To keep a poinsettia year round, you will need to move it into the outdoor garden once there is no danger of the temperature falling below 45 degrees Fahrenheit (7.22 degrees Celsus). The best place to plant your poinsettia outdoors is a spot where it will get sunshine in the morning. When you move your poinsettia outdoors, also cut the plant back to six inches above the ground, but make sure to leave a third of the foliage intact.
If you like, you can pinch back the tips of new shoots once they emerge to encourage bushier growth from your poinsettia. Water the plant whenever the top inch of soil is dry, and fertilize it to keep it healthy while outdoors.
Before temperatures fall to reach 45 degrees Fahrenheit in the fall, you need to dig the poinsettia up and bring it inside. You will likely need a larger container than you used the previous year after the poinsettia has had the summer to grow in the outdoor garden.
When you bring the poinsettia indoors, find a spot where it will get 14 hours of uninterrupted darkness followed by 10 hours of sunlight each day. If you don’t have a room that stays dark for 14 hours a day, you can place a cardboard box over your poinsettia to keep it in the dark. This darkness and light routine is needed for the poinsettia to bloom again. After two months of 14 hours of darkness followed by 10 hours of light, the plant should begin to produce colored bracts again.