Q: How do I get my Christmas cactus to bloom? I have two large ones in an east window. They bloom very little and never at Christmas. — Phyllis Olsen, Crete, Ill.
A: Less light is the answer. Your lamp in the room at night or maybe a strong streetlight outside is the culprit. If you keep them totally dark for 12 to 14 hours a day for three to four weeks in the fall, they will start to form flower buds; about six to eight weeks later, they will bloom. Although short bursts of light won’t ruin your chances, the less light the better. But there’s still no guarantee they’ll bloom precisely on December 25. Old-fashioned kinds really did bloom then, and you may have one of those. They were derived from the wild species Schlumbergera russelliana, which has gently lobed leaves. Other varieties, with spikes on the sides of the leaves, are mostly hybridized from the species S. truncata, and they bloom closer to Thanksgiving. A cross between the two, S. x buckleyi, is the most common of holiday cactus. All these plants begin to set flower buds in late September, when the nights become longer than 12 hours.
I suggest growing your plants outdoors in summer, if you can. I inherited my grandmother’s plant, which does bloom right at Christmas, and I move it outside in early summer and leave it there until the threat of hard frosts makes me bring it indoors. Be aware, however, that outside even a nearby streetlight or porch light may thwart bud set. Indoors, it can be harder still. You need to grow the plants near a window in an unused room and maybe draw the blinds at night, or else shift them to a closet or cover them with a thick bag…every…single…night. The night temperature for bud set needs to be 55ºF to 60ºF. That, too, can be tough to achieve indoors, but you may counteract the need for cooler nights by increasing the dark period to 14 hours. After about a month of this, you’ll begin to see pink pinheads forming on the ends of the branches. Once the first buds get about 1/8 inch long and you see plenty of smaller ones at the branch tips, the hassle is over. You can move the plants back where you like to grow them and enjoy watching the buds elongate into glorious flowers over the next couple of months.
Just because you bought a plant in bloom at Christmas doesn’t mean that’s when it wants to bloom for you. Plant breeders have worked hard to develop exotic flower shapes and colors, confident that greenhouse growers can easily manipulate light to produce plants in flower whenever the market is ready. At home, give your plant the treatment I recommend, and it will soon fall into its own natural cycle, blooming profusely when it’s ready. If you’re dead set on a plant that blooms during a certain week of the year, however, check the mail-order catalogs that specialize in these holiday cactus for descriptions of specific varieties and their bloom times. One more thing: As you grow cactus through the year, keep them in filtered, not direct, bright light. A potting mix of 60 percent peat with 40 percent perlite is good; it’s well drained, moisture-retentive, and very high in organic matter. Although these really are cactus, don’t think hot, dry desert as you tend your plant; think cool rain forest. That’s where they grow in the wild, among the rocks on cliff faces and on tree branches high above the forest floor.