You can enjoy the challenge of colouring poinsettias for a second season by getting a head start on next Christmas. It is not difficult, but there are rules that have to be taken into account and acted upon!
Preparation is all important to ensure that your Poinsettia is ok for next Christmas. The preparation should start taking place immediately it has finished its colourful season, but have no worries - if you miss out on that you can catch up later.
About 2 weeks after receiving the plant, fertilize it with a complete fertilizer. This two week ‘rest’ period is on the basis that the nursery that grew your plant, would have been feeding on a regular basis prior to sending it to your retailer. The Poinsettia plant should then be fertilized every 7-10 days until loses its bracts (coloured leaves).
Be sure to keep the soil moist until the plant drops its leaves. Then let the soil dry out and don't water it until the stems begin to shrivel or crack. At this stage keep the plant in a cool, sunny place such as a basement window ledge. Basically, this will be sending the poinsettia into dormancy.
The other alternative is to keep the plant growing through the summer months – when it will at least double in size! You can keep it smaller by taking cuttings in early summer.
If using the dormancy method - Late April or early May, bring the plant out of its 'resting' stage. Cut the stems back to 3-5" from the soil and re-pot the plant if necessary. After re-potting, water whenever the soil begins to dry out - not too much until the new growth starts.
Once the new shoots are an inch long, apply a complete fertilizer. When the new growth is 4-6" long, prune the plant to form the desired compact shape.
In mid September, your poinsettia will need complete darkness every day from 5 pm to 8 am so put a cardboard box over the plant to provide the necessary "short day". Continue the "short days" until the plant's bracts show colour sometime in late November or early December. The night temperature should be 60-65F to ensure flowering of the plant.
In areas of mild or hot summers, Poinsettias can be grown on outdoors until autumn/fall when time to bring indoors before the frosts. They are surprisingly hardy. I have found one growing in the open ground near the summit of Thailand's highest mountain - also in several other high altitude areas! Not always totally frost free areas. I suspect that once established, the Euphorbia can probably survive a degree or so of frost! It will need a few seasons of frost-free growth first though - I think.
If you use this method – you will need a large cover to start the ‘day length’ process.