‘Tis The Season, Almost’

Kim Bagby, VCE Master Gardener Intern, Virginia Master Naturalist Considering how much I enjoy my houseplants, you’d think I’d be a better caretaker of them. Spoiler alert: I’m not that great at it. But I try. I have a handful of houseplants that I’ve had for decades. One of them is what I thought was a Christmas Cactus, and it took what I thought was fatal damage one frigid night in Iowa in a cross-country move. While it collapsed after the freeze, I managed to save enough of its parts to re-root some and it’s growing again. But while it flowers regularly enough, it hasn’t ever made it to Christmas to do so. Turns out it isn’t a Christmas Cactus at all, it’s a Thanksgiving Cactus, Schlumbergera truncata.
You easily distinguish a Thanksgiving cactus from a Christmas cactus by its sharp points on the leaves. There are actually three different cactuses that fall into a group frequently called Holiday cactuses. Two of them I’ve already mentioned. The third one is the Easter Cactus. All three have segmented, flattened stems called phylloclades (from the Greek phyllo, leaf, and klados, branch), which perform the functions of leaves as well as support the stems and flowers. All three are lovely plants to have in your home. Mine spend the summer out on the deck, but since they’re only hardy in the far southern United States, they must come in for the winter. These aren’t the spiny desert plants most of us think of as cactuses. These are tropical or subtropical Brazilian epiphytes, plants that grow on other plants but aren’t parasitic. Epiphytes use other plants only as supports and otherwise provide for all their own needs. The ability to grow high on other trees up in the forest canopy helps these cactuses get their sunlight and there’s plenty of detritus up there to help them grow. Dead leaves, bits of sticks, and that kind of thing make little compost heaps in the crotches of branches and that’s where these cactuses like to live. It’s a loosely packed, well-aerated compost-y soil where they can anchor themselves. That’s why our holiday cactus houseplants appreciate good drainage. If the soil in their pot is too dense, their roots won’t be able to get enough air and they’ll suffer. Likewise, if that dense soil’s holding a lot of water, they can drown. You can use a commercial succulent mix for potting them, or you can make your own. A good option is three parts regular potting soil mixed with two parts perlite. Schlumbergera enjoy being a little potbound, but repotting every few years, into a pot that’s just a little bigger than the one they’re coming from, works fine. Wait to repot until late winter or early spring after they have finished blooming, but don’t repot after that so they can work on next season’s bloom, undisturbed. Wait till the surface of the soil is dry to water. If you think about it, those little compost heaps they grow in probably get a lot of rain, but they drain quickly and dry out. Water deeply when the soil’s dry to the depth of your first knuckle and let the water drain out. Don’t let any water stand in the saucer under the pot or the plants may develop root rot. As for light, we can think back again to that rainforest canopy. It provides bright but indirect light. A rough rule of thumb is that your hand should cast a blurry shadow, and the light should be bouncing off a wall or filtered through a shade or something similar—too much direct sunlight will burn these cactuses. Save that nice, bright southern exposure for something that really needs it. Clemson University’s College of Agriculture, Forestry and Life Sciences recommends fertilizing monthly with a soluble all-purpose fertilizer like you can get at the store, something in the range of 20-5-15 or so, at half-strength. Supplement that during the growing season with magnesium sulfate (Epsom salts) mixed at one teaspoon per gallon of water. Use this magnesium solution once a month, but not the same week as when you use the regular fertilizer. Start fertilizing in the spring with the new growth, and stop in late summer to get good flower bud production later. I’ve never tried forcing blooms on any of my cactuses, but Clemson says to keep them cool, about 68°F, with at least fourteen hours a day of complete darkness starting in September and going for six weeks. This should allow good bud set. Once the buds have developed, you can go back to your normal indirect sunlight. Pinching the plant back in September to remove phylloclades less than half an inch long may also help with budding. The shorter phylloclades are immature and won’t set buds, but pinching them off helps the more mature stem segment “behind” them set buds. Likewise, pinching the plant back earlier in the summer, in June, helps promote branching and thus more opportunities for flowers. Propagating these plants isn’t hard. In early summer or so, May or June, pinch or twist off a piece three to five segments long and dry it in the shade for a day or two to develop a callus over the wounded area, which is a plant’s defense response to seal the wound site and prevent water loss and infection. Then plant the piece about an inch deep, water it well, and cover both plant and pot in sealed clear plastic bag to make a little greenhouse. Keep it in the greenhouse in bright indirect light till new roots form, three to eight weeks. I generally start small, one cutting to a pot, but the Clemson source says you can start three cuttings in a four-inch pot, or five in a six-inch pot. Oh, and as to how to tell the different cactuses apart? It’s (almost) all in the phylloclades… The most common of these houseplant cactuses is probably the Thanksgiving cactus, S. truncata, the species I’ve had for over thirty years. Thanksgiving cactuses have phylloclades with scalloped edges that have fairly sharply delineated points, two to four to a side. Christmas cactuses, S. x buckleyi, also have scalloped edges to their phylloclades but they’re more rounded: no points. These are harder to find as they’re more brittle and don’t transport well, and so are not as profitable for retailers. Both S. truncata and S. x buckleyi have elongated kind of tubular flowers that generally come in reds, peaches, oranges, cream, and sometimes purples. The Easter cactus (S. gaertneri) has phylloclades that are only very slightly scalloped if at all, and their flowers are quite different than those of the other two. These flowers are more of a starburst shape, generally in crimsons with yellow stamens inside. So, there you have it! Probably more than you ever wanted to know about Holiday cactuses. Many thanks to our Virginia Cooperative Extension Southside Master Gardeners and Virginia Master Naturalist Southern Piedmont Chapter, and to all the people who give their time to and through these groups to educate us! The Virginia Cooperative Extension Halifax Extension Office is open from 8 a.m. to 5 p.m. Monday-Friday. Masks and physical distancing are required. If you have gardening questions, you can continue to reach an extension master gardener or extension staff member by sending an email to wmccaleb@vt.edu or calling the Halifax Extension Master Gardener Help Desk at 434-830-3383. Be sure to give us your first and last name, telephone number and the nature of the call. The help desk phone is routinely checked Monday-Friday. Someone will get back to you, although it may be from a different telephone number.