Thelocactus macdowellii
| Light | Bright light; full sun to very light shade |
|---|---|
| Water | Sparingly; let the soil dry completely between waterings, dry rest in winter |
| Soil | Very free-draining mineral mix (see Soil and potting mix) |
| Temperature | Keep above freezing; hardy to around −5 °C if bone dry (roughly USDA zone 9) |
| Propagation | Seed; offsets on the rare occasions clustering occurs |
| Toxicity | Non-toxic to cats and dogs |
Thelocactus macdowellii is a small globular cactus from northeastern Mexico, prized for its dense covering of white radial spines and its large, vivid magenta flowers — a combination that makes it one of the showiest species in the genus Thelocactus. The pale spination almost hides the green body beneath, so that a well-grown plant in bloom looks like a snowy ball crowned with bright pink funnels. It has long been shuffled between genera and is often still sold under synonyms such as Echinomastus macdowellii or Neolloydia macdowellii.
Description
Thelocactus macdowellii forms a rounded, usually solitary globe up to about 10 cm across — sometimes broader than tall, and only rarely offsetting to make a small cluster with age. The body is arranged into low, spiralled tubercles, each carrying an areole set with numerous slender radial spines and a few stouter, distinctly longer centrals. The spines are white to pale yellowish and so densely arranged that they overlap across the plant, obscuring much of the green skin and giving the whole cactus a soft, woolly-white look.
Flowers open from near the crown, chiefly in late winter and spring. They are large for the size of the plant — bright magenta to rose-pink with a paler throat and a satiny sheen — and several may open together over a sunny spell, making a striking contrast against the white spines.
Distribution and habitat
The species is a narrowly restricted endemic of northeastern Mexico, known from a small area on the border of Coahuila and Nuevo León, east of Saltillo, where it grows on limestone hills in arid Chihuahuan Desert scrubland at roughly 1,300–1,600 m. Plants sit among rock and low vegetation in gritty, sharply drained substrates, exposed to strong sun and receiving most of their moisture in short seasonal bursts.
Because its natural range is so small, the wild population is inherently vulnerable, and the species is assessed as Near Threatened on the IUCN Red List. As with all cacti, the family is listed under CITES Appendix II, so international trade is regulated. Nursery-propagated plants are readily available and legal to own and trade; wild collection is neither necessary nor acceptable.
Cultivation
Thelocactus macdowellii is not difficult given the two things it insists on: sharp drainage and plenty of light. Grow it in a very free-draining, mostly mineral mix in a pot only a little larger than the plant, and give it the brightest position you can — full sun suits it and helps keep the spination dense and white. Weak light causes the body to stretch and the spines to thin, spoiling the plant's compact character.
Water thoroughly during the warm growing season once the soil has dried out, then allow it to dry again before the next drink. Keep the plant completely dry and cool through winter; this dry rest both prevents rot and encourages the early-spring flowering. Overwatering, especially in cold or poorly drained conditions, is the main cause of loss. See Watering and Repotting for general technique.
Propagation
Seed is the usual method — indeed almost the only one, since the plant seldom offsets. Sown on a warm, gritty surface and kept humid until germination, the seedlings grow at a moderate pace and are best left undisturbed for their first season. On the rare occasions a plant has produced offsets these can be removed once they have a reasonable size, allowed to callus, and rooted on their own — see Propagation — seed and Propagation — offsets. Some growers also graft seedlings to speed early growth, later returning them to their own roots.
Common problems
- Rot — nearly always from overwatering or a slow, water-retentive mix; the plant softens and discolours from the base.
- Etiolation — too little light makes the body elongate and the spines sparse, losing the tight white cover that is the species' main appeal.
- Pests — red spider mites (fine webbing, bronzed skin) and mealybugs (white fluff among the spines and roots) are the usual offenders; see Pests and diseases.
See also
- Thelocactus — the genus overview
- Grafting · Soil and potting mix · Watering · Propagation — seed · Propagation — offsets