In shades of white, golden yellow, pink, rose, coral and red, the flowers of holiday cacti look like exotic birds in flight. It is no wonder that these fall and winter blooming plants have become holiday favorites.
People group holiday cacti together under the term “Christmas cactus.” Actually, most holiday cacti are derived from the Thanksgiving or crab cactus, Schlumbergera truncata, family Cactaceae. The true Christmas cactus is Schlumbergera bridgesii. The main difference between the two is the shape of the leaf-like stem segments. In the Thanksgiving cactus, the edges of the segments are toothed or pointed; in the Christmas cactus, the segments are smaller and have rounded lobes. The Thanksgiving cactus tends to grow more upright, while the Christmas cactus tends to be more pendulous, or drooping.
Both species originated in Brazil, where they grow as epiphytes in plant debris trapped among tree branches or in decaying humus on the ground. Therefore, plants grown in the home do best in a light, ‘humusy’ or peat-based potting soil and containers with holes for drainage. Avoid potting in heavy mineral-type soils (garden soil), where over watering will quickly lead to root rot and plant death. The once recommended practice of keeping holiday cacti quite dry in the fall to stimulate flowering probably grew out of greenhouse growers’ attempts to prevent fungal diseases in plants grown in heavy soils under cool, moist conditions.
Though holiday cacti naturally flower around Thanksgiving and Christmas, commercial growers take no chances — they manipulate light levels and temperatures to guarantee timely flowering. Holiday cacti bloom in response to short days and/or cool temperatures. In the greenhouse or in the home, expose them to nine hours of light and 15 hours of darkness each day, starting in September, to cause plants to set flower buds. Holding temperatures between 50 and 59F. will stimulate flowering despite day length. Kentucky gardeners that re-flower their holiday cactus each year, simply leave the plants outdoors through September and protect the plants from frost if necessary. When the plants are brought indoors in early October, flower buds are set and the plants flower around Thanksgiving.
Temperature above 75F, sudden changes in temperature or light levels, and over watering plants in heavy soil will cause unopened flower buds to fall off. Besides controlling the growing environment, greenhouse operators use chemical growth regulators to increase stem branching and flower bud formation. That is why the plants you see in florist shops and greenhouses have many short lateral branches loaded with flower buds. One to two flowers at the end of each stem are normal, but treated plants may have three to five.
Submitted by Alexis Sheffield, Agent for Horticulture, Boyle Co. Cooperative Extension