How to get over wintered poinsettias to turn red for the holidays
Although - sadly - the vast majority of poinsettias get tossed out after the holidays, there are those who like to keep them going as a house and patio plant. That's cool!
There is a very simple but specific process required to get the top growth to turn holiday red, if you have an over wintered plant.
Here are the three key steps:
1) Make sure your plant is generally healthy. If pot bound, repot using a composty organic potting mix. Water regularly and deeply, but ensure your plant has great drainage. Feed monthly with a general-purpose organic plant food. Avoid heat vents/forced air as much as is possible.
2) Provide about 14 hours of total darkness, starting 6 - 8 weeks before desired colourful period. When growing poinsettias for colour, darkness means complete darkness; even a crack under the closet door or a small night light will prevent the proper process for turning colour. The best thing is to put the plants in a room with no natural light sources, a closed door, and no artificial light sources near-by.
3) During the day, provide lots of direct sunlight. When not in dark seclusion, poinsettias require a nice bright location. If not available, full spectrum grow lights will do the trick.
Remember though: 14 hours of darkness to 10 hours of light - very important.
So there you have it: an easy but also interesting project, and a good reason to keep your holiday poinsettias year-to-year. Speaking of that, after the holidays poinsettias naturally drop leaves and take a little break. It is a good idea to give them a hard-prune at that point (cut back by about half). As they regrow in late winter you can up-size the container for a bigger and better show the following season.