To understand the Christmas cactus, think "jungle," not "desert." These spineless, succulent cacti (Schlumbergera, formerly Zygocactus) all come from the Brazilian rainforest. Come the end of the year, around holiday time, nearly every branch tip will end in one or two blossoms in a wide range of translucent colors, depending on the variety: white and yellow through orange, pink, red, and magenta. Individual flowers last about a week, and one plant's display can go on for three weeks.
Hybridizers have done much mixing and matching in the last few decades, but most of the kinds available are hybrids of just two: S. russelliana, with gently lobed leaves, which blooms around Christmas, and S. truncata, which has leaves with fleshy spikes on the sides (hence the name crab-cactus) and blooms closer to Thanksgiving.
Sometimes you'll see Christmas cacti referred to as Buckleyi hybrids, or you may see the species S. bridgesii. Both are outdated names for hybrids with predominantly S. russelliana traits. The newly discovered "princess" or "queen" cactus is S. orssichiana. It can bloom two or three times a year and is still something of a rarity, though commercially available.
Though Schlumbergera truly are cacti, they aren't desert plants. They don't like either full sun or dry conditions. In their native Brazil, they grow high up in trees, in pockets of leaf mold and other organic matter that accumulates in cavities along branches. Give them bright indirect light and evenly moist soil -- like what they'd experience in a tree -- and they'll live happily for decades.
They survive near-freezing temperatures and long dry spells, making them nearly indestructible houseplants. Surviving isn't thriving, however, and getting them to pump out the lush and delicately colored flowers that originally tempted you to bring them home takes a little special attention. But with just a modicum of care, they will reward you with an abundance of color that few winter bloomers can match.
Does that mean that you can count on the blooming plant you bought at the nursery or supermarket on December 15 to repeat the performance on the same date next year? Not at all. Growers manipulate light and temperature to push plants forward or hold them back, depending on the weather and market conditions.
If you buy a plant in bloom, it will be a year before you'll know exactly when it will flower under your own conditions. If you want bloom in time for the holidays, you are better off buying from a mail-order catalog that lists approximate bloom times or from a nursery or greenhouse that grows its own plants; a few companies in all regions still do this. The majority of blooming plants available this time of year are grown in California, Florida, Denmark, or Holland.
Older varieties of holiday cactus tend to have gracefully drooping branches, which make them prime subjects for hanging baskets. The flowers-mostly in pink, red, and white-face downward. The petals, which look like two tubes, one inside the other, curve severely back on the tubes. Many of these varieties are still available. Modern hybrids have more erect branches, so the blossoms face up. The flowers are more compact and come in a wide range of colors that now include pale yellow, orange, and even purplish tones.
Holiday cacti are infamous for dropping their buds when brought indoors. The cause is usually a drastic change in temperature, thanks to proximity to a fireplace, woodstove, radiator, or hot-air duct. Ethylene gas can also be the problem; keep blooming plants away from space heaters, gas stoves, or ripening fruit. Ordinary home temperatures of about 70°F are fine, though cooler nights are beneficial. Water blooming plants to keep the soil evenly moist but not saturated. An easy test is to insert a wooden toothpick: if it comes away clean and feels fairly dry, it's time for more water. Withhold all fertilizer.
After blooming cut back on watering slightly, but don't let the leaves begin to shrivel. They certainly can survive severe drying, but it's not conducive to prime performance. The ideal location after flowering is a cool room (above 40°F) with bright indirect light, not in a sunny window.
When new growth begins in spring, fertilize the plants at each watering with a soluble fertilizer at one-third to one-half strength. As soon as nights are dependably above 40°F, move the plants outdoors under 80 percent shade. Hanging in a tree is an ideal spot, though they will also grow fine indoors in bright light out of direct sun.
If you want to buy special varieties through the mail, the time to order is late winter or spring. Set the young plants in a pot that is slightly larger than the rootball. Holiday cacti like acidic soil high in organic matter, and very good drainage. A potting mix of 60 percent peat with 40 percent perlite is perfect.
Refresh the soil every three to four years by removing the plant from the pot, cutting away the outer one-quarter to one-third of the roots and soil, then repotting it in the same container with fresh potting mix. Plastic pots are perhaps better than clay for growing these cacti, because plastic keeps the soil from drying out too quickly.
The end of summer is the time to start inducing flower buds for the coming holidays. The crucial time is the fall equinox during the third week in September, when nights become longer than 12 hours. In early September, stop fertilizing until flowering is finished. Cool nights outdoors are also ideal, though not critical, to bud set.
Once the longer nights arrive, the plants must have total darkness every night for at least three weeks. Keep the plants outdoors or move them to a room where you can maintain darkness for the bud-forming period. Short bursts of light don't matter much, but avoid them if possible. Commercial growers prevent bud formation by using only 40-watt light bulbs for 4 hours each night. Outdoors, street lights or even porch lights can inhibit bud set. Indoors, draw the window shade if outdoor lights shine in.
Two to three weeks after the equinox, pinpoint buds will start showing on the branch tips. When the buds reach about 1/8-inch long, the new crop of flowers has set, and the absence of light at night is no longer critical. When you move plants indoors, however, try not to shock them with drastic changes in light or temperature. If you are growing the plants outdoors through the fall, very cool nights will intensify colors. Pale yellows, for example, may develop orange or reddish tones.
Display plants when the buds are well enlarged and showing strong color. Don't worry if a few buds drop. Place the plants on a pedestal or table in good light where the flowers can show their colors best.
Jack Ruttle is a former editor at National Gardening.
Photography by Suzanne DeJohn/National Gardening Association
Article published on June 23, 2008.