Christmas Cactus by Stephanie Suesan Smith, Ph.D., Master Gardener.
Christmas cactus has segmented stems that are flat and have spines along the edge of the sections. Most of the year, it is just a slightly odd green plant. However, when it blooms the blooms are beautiful and unusual, and more than make up for the wall flower appearance of the stems.
The Christmas cactus is native to the South American rain forests. This means that it expects a rather high degree of humidity. It would be impractical to keep your house at a humidity of 50-60%, so use this work around: Take a saucer one size bigger than the saucer you use for the plant. Place it where you want the Christmas cactus to live and fill it with pebbles. Fill the saucer with water but leave the tops of the pebbles dry. Place the Christmas cactus on top of this saucer. As the water evaporates from the pebble tray, it creates a high humidity zone around the Christmas cactus and keeps it happy. Make sure you refill the water around the pebbles on a regular basis so the humidity around the Christmas cactus does not get too dry.
The Christmas cactus is picky about soil moisture, too. Water the cactus by pouring water at the base of the cactus until it begins to fill the saucer under it. Discard the water in the saucer and put it back under the plant. Do not water again until the top inch of soil is dry.
You can use any balanced water soluble fertilizer that is for potted plants. Be sure and follow the label instructions.
Blooming is controlled by how much light the Christmas cactus gets. The plant should be in indirect light most of the year. However, if you want the Christmas cactus to bloom at Christmas, you will have some work to do. You will have to duplicate the length of day and night that it would encounter in the wild.
Christmas cactus will only bloom when kept in complete darkness for thirteen hours a night if the temperature is between 55-70 degrees or in complete darkness for fifteen hours of dark a night if the temperature is above 70 degrees. Eight weeks before you want flowers, you start putting the plant in a totally dark space for the time indicated above. Do not fertilize it or overwater it. When you see buds on the Christmas cactus, you should start giving the plant water every day, but smaller amounts of water each time you water it. Too much water will cause the buds to drop.
When the cactus starts to bloom, set in an area of indirect sunlight. The cactus blooms for four to six weeks. Each individual flower lasts six to nine days. After the Christmas cactus quits blooming, prune back every stem to a uniform size. Use the sections you pinch off to start new plants. Go back to the regular fertilizer and water routine when new growth occurs on the stems you left on the original plant.