Named for its habit of flowering around the holidays, a Christmas cactus (Schlumbergera bridegsii) is a native in tropical forests where its pendulous branches hang from trees and produce sprays of showy flowers. Although it can grow outdoors in U.S. Department of Agriculture plant hardness zones 10B through 12, it's usually grown as a houseplant. If you have a Christmas cactus and it's being stubborn about re-blooming, you probably need to adjust its growing conditions to correct the problem.
During spring and summer, after a Christmas cactus has finished blooming, it grows best in bright indoor light such as in a sunny south-facing window. If you grow it in the garden year-round or move the plant outdoors when weather warms, keep it in a partially shaded spot to avoid sun scorch of the leaves. If you see leaves that turn reddish, this indicates the plant's getting too much light and needs a bit more shade.
The period of time that the cactus is exposed to light becomes critical in fall, when it starts setting its next group of flower buds. Beginning in mid-October and for about the next six weeks, a Christmas cactus requires about 12 hours of uninterrupted darkness each night to produce flowers for the holiday season. Keep the plant in an unused room where lights are normally off every night, placing it in a window where it gets sun during the day. You can also move the plant into a dark closet each evening, bringing it back out to a bright spot in the morning. If you live within its outdoor range, it should set buds when left outdoors year-round, provided it's shielded from any outdoor lights left on at night.
Temperature for Flowering
Giving a Christmas cactus the correct temperature is also important to help it flower for the holidays. In the fall, keeping it at about 60 to 68 degrees Fahrenheit helps ensure it produces the maximum number of flower buds, provided it gets 12 hours of darkness daily. It's also important to shield the plant from temperatures above 90 degrees in fall, because this can cause the plant to drop flower buds once it develops them.
If you find it difficult to provide the required amount of darkness during the critical fall period, you can keep the plant in a spot -- such as an unheated room or garage -- where nighttime temperatures stay between 50 and 59 degrees. These cool temperatures override the plant's need for darkness and allow it to develop flower buds, although it might grow more slowly and some buds could drop at 50 degrees.
Other Special Care
Because it's a tropical native, a Christmas cactus grows best in an environment with a humidity of 50 to 60 percent. Placing the plant on a pebble-filled tray containing water that reaches just below the pebble tops can help meet this need.
A Christmas cactus also needs regular watering, whenever the top inch or two of soil is dry, but avoid over-watering because this can cause the plant to drop flower buds. Once the plant has buds, avoid moving it to a new spot because a change in environment can also cause bud drop.
Fertilizing the plant monthly also helps promote strong growth and heavy flowering. Start in late winter or early spring, when you see new green growth, then continue through summer, but stop in late summer to promote good bud production. Use a 20-20-20 formula, diluted half-strength, or 1/4 teaspoon per gallon of water, but also check the product label for further instructions. Providing extra magnesium also helps boost flowering. Dilute 1 teaspoon of Epsom salts in 1 gallon of water and use this once monthly at a different time than fertilizer.