Prepare Christmas cactus for holiday bloom

A recent postcard from Bangor gardener Jasper Owens presented a familiar seasonal question: Why are there no flower buds on his Christmas cactus, a gift that he received in full flower last year and that doubled in size over the summer? My best guess is that his plant was not properly conditioned to flower at Christmastime.

Christmas cacti are short-day plants, blooming when given a long, uninterrupted dark period, about 12 hours each night. Commercial growers keep the plants in cool greenhouses (50 degrees F) where they receive between 12 and 14 hours of total darkness each day.

You can induce flowering at home by starting the dark treatment in about mid-October to have your plant in full bloom by Christmas. Simply place your Christmas cactus in a cool room with bright, indirect daylight and no artificial light. Christmas cacti will even bloom without this dark treatment if they are subjected to temperatures of about 50 to 55 degrees F beginning in mid-October. In either case, your plant can be returned to its previous location once flower buds have formed.

The next hurdle is preventing flower bud drop, a common problem caused by several conditions including overwatering, lack of humidity, or insufficient light. Christmas cacti given as gifts should include cultural information about optimum growing conditions to avoid loss of the flower buds.

We typically think of cacti as being both heat- and drought-tolerant, but Christmas cacti are not at all similar to desert cacti; they are epiphytes, found in the wild growing in the cool, moist shade of tree canopies. They will keep their blossoms longer at lower temperatures and in evenly moist soil. Plants will wilt if subjected to long periods without water.

Your plant will thrive in a well-lit location away from heat vents, fireplaces or other sources of hot air. Drafts and temperature extremes can cause the flower buds to drop from the plant before they have a chance to open. Water thoroughly when the top inch or so of soil feels dry to the touch. The length of time between watering will vary with the air temperature, amount of light, rate of growth and relative humidity.

Christmas cacti are not happy with the extremely low humidity of most winter homes; they thrive at 50 to 60 percent relative humidity. A humidity tray is a good method of adding moisture to the air around your plant. Fill a waterproof tray or pot saucer with gravel, adding water halfway up the gravel. Place the pot on the gravel surface.

If you are up to the challenge of keeping the plant after the holidays, you should fertilize it with a water-soluble houseplant fertilizer with nitrogen content no higher than 10 percent. Start feeding about a month after flowering; like most of us, Christmas cacti welcome a period of rest after the holiday season. Follow the label directions for how much and how often to feed.

While Christmas cacti can adapt to low light, more abundant blooms are produced on plants that have been exposed to greater light intensity. Keep your plant in a sunny location indoors and consider moving it outdoors in summer, placing it in a shady or semishady spot. When it is time to bring the plant back inside in the fall, certainly before even the hint of frost, slowly acclimate the plant to life indoors by gradually increasing the number of hours it spends indoors each day.

Bringing a Christmas cactus into bloom at the right time every year is a challenge taken up by many gardeners. I hear tales of individual plants that have been in the same family for generations.

Send queries to Gardening Questions, P.O. Box 418, Ellsworth 04605, or to rmanley@ptc-me.net. Include name, address and telephone number.