At the beginning of spring, in March or April, cut the poinsettia back to about 8 inches. This will encourage it to begin a new growing cycle and get it ready for transplanting. Keep it watered and fertilize it once a month or so until early summer, when it’s time for transplanting.
Dip the ends of each cutting into a rooting hormone, then insert it into a pot filled with potting soil or a vermiculite mix. Keep the soil in the pot moist, but not wet, for several weeks while the cuttings make roots.
If the summer temperature is warm enough and never drops below 65 degrees at night, you may keep the poinsettia outside during the growing season. Place the plant in an area that gets partial shade. [5] X Research source
Move the poinsettias to an area where they receive total darkness for 14 to 16 hours a day in late September or early October. A cool closet is the best location, but if one is not available, you can also put the plants into a large box for their uninterrupted dark hours. Any light exposure during this time will delay the color change. Keep the plants in total darkness at the time when temperatures are coolest. The best hours are from 5:00 p. m. to 8:00 a. m. Poinsettias flower best when nighttime temperatures are 55 to 60 degrees Fahrenheit (12 to 16 Celsius). Bring the plants out of darkness every morning and put them near a sunny window where temperatures stay around 70 degrees Fahrenheit (21 Celsius).
Prune the plants heavily to 8 to 10 inches (20 to 25 cm) tall. This is a good time to take cuttings for plant propagation. Cut back on watering for a couple of months until it is time to start new growth in the spring. Allow the top several inches of soil to dry out before watering them.
