Christmas cactus is the common name for a number of cactus varieties in the genus Schlumbergera.
Other members of the same group are known as Thanksgiving cactus, Easter cactus, crab cactus and holiday cactus.
The ancestors of all of these originated in the cloud forest of Brazil at altitudes of 2000 to 9000 feet above sea level. Plants are epiphytic or lithophytic, meaning they grow on moss-covered tree branches or in rock crevices, often in small pockets of soil formed from decayed leaves and other vegetation.
Most species of Schlumbergera have stems that resemble leaf-like pads joined one to the other with flowers that appear at the joints and tips of the stems. In Brazil, the genus is referred to as Flor de Maio (May flower) because they flower in that month in the southern hemisphere.
Specimens of the wild cactus were collected in the early 1800s and taken to Europe where they were hybridized with each other and with other cactus varieties resulting in the current wide array of colors, foliage form and bloom period. Blooms rang in color from red, rose, purple and lavender to peach, orange, cream, white and metallic gold.
The natural distribution of Schlumbergera species has become confused because European cultivars were deliberately introduced into some areas by the Brazilian Agricultural Department, including the Parque Nacional da Serra dos Órgãos where many of the cultivated Christmas cactus ancestors were collected.
Flowers of many Christmas cacti exhibit different colors depending on the temperature during bud formation and growth. Temperatures below 57 °F produce pink tones in normally white and yellow cultivars, and deepen the color in pink and red cultivars. The availability of iron to the plant has also been suggested to affect flower color.
Christmas cacti are adapted for pollination by hummingbirds including tubular flowers with abundant nectar, and colors towards the red end of the spectrum. Most species require cross-pollination from a separate plant to set seed. The fruits of Schlumbergera do not open spontaneously when ripe, and appear to be adapted for distribution by birds, which eat the seeds and pulp contained in the fruit.
The holiday cacti grow best in light shade. Full sunlight is beneficial during fall and winter, but bright sun during the summer months can make plants look pale and yellow. Ideal spring and summer growth occurs at temperatures between 70 to 80 °F.
During the fall, these cacti depend upon shorter day lengths of eight to 10 hours and cooler temperatures to set their flower buds. Do not let temperatures rise above 90 °F once the flower buds are set in the fall. Continuous warm temperatures can cause flower buds to drop.
Fourteen hours or more of continuous darkness each day is required before flower bud set will occur. Long nights should be started about the middle of September and continued for at least six continuous weeks. If your plants live on a windowsill, consider moving them to the closet each night.
Pinch back the stems in early June to promote branching and more terminals for more flowers.
Water only when the soil is dry to the touch. The holiday cacti are tolerant of dry, slightly under-watered conditions during the spring and summer. It’s ok if the stems shrink a little. Do not let the soil become waterlogged, especially during the dark days of winter. Never let water stand in the saucer beneath the pot.
Fertilize plants monthly when the days are growing longer but when they begin to shorten stop. I use orchid fertilizer on mine. Some experts recommend a dose of Epsom salts in the summer especially if the green begins to fade.
The holiday cacti flower best when kept somewhat pot bound. Repotting is necessary only about once every three years.