Poinsettias require 12 hours of complete darkness in 60-65 F temperatures per 24 hour cycle, in order to maintain their colorful blooms.
Holiday Hues: Keep poinsettias in the dark to encourage new blooms
K-State's Upham shares tips for reblooming poinsettias saved from last year
Sept. 15, 2022
By Maddy Rohr, K-State Research and Extension news service
MANHATTAN, Kan. — It is never too early to begin preparing for the Christmas season, starting with the floral symbol of the holidays. Kansas State University horticulture expert Ward Upham said there is a unique way to rebloom poinsettias saved from last year.
He said that poinsettias are considered “short-day” plants, meaning they require short days and long nights to flower.
“Originally, it was thought that short-day plants needed a short duration of daylight in order to flower,” Upham said. “Now we know that flower formation is actually triggered by long periods of uninterrupted darkness.”
The red and green plant requires 12 hours of complete darkness in 60-65 degrees Fahrenheit for every 24-hour cycle. Upham recommends placing the plant in a closet or covering it with a cardboard box to ensure uninterrupted darkness.
“If you use a cardboard box, tape all the seams with duct tape to cut off any light,” he said.
During the day, poinsettias can be placed in the sunniest part of the house at 65-75 F. Exposing the plant to the sun is necessary for energy conversion, which impacts the color of the flowering.
It is recommended to start the dark treatment early, as it takes 8-11 weeks for the poinsettia to flower. Upham suggests starting in late September or early October. The first six weeks trigger the reblooming process and the remaining time is when the flowers begin developing.
While this can take significant effort and scheduling, Upham said for every night you miss during the first six weeks, add two days to the bloom time. After the buds have set, there is no longer a need for the dark treatment.
Upham and his colleagues in K-State's Department of Horticulture and Natural Resources produce a weekly Horticulture Newsletter with tips for maintaining home landscapes and gardens. The newsletter is available to view online or can be delivered by email each week.
Interested persons can also send their garden and yard-related questions to Upham at wupham@ksu.edu, or contact your local K-State Research and Extension office.
