Poinsettias

To keep your poinsettia as a green plant:

In late April (or as soon as your poinsettia loses its colourful bracts), prune your poinsettia, cutting all the stems back by a third. Be sure to spray a mist of water on the plant to limit the seepage of latex and prevent it from drying out. Repot the plant in an aerated, well-drained pot.

Give it only enough water to prevent the soil from drying out completely. Apply a soluble fertilizer, such as one labelled 20-20-20,* once a month. After pruning, a temperature of 20°C is ideal.

Once growth resumes, give water as necessary, allowing the soil surface to dry out between waterings. During the growing period after that, indoor temperatures of 20-24°C are adequate. Once new leaves appear, increase the frequency of fertilizing (every two weeks) to promote vigorous growth.

Once all risk of frost has passed (and ideally once temperatures remain above 13°C), take the plant outside for the summer. If you cannot take your plant outside, give it as much light as possible, even direct sunlight. When outside, especially under full sun, be sure not to let it wilt!

Prune the plant again in late July or early August.

To make your poinsettia reflower:

Remove any weak stems, keeping only the most attractive ones. To get a poinsettia to bloom again in time for the holiday season, it needs a period of total darkness every day. From mid-September onwards, give the plant a minimum of 14 to 16 consecutive hours of complete darkness every single day of the week, for eight to ten weeks or until the bracts start showing colour.
If the plant isn't given total uninterrupted darkness, the bracts may be malformed or the flower buds may drop off.

Remember that the plant needs bright light in the daytime.

It is also important to keep the temperature at 20°C to encourage good-sized bracts to form. Once the bracts are fully coloured, you can lower the temperature to 15-18°C to intensify the colour. Fertilize with a 20-20-20 product every two weeks. Starting in November, fertilizer with more potassium, like 15-15-30, will help promote flower formation.

Useful link: http://www2.ville.montreal.qc.ca/jardin/en/info_verte/poinsettia/soins.htm