There are two major reasons you’d want to simulate darkness: either you bring potted plants indoors and want blooms, or you’re growing ornamentals outdoors in mild climates. No matter your growing situation, you’ll want to give short-day blooming species the care they need to perform at their best.
Before we dive in, there are a few terms we should familiarize ourselves with. The first is photoperiod; a photoperiod is the amount of daily light a plant receives. Some species flower with long days and short nights, while others open their buds with short days and long nights. Most winter-blooming species need short days and long nights since these are the natural daylight lengths they have in their place of origin.
Some of these plants are classic winter species that growers force into bloom with greenhouses and simulated daylight lengths. You can do the same and have cacti, hellebores, and coleus flowering throughout the winter. Other species grow in mild climates with cool but not freezing seasons. We’ll cover how to care for each one so they flower proliferously this year, no matter where you live!
Christmas Cactus
Bright blooms appear after nights lengthen and temperatures cool.
A Brazil native plant, the Christmas cactus is a lovely low-growing succulent with surprising blooms. They dangle all over the plant’s branches when days shorten and nights lengthen. Many gardeners keep them as houseplants, although they’re perennial for those living in USDA hardiness zones 10 through 12. Christmas cactus likes partial shade outdoors and bright indirect light indoors.
Flower buds appear with short photoperiods. To force Christmas cactus flowers, place your potted plant in the dark for 12 to 14 hours a day. Your plant also needs cool temperatures during the flowering period, from 50 to 55°F (10 to 13°C). Start this process in November to ensure the flowers appear by Christmastime.
Many varieties and species of Christmas cactus exist for growers to try. Try the longtime favorite hybrid Schlumbergera x buckleyi for rosy purple-red blooms. If you’d like longer flowers try the similar species S. russelliana. All types need long nights in the fall for winter blooms.
Thanksgiving Cactus
Vibrant blooms emerge after cool nights and proper care.
The Thanksgiving cactus is a close relative of the Christmas cactus that also goes by “crab cactus.” It sprouts scarlet flowers with long tubes, and its foliage has lots of teeth along the edges. There are many cultivars available with blooms in shades of white, orange, and pink.
Thanksgiving cactus needs 13 hours of darkness daily for a month to bloom. It also prefers cool temperatures during this time to mimic its native habitat. For blooms at Thanksgiving, start putting this species in a dark location in October. If you start putting it in the dark in November, blooms won’t appear until December.
Thanksgiving and Christmas cacti need consistent moisture, bright light, and high humidity. Place them in a bathroom, brightly lit kitchen, or windowsill, and they’ll thrive throughout the winter. If their leaves are yellow or fall off, cut back on watering to let the roots breathe.
Poinsettia
Bright red bracts bring festive cheer with proper seasonal care.
Poinsettias add an iconic charm to any home during Christmastime. They bloom tiny flowers with long, red leafy bracts underneath. Underneath the red bracts lie green stems and foliage that contrast beautifully with the flower clusters. Poinsettias need warm temperatures year-round to thrive, but they grow well as houseplants.
To force poinsettias to bloom, give them cool nights for 14 hours daily with 10 hours of sunlight. A dark closet, garage, or grow tent works well to keep light out. Start forcing blooms in early October and maintain the process for 10 weeks. After 10 weeks, flowers and red bracts should appear, and you can move your potted plant to a bright, sunny window.
Chrysanthemum
Cool temperatures and longer nights encourage vibrant seasonal blooms.
Chrysanthemums grace the shelves of nurseries and grocery stores every fall and winter. They’re classic cool-season blooming perennials with flowers in dozens of colors, shapes, and sizes. Chrysanthemums, or “mums,” need low temperatures with long nights to initiate flower buds, and you can force them to flower early or late, depending on your preferences.
For winter blooms, start placing mums in the dark for longer than 12 hours daily a month before you’d like blooms. They require cool, moderate weather to start flowering and will struggle in hot climates. If it’s warm outdoors but cool indoors, you can simulate blooming by bringing them indoors. The shift from hot outdoor conditions to inside ones, alongside long nights, initiate chrysanthemum budding.
Coleus
Vibrant, multicolored leaves brighten up any garden or patio.
Gardeners plant coleus for its foliage, not its flowers, but this perennial does produce flowers. The flowers form on spikes at stem tips. They’re purple and white, and they form on the plant’s entirety while days are short and nights are long. Where they’re hardy in zones 10 through 11 they flower in the winter under mild weather. Try sowing ‘Rainbow Blend’ seeds to see four different foliage types!
To force coleus to bloom, give them cool but not freezing temperatures and nights longer than 12 hours. After three to four weeks, they should show signs of flowering. Small, scaly flower buds emerge with small leaves around them. Outdoors, use a light-proof covering in the fall to force coleus to bloom earlier than normal.
African Violet
These charming plants thrive in bright, indirect light indoors.
African violets populate offices, kitchens, and windowsills throughout North America. They’re a popular houseplant because they appreciate moderate weather that’s common in our homes. They like moist, fertile soil, bright indirect light, and consistent water to thrive.
Force African violets to flower by granting them 8 to 12 hours of light and 8 hours of darkness. Do this daily for four weeks or longer, and they’ll push blossoms out of their crown. African violets need this treatment throughout flowering, and they’re a bit high maintenance.
An easy way to ensure they receive sufficient light is by setting up a grow light on a timer. Alternatively, leave yours offset from a bright window in fall and winter. This is a foolproof way to have violets in the middle of December!
Hellebore
These perennials thrive in shady conditions with bright light.
Hellebores, or Christmas roses, are winter-blooming perennials for frigid gardens. Dozens of varieties exist, despite their relatively recent introduction to North American gardens. Choose varieties with nodding flowers in black, yellow, white, red, or purple, or with multiple colors.
They’ll naturally bloom in winter outdoors, although they’re marginally cold hardy below zone 5. In colder areas, bring your hellebores indoors and control their photoperiods to ensure you have seasonal flowers when you’d like them.
Hellebores flower as days start to lengthen after the winter solstice. They need a cool period matching autumn’s conditions, then a period with lengthening days. Start by placing your hellebore where it has 12 chilly hours or more of darkness for four weeks or longer. When you’d like flowers, bring them where they get 13 hours or more of light with cool temperatures. Ensure their light is bright and indirect, as hellebores need shady conditions to thrive.
Primrose
Low-growing perennials burst into vibrant color during winter.
Primroses are low-growing herbaceous perennials perfect for temperate gardens. They enter dormancy when hot summers arrive, and they pop out of the ground under mild, moist conditions. Find flowering varieties with shades of pink, lavender, blue, yellow, and white. Grow primroses outdoors in zones 4 to 8, and they’ll flower in fall or spring.
Primroses enter dormancy again once freezing winters arrive, and the frost can damage their foliage. Bring them indoors in the fall to start forcing flowers. They perform well in pots with fertile, free-draining soil. Give them short days between eight to ten hours long, with nights 16 to 14 hours long. Primroses prefer mild, cool areas while they flower to avoid bloom or leaf drop.
Cape Primrose
Long, fuzzy leaves support elegant blooms year-round.
Cape primroses sprout leaves similar to primroses. They’re long, fuzzy, and green. In the center of the leafy rosettes sprout flowers that resemble lady slipper orchids, hence this plant’s other name “lady slipper.” These perennials prefer conditions similar to primroses, thriving with cool weather, moist soil, and shade or bright indirect light.
Cape primroses flower throughout the year with days 12 hours or longer. Force them to bloom in the winter by placing them under filtered or bright indirect light for half the day and darkness the other half.
If you garden with short, dark days during winter, use grow lights to help them flower. Turn them on for 12 hours or more a day, and they’ll produce blooms in two weeks or more. Giving the plant a significant temperature difference of 70°F (21°C) in the day and roughly 55°F (13°C) at night will help immensely.
