Thanksgiving cactus

Identification and origin

Schlumbergera truncata is a true cactus (family Cactaceae), but an epiphytic one: it comes from the humid forests of the coastal mountains of southeastern Brazil, where it grows on trees or among rocks and its flowers are pollinated by hummingbirds. It was long classified as Epiphyllum truncatum and later as Zygocactus truncatus.

It has no leaves; its flat, segmented stems (cladodes) do the job instead. Each segment is 2–2 in long and 1–2 in wide, with two to four pointed, forward-facing teeth along the edges that earn it the name crab or claw cactus; the epithet truncata (“cut off square”) refers to the squared-off tips of the segments. The plant forms a branching clump 9–12 in tall and 12 in – 2 ft wide, with stems that arch and then cascade.

The tubular, asymmetrical flowers, 1–3 in across, are held horizontally or upright rather than hanging; depending on the cultivar they are red, pink, purple, orange, cream, white or yellow, with yellow anthers. Bloom starts in late fall and lasts about five weeks, sometimes with a repeat in late winter. To tell it apart from the Christmas cactus (S. bridgesii): the Christmas cactus has rounded segment edges without teeth and drooping flowers. Names such as Thanksgiving, Christmas and Easter cactus only loosely reflect the time of year each one blooms. Well cared for, a holiday cactus can live more than 100 years and be passed down through generations.

Light

Bright but filtered light. Light shade suits it during the growing season, while full sun is beneficial in midwinter. In summer, strong sun makes the stems pale or yellowish and can scorch them. It can summer outdoors in part shade (three to six hours of sun), ideally in the dappled light under a tree, much like its native habitat.

Flowering depends on night length: it is a short-day plant. Starting in early fall, it needs at least 13 hours of uninterrupted darkness every night for about eight weeks. A lamp switched on in the evening, a streetlight or car headlights are enough to interrupt that signal, so keep it in a room that stays dark at night.

Watering

Water when the mix feels dry to about 1 in deep, let it drain (in the sink, for example) and empty the saucer. It tolerates being a little underwatered but not soggy soil, especially during the dark days of winter; don’t let it dry out completely either. While buds form and the plant blooms, keep the mix evenly moist: drying out makes buds drop. After flowering, water less often, then increase again in spring or early summer as growth resumes. Unlike most cacti, it appreciates high humidity.

Potting mix

A light, airy, very free-draining mix: it does poorly in heavy, wet soil. For example, one part potting soil, two parts peat moss (or a substitute) and one part coarse sand or perlite. The pot must have drainage holes. In summer, feed monthly with a balanced fertilizer at half strength; in fall, while buds are forming, switch to a fertilizer low in nitrogen and high in phosphorus and potassium, then cut back in early winter.

Temperature

It grows best between 61–81 °F, with ideal growth around 70–81 °F in spring and summer. To bloom it needs cool nights: with nights around 55 °F, it sets buds in five to six weeks almost regardless of day length; at 61–64 °F at night, it also needs its long, dark nights; with nights above 64 °F, it is unlikely to bloom. Once buds have set, avoid temperatures above 90 °F or below 50 °F, as well as air currents and sudden changes, which make buds drop. It cannot take frost: if it summers outdoors, bring it in before the first frost. In the ground it is grown only in USDA zones 10–12.

Repotting

It blooms best when kept somewhat pot-bound. Repotting every two to three years into a pot just slightly larger is enough, and a healthy plant can stay in the same container for several years. Don’t disturb it once buds have appeared.

Pruning

After flowering, prune to encourage branching: with small, clean pruners or a knife, cut where two segments join. Each cut produces one or two new segments. Also remove diseased, broken or dead stems, and shorten the longest ones if a large plant can no longer support the weight of its branches.

Propagation

Very easy from cuttings. Remove a section of two to five segments at a joint, let the cut end dry and callus overnight to a few days, then insert it about 1 in deep in moist perlite or coarse sand, dipping it in rooting hormone first if you like. Place it in bright light at around 70–81 °F and keep the medium moist but not wet. Roots form within a few weeks, sometimes six to eight; segments also root in water. Then pot the rooted cutting in cactus mix.

Common problems

The same symptom can have several causes: look first at light, temperature and watering.

• Bud drop: sudden changes in temperature, light or humidity, air currents, a mix that is too dry or too wet, moving the plant while buds develop, or simply more buds than the plant can support.
• No flowers: nights too warm, nights interrupted by artificial light, or a lack of nutrients in fall.
• Wilted plant even though the soil is wet: roots rotting from too much water; let it dry out and check the drainage.
• Pale or yellowish stems: too much sun.
• Brown, corky base: normal aging of the stems.
• Diseases: root and stem rots (gray-green plant that wilts, stems rotting at the soil line), bacterial soft rot (black, wet, slimy lesions starting at the soil line; the plant should be destroyed), and gray mold where air circulation is poor.
• Pests: mealybugs, soft brown scale (small, sticky brown bumps), spider mites and aphids.

Toxicity

No toxicity reported: it is considered non-toxic to cats and dogs, and the closely related Christmas cactus (S. bridgesii) is listed as non-toxic to dogs, cats and horses. As with any plant, a pet that eats some may still have passing vomiting or stomach upset.