Turbinicarpus subterraneus
| Light | Bright light with a little shade from the fiercest afternoon sun |
|---|---|
| Water | Very sparingly; allow the mix to dry completely, keep dry in winter |
| Soil | Deep, gritty, fast-draining mineral mix (see Soil and potting mix) |
| Temperature | Keep above freezing; roughly USDA zones 9b–11 |
| Propagation | Seed (primary) |
| Toxicity | Non-toxic to cats and dogs |
Turbinicarpus subterraneus is a small, slow-growing Mexican cactus notable for the way it hides most of itself below the surface: only a modest, cylindrical head sits above ground, drawn down onto a long, swollen, carrot-like taproot that stores water through the dry season. In spring it opens comparatively large, magenta to purplish-pink flowers that dwarf the little body. Some botanists place it in the segregate genus Rapicactus (as Rapicactus subterraneus), and it has also been treated under Gymnocactus and Pelecyphora; here it is kept within Turbinicarpus.
Description
Turbinicarpus subterraneus produces a small, solitary (occasionally clustering) head only a few centimetres across, cylindrical to shortly club-shaped, in dull green to greyish-green. Below it the plant tapers into a thick, elongated taproot that is often longer than the visible body — an adaptation that lets the head retract almost flush with the soil during drought. The tubercles are low and soft, each tipped with a small areole bearing a few fine, weak, pale spines that give the plant a gentle, unarmed look rather than a fierce one.
The flowers are the main event: relatively large for the size of the plant, funnel-shaped, and a bright magenta to purple-pink, opening near the crown over sunny days in spring. Small dry fruits follow, splitting to release fine seed.
Distribution and habitat
The species is endemic to northeastern Mexico, where it grows on limestone-derived, gritty or clay-rich soils in arid scrub and grassland. Plants sit tucked among rock and low vegetation, with the contractile taproot pulling the body downward so that in the dry season little more than the flat top is visible — a strategy that protects it from heat, cold and grazing alike.
Like all cacti, T. subterraneus is listed under CITES (the genus falls under Appendix I), and wild populations are vulnerable to illegal collection. Nursery-propagated, seed-grown plants are the responsible source; wild collecting is neither necessary nor legal.
Cultivation
This is a connoisseur's plant that rewards restraint. The single biggest risk is overwatering, which quickly rots the fleshy taproot. Grow it in a very free-draining, mostly mineral mix in a pot deep enough to accommodate the long root — a taller container suits it far better than a shallow one. Give it bright light with only light shade from the most intense summer sun.
Water thoroughly only once the mix has dried right through, then let it dry again; taper off in autumn and keep the plant completely dry and cool over winter, which both prevents rot and encourages the following spring's flowers. See Watering and Repotting for general technique, and handle the brittle root carefully when potting on.
Propagation
Seed is the standard and most reliable method. Sown on a warm, gritty, mineral surface and kept humid until germination, seedlings are slow but undemanding once past the earliest stage. Because the species rarely offsets, vegetative propagation is uncommon; where growers want to speed up a young plant, seedlings are sometimes grafted onto a vigorous rootstock and grown on. See Propagation — seed for a full walkthrough.
Common problems
- Root and basal rot — nearly always from overwatering, a slow-draining mix, or a pot that stays wet around the taproot; the body softens and discolours.
- Etiolation — too little light makes the head elongate and pale, losing its compact form.
- Pests — mealybugs (white fluff in the areoles and on the roots) and red spider mites are the usual offenders; check the root as well as the crown.
See also
- Turbinicarpus — the genus overview
- Grafting · Soil and potting mix · Watering · Repotting · Propagation — seed · Pests and diseases