Ariocarpus retusus
| Light | Bright light; some protection from the fiercest afternoon sun |
|---|---|
| Water | Very sparingly; allow to dry completely, kept bone-dry through winter |
| Soil | Very gritty, mostly mineral mix (see Soil and potting mix) |
| Temperature | Keep above freezing; USDA zones 9b–11 |
| Propagation | Seed (primary); grafting to speed slow seedlings |
| Toxicity | Contains trace alkaloids; not a food plant — see Legal status |
Ariocarpus retusus is a slow-growing "living rock" cactus from the arid highlands of northern Mexico, and one of the largest members of the genus Ariocarpus. It forms a low rosette of stiff, pointed, grey-green to bluish tubercles that blend almost perfectly into the surrounding rock and soil. Woolly white to pale-pink flowers open from the crown in autumn.
Description
Ariocarpus retusus grows as a solitary, flattened rosette up to about 25 cm across, though most cultivated plants are considerably smaller. The body is built from firm, triangular tubercles arranged in a spiral, each ending in a blunt or sharply pointed tip and coloured a chalky grey-green, sometimes with a bluish or brownish cast. Between the tubercles sits a persistent tuft of white wool, most dense toward the centre of the plant where new growth and flowers emerge.
Beneath the ground the plant anchors itself with a large, swollen taproot that stores water and moisture reserves, allowing it to shrink down into the soil during prolonged drought until little more than the tubercle tips are visible. Flowers appear from the woolly crown in autumn, funnel-shaped and 3–5 cm across, ranging from white through to pale pink. The species is variable across its range, and several forms and varieties have been described based on tubercle shape and size.
Distribution and habitat
The species is native to the Chihuahuan Desert region of northern Mexico, where it occurs across a broad area of the central and northeastern states on limestone hills, gravelly flats and rocky slopes. Plants typically grow among rock and low scrub in full exposure, their cryptic bodies sitting low against the substrate.
Like all cacti the species is listed under CITES, and Ariocarpus as a genus is afforded the strictest protection (see Legal status). Wild populations have been reduced by illegal collecting, so responsible growers rely exclusively on nursery- and seed-raised material.
Cultivation
Ariocarpus retusus has a reputation for being difficult, but the challenge is almost entirely about restraint with water. It demands a very free-draining, mostly mineral mix, ideally in a deep pot to accommodate the taproot. Grow it in bright light with a little shade from the most intense afternoon sun, and water thoroughly only when the soil has dried out completely — then wait. Through winter the plant should be kept cool and completely dry, which both prevents rot and encourages autumn flowering.
The species is exceptionally slow, and overwatering is by far the most common cause of loss: the swollen root and body rot readily if kept damp. Many growers speed up young plants by grafting seedlings onto a vigorous rootstock, growing them on their own roots later once established. See Watering and Repotting for general technique.
Propagation
Seed is the standard and usually the only practical method. The fine seed is sown on a warm, gritty surface kept humid until germination, after which seedlings are grown on very cautiously. Because the plant is solitary and rarely offsets, vegetative propagation is uncommon apart from grafting to accelerate growth. See Propagation — seed for a full walkthrough, and Grafting for the grafting technique.
Common problems
- Rot — almost always from overwatering, a slow-draining mix, or water sitting in the woolly crown; the taproot or body softens and browns.
- Slow establishment — freshly imported or bare-rooted plants can take a long time to re-root; keep them dry and warm and be patient.
- Pests — root mealybugs on the taproot, spider mites, and scale can all trouble the species; check the roots at repotting. See Pests and diseases.
Legal status
Ariocarpus retusus is reported to contain trace phenethylamine alkaloids. It is not among the widely cultivated cacti most associated with such compounds, but the whole genus Ariocarpus is treated with particular caution in conservation law. The genus is listed under CITES Appendix I — the strictest level of protection — which places tight restrictions on international trade in wild-collected plants. Artificially propagated, nursery-raised specimens are legal to own and trade in most jurisdictions, subject to the relevant CITES and local documentation for cross-border movement. Growers should check the regulations that apply where they live, and should never collect plants from the wild.
This entry is a horticultural reference only and does not describe any use of the plant beyond cultivation.
See also
- Ariocarpus — the genus overview
- CITES · Grafting · Soil and potting mix · Watering · Propagation — seed · Pests and diseases