If your poinsettias spent the summer outdoors, they should be back indoors by late September or early October, according to Ross Penhallegon, horticulture agent with the Oregon State University Extension Service.
Sensitive to day length, the poinsettia needs a certain minimum amount of darkness each 24-hour period to stimulate blooming in the winter. With the shorter fall and winter days in Oregon, the poinsettia has a natural tendency to want to bloom in the fall, when there are about equal amounts of dark and light.
However, if the plants are exposed to lights inside and around the home, the plants won't receive enough darkness to start blooming and could stay vegetative through the winter.
To make a poinsettia bloom in the early winter, indoor gardeners simply need to adjust the amount of light and darkness to "fool" the plant, said Penhallegon.
Start in mid- to late October. Place your poinsettias in a completely dark area from 5 p.m. to 8 a.m. daily, until red colour starts to develop on the top leaves or "bracts."
"This can take quite a while," warned Penhallegon. "People often forget to cover the plant one or two nights. Interrupting the darkness by even a few minutes may cause failure of bloom."
Bring the plant to ordinary light after the bracts show color. Poinsettia growers may have better luck if the bracts are almost fully expanded before bringing the plant out. Once the plant has large, coloured bracts, the artificial light inside a house will not inhibit a poinsettia's blooms.
Penhallegon offered a general time line for "colouring" the poinsettia:
- Mid- to late October - Begin giving poinsettias long nights (darkness from 5 p.m. to 8 a.m.).
- Mid-November - Colour should be showing in the bracts.
- Early December - Bract colour should be almost complete. Plant can be brought out into ordinary light.
- December until February - During this "forced" bloom, keep the temperature between 60 and 70 degrees.
- Let the plant receive as much sunlight as possible during the day. Water regularly and thoroughly.
Throughout the winter, fertilize once a week with a light, complete water-soluble fertilizer. Over-fertilization will cause the leaves to drop. Keep hot or cold drafts away from the plant. The leaves may wilt if the plant is too dry, too wet or exposed to a draft. Never allow a plant to stand in water. Yellowing of foliage may indicate insufficient light, over-watering or lack of nitrogen. Generally, a slight correction of the environment will correct any of the above symptoms.
After blooming, the plant matures in early spring. Bracts and leaves will begin to fall naturally.
Discontinue fertilizing and reduce watering. Cut the plant back to six inches and store in a cool dark area. Water only enough to prevent the stem from shrivelling. In the spring, the plant can once again be set outdoors or maintained as a houseplant. Water it regularly and fertilize monthly. If grown outdoors, be sure to check for insect pests and treat if needed. Next autumn, start the cycle again.
Source: Ohio State University