the pot outdoors, it won't tip in winds or dry out as quickly since the roots are protected by soil and kept cooler, etc. Do plant the pot in a spot that usually isn't hit by winds and doesn't flood during a rainstorm.

You may wish to pinch back the grow on the poinsettia this summer so it doesn't become a lopsided or leggy monster by fall. You ideally want a full, rounded plant around 3' around and tall, correct?

So far so good, my poinsettias have at least doubled in size and look great!  I am still trying to decide where I am going to put them in the fall to make them turn red.  Maybe my garage?