The Passionate Gardener: Keep Your Poinsettias Blooming

Ron Kushner, for the Shuttle
What U.S. Ambassador Joel Poinsett saw in Mexico in 1825 — or something like it.

Poinsettias are leafy plants, typically with dark green leaves topped with colored, modified leaf bracts which are often incorrectly considered flowers or flower petals. The real flowers are tiny, mostly berrylike structures in the center of each colored leaf bract.

The common name for this popular winter-holiday plant was given in honor of Joel Poinsett, a gardener and botanist from South Carolina who in 1825, was the U.S. Ambassador to Mexico where he found the native Mexican plants growing.

The botanical name is Euphorbia pulcherrima. The genus was named by taxonomist Carl Linnaeus for Euphorbus, a Greek physician of the 1st century BC. The species name “pulcherrima” means “very beautiful.”

Contrary to widely circulated misinformation, poinsettias are not poisonous to humans or animals. If ingested in enormous quantities, they might cause mild stomach discomfort. The white sap of some other Euphorbias can cause skin irritation, but poinsettias, snow on the mountain (E. marginata), crown of thorns (E. milii) and other popular members of the very large “spurge” family don’t deserve this rap.

When the poinsettia’s bracts age and lose their aesthetic appeal, there is no reason to throw out the plant or toss it into the compost. With proper care it will re-bloom! The process can be a bit of a challenge but it can be done.

By late March or early April, cut your poinsettia back to about 8 inches in height. Continue regular watering and fertilize with a balanced, all-purpose fertilizer. By the end of May you should see plenty of new growth.

Place your plants outdoors after all chance of frost has passed, normally after mid-May. Night temperatures should be above 55 degrees F. Some morning sun is fine but the plants do best in indirect sun. Continue regular watering and fertilize every two to three weeks.

About June 1, transplant into larger pots (no more than 4 inches bigger in diameter than the original pot). Use any soil-less potting mixture sold in garden centers. Some pruning may be required during the summer to keep the plants bushy and compact. Late June or early July is a good time to do this; don’t ever prune after Sept. 1.

Poinsettias set buds as the autumn days get shorter. In their native Mexico, they naturally come into bloom during November or December, depending upon the individual cultivar.

To get this to work at home, starting Oct. 1, keep the plants in complete darkness for 14 continuous hours each night, followed by 6 to 8 hours of bright sunlight daily. To achieve the required darkness, it is best to move the plants to a totally dark room or cover them with a large box. Stray light of any kind, such as from a household lamp or even a streetlight, could delay or entirely halt the re-flowering process. It’s also important to control night temperatures: no lower than 60 degrees nor higher than 70.

Continue normal watering and fertilizing and the nighttime darkness regime for 8 to 10 weeks and you should have tiny blooms — and large colorful bracts — for the holiday season.

Fertilizer is not necessary while the plants are in bloom. Water thoroughly, saturating the soil completely, allowing them to drain. Do not allow the pots to sit in water in saucers. If you see leaves or bracts wilting, water immediately.

Poinsettias thrive in humid air. In dry, interior environments, placing pots on a pebble tray or misting the leaves frequently will help. Keep plants away from drafts, fireplaces and ventilating ducts.

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