Bill Starr: Christmas cactus survives, despite owner

Published 4:00 pm Monday, December 21, 2015

Last Christmas season I won a Christmas cactus as a door prize; surprisingly, this  plant is still thriving in my office despite me. I really have neglected this plant and it seems to still flourish. These are the kind of plants I like.
Thanksgiving cactus (Schlumbergera truncata) and Christmas cactus (Schlumbergera bridgesii) are popular, fall- and winter-flowering houseplants native to Brazil, and are available in a wide variety of colors including red, rose, purple, lavender, peach, orange, cream and white. These Schlumbergera species grow as epiphytes among tree branches in shady rain forests, and their pendulous stems make them a great choice for hanging baskets.
The secret of good flower bud production during the fall involves temperature regulation and photoperiod (length of day and night) control. To initiate flower buds the plants need the following.
• Bright light (mine sits in a south facing window sill).
• Long nights. Fourteen hours or more of continuous darkness each day is required before flower bud set will occur. Long nights should be started about the middle of September and continued for at least six continuous weeks for complete bud set. Note that as little as two hours of interrupted lighting will inhibit flower bud set. Buds normally will be visible in three to four weeks. The photoperiod has no effect on flowering once the buds are set.
• Fall growing temperatures should be kept between 60 and 68 degrees .F, but as close to 68 degrees F, as possible for maximum flower production. Plants grown with night temperatures between 50 and 59 degrees F. will set flower buds regardless of day length, but growth will be slower and bud drop may occur at 50 degrees F.
• Pinching back the stems in early June to promote branching and more terminals for more flowers.
The night time temperature in my office must be just perfect for the development of the flowers on this cactus. Mine must be a Thanksgiving cactus because it actually bloomed two weeks ago.

Watering and fertilizer
Water the growing medium when it is dry to the touch. The holiday cacti are tolerant of dry, slightly under-watered conditions during the spring and summer (pretty much all the time for the one in my office). Do not let the soil become waterlogged, especially during the dark days of winter, but do not let the soil completely dry out either  (this only happened  about 20 times this year). However, following bud set in the fall, the growing medium must be kept evenly moist to prevent flower bud abscission. Never let water stand in the saucer beneath the pot.
Fertilize plants monthly from the time new growth starts in late winter or early spring, and throughout the summer using a one-half strength soluble fertilizer, such as a 20-10-20 or 20-20-20 with trace elements. Holiday cacti have a higher requirement for magnesium than many plants. Fertilize monthly during the growing season with Epsom salts (magnesium sulfate) mixed at 1 teaspoon per gallon of water, but do not apply the same week as the regular fertilizer. Stop fertilization during the late summer for greater flower bud production in the fall.
The holiday cacti flower best when kept somewhat pot bound. Repotting is necessary only about once every three years and is best done in the spring. The potting medium must be well-drained with good aeration, as these epiphytic cacti do not grow well in heavy, wet potting mixes. A good mix may contain 60-80 percent potting soil with 40-20 percent perlite. Choose a good brand of potting soil which is pH balanced.
The Thanksgiving and Christmas cacti commonly drop unopened flower buds, which may be induced by an excessive number of buds or a sudden change in temperature, light or other environmental factors, such as drying out of the growing medium (this is what happened to mine, but it still had lots of blooms). Lack of flowering is often due to light interrupting the long night period (14 hours) that is required for flowering initiation to occur. Street lights, car lights, or indoor lighting can disrupt the required dark period.
I have been impressed with how tough this plant can be; I guess I will try to do better for 2016, and see if it blooms even more, but then again, I probably won’t! Happy gardening!

Bill Starr is Sumter County Extension coordinator/ANR Agent, University of Georgia Cooperative Extension Service. Contact him at 229-924-4476.