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March 5, 2012

Christmas cacti need special conditions in the fall to encourage flowers in the winter and occasional repotting to renew their potting soil.

Yard and Garden
March 5, 2012

Q. :

I have a very old Christmas cactus. I bought it at an auction about 45 years ago and it was old then. It has wavy edges, not pointed. I put it in darkness during August and just now the one bud is coming into bloom. What are the light requirements to get it to bloom? My grandmother used to put hers in the basement in August and withhold water. Do you have any suggestions?

Carolyn M.

A. :

You have a very special plant. Such old plants are a great addition to a gardener’s collection. If I understand correctly, your plant did not respond in the same manner as it has in the past.

Did you repot the plant late last year? If you repotted it late in the season, that may have stressed the plant enough to reduce flowering. However, cacti can tolerate such stress with little problem, so I also wonder if it has been a long time since it was repotted and given fresh potting soil. Extremely long times without repotting may interrupt flower formation by allowing depletion of nutrients in the potting soil or due to accumulation of surplus mineral salts in the soil. If repotting is necessary, repot your plant in the spring after flowering and just before new growth begins. If growth has begun, it is OK to repot.

Good light and nutrition with adequate water are needed to keep the plant healthy enough to flower. Even though this plant is a cactus, it is native to the tropical rain forests where it experiences frequent moisture with periods of drying between precipitations, and a long, cool dry period in the autumn. During the growing season, do not let it dry excessively.

Christmas cacti (and Thanksgiving and Easter cacti) require special conditions in the autumn to produce their flowers. A dark treatment should begin about the time of the autumnal equinox in September. This dark treatment means long nights (14 hours or longer without any light at all). Interruption of the night by lamps in a room or streetlights outside a window can delay or prevent flowering, especially early in the autumn.

Temperature and limited irrigation seems to be at least as important, or more so, for inducing flowering in these cacti. Temperatures as low as 40 degrees or even a little lower (especially at night) are very important. High night temperatures (above 65 degrees) during the floral induction period can prevent flowering. Your grandmother's technique of putting it in the basement provided the cool temperatures that were needed. I assume the basement was dark most of the time. The cactus was able to tolerate the darkness because it is such a tolerant plant, but it would be better for the cactus to have light during the (short) days. These days should be 10 hours or less.

For other gardeners who are interested in these interesting plants, there are three species of cacti in 2 genera that look quite similar, but have different seasons for blooming. The Thanksgiving cacti and Christmas cacti bloom in the autumn or mid-winter as their names imply. They may have pointed edges or wavy edges on their flattened stems that serve as leaves. An interesting, and less known, relative of these plants, the sunrise, or Easter cactus looks like the Christmas and Thanksgiving cacti with wavy edges to the stems, but it produces a large flower with a typical cactus-flower shape. It blooms in the spring. It requires a very dry period in October and November to flower properly. Drying is also beneficial to the Christmas and Thanksgiving cacti.

When these cacti experience cool, dry autumns with long nights and short days, they are very consistent producers of beautiful flowers in the appropriate season.

For more gardening information, visit the NMSU Extension publications Web site at http://aces.nmsu.edu/pubs/_h, or to read past articles of Yard and Garden go to http://aces.nmsu.edu/pubs/periodicals.html

Send your gardening questions to :

Yard and Garden, Attn: Dr. Curtis Smith
NMSU Agricultural Science Center
1036 Miller Rd.
SW, Los Lunas, NM 87031.

Curtis W. Smith, Ph.D., is an Extension Horticulture Specialist emeritus with New Mexico State University's Cooperative Extension Service. NMSU and the U.S. Department of Agriculture cooperating.