By Larry Hodgson
Short-day plants present a complication for houseplant growers.
These plants require uninterrupted nights more than 12 hours long before they can bloom. Their normal blooming season is autumn or winter, but they often fail to bloom indoors because their owners turn lights on at night, artificially extending the day length beyond 12 hours.
Popular Short-Day Houseplants
- Christmas cactus and Thanksgiving cactus (Schlumbergera spp.)
- Christmas kalanchoe (Kalanchoe blossfeldiana)
- Chrysanthemum (Chrysanthemum × morifolium)
- Orchids (some species, including Cattleya warscewiczii, C. mossiae and Dendrobium phalaenopsis)
- Poinsettia (Euphorbia pulcherrima)
- Queen’s tears bromeliad (Billbergia nutans)
- Rhizomatous begonias (Begonia heracleifolia, B. × erythrophylla, etc.)
- Rieger begonia (Begonia × hiemalis)
When to Begin
Logically, you would start giving your short-day houseplants the shorter days they need to bloom when Mother Nature does: starting at the fall equinox. That would be around September 21 (September 22 in 2021) in the Northern Hemisphere and around March 21 in the Southern Hemisphere. (The exact date varies from year to year.) Of course, you could delay the treatment for a few weeks without doing any harm, although deferring too much will delay flowering.
Tips on Providing Short Days
You can do any one of several things to ensure that short-clay plants will bloom indoors:
- If you have a sunny room that you never use at night, use it to house the short-day plants during the fall. Unplug the lamps in the room or remove their light bulbs so that no one can turn the lights on accidentally during the night—even a short interruption of their 12-plus hours of darkness is often enough to cause them to interrupt the flowering process.
- Place the plants in front of a brightly lit window, then block the light from reaching them from inside the house with a panel or even just other plants. That way, they’ll receive good light during the day, but none in the evening or at night.
- If you live in a region where there’s no danger of frost, leave the plants outside during the fall until they start to show color or produce buds, then bring them indoors.
- Place the plants under artificial lights (LED or fluorescent), set a timer to keep the lights on no longer than ten hours a day, and then enclose the growing area with a box or some other opaque material to prevent any outside light from reaching in. Or set up your time-controlled light garden in a large closet.
- One age-old but time-consuming technique is to put short-day plants in a closet or opaque box early each evening and then move them back to a brightly lit spot each morning until they start to show color. The typical schedule is putting them in darkness at 6 p.m., then back into the sun at 8 a.m. The catch is you have to repeat these actions daily for up to three months, including weekends! If you miss just one night, that may prevent blooming!
Beware of False Information
If someone suggests you shut your poinsettia or Christmas cactus in a closet for two months to force it to bloom (and you see the strangest things on the Facebook these days!), just ignore them. That will seriously weaken your plant or even kill it. Instead, all these short-day plants need good light during the day.
Once you see flower buds, even very tiny ones, you no longer need to do anything more to meet the plant’s need for short days. Flowering has been initiated and will continue its course. You can just expose it to the same lighting conditions as your other plants.
When You Do Give Them Light, They Need Plenty of It
Even though short-day plants need nights over 12 hours long, it doesn’t mean they don’t need sunlight during the day! All these plants need bright light with plenty of sun during the day to bloom well, especially since, in many areas, the sun is vastly weaker in the fall, so a daytime spot on even the sunniest windowsill would be perfect.
Text adapted from the book Houseplants for Dummies by Larry Hodgson.