Mammillaria sempervivi
| Light | Bright light; a little shade from the harshest summer sun |
|---|---|
| Water | Sparingly; let the mix dry out fully between waterings, keep dry in winter |
| Soil | Gritty, fast-draining mineral mix (see Soil and potting mix) |
| Temperature | Keep above freezing; roughly USDA zones 9b–11 |
| Propagation | Seed; occasional offsets |
| Toxicity | Non-toxic to cats and dogs |
Mammillaria sempervivi is a small, flattened-globular cactus from central and northeastern Mexico, easily recognised by its angular, keeled tubercles and the dense pads of white wool packed into the axils between them. In spring it produces a neat ring of small cream to dull-pink flowers near the crown. It belongs to the large and popular genus Mammillaria, and its geometric, wool-filled body makes it a favourite among collectors of compact "pincushion" cacti.
Description
Mammillaria sempervivi forms a solitary, somewhat flattened globe that in age may clump slowly from the base. The body is firm and blue-green to grey-green, its surface built from stout, four-angled (keeled) tubercles arranged in neat spirals. A defining feature is the copious white wool held in the axils — the grooves between the tubercles — which can be so dense that it nearly veils the body and gives the plant a soft, snowy look.
The areoles at the tubercle tips carry short, stiff radial spines and, on many plants, one or two darker central spines; young growth is often woollier and more spined than the smooth older body. Flowers are small and funnel-shaped, cream to dull pink with darker midstripes, opening in a ring around the crown in late winter and spring. These are followed, as in most Mammillaria, by slender club-shaped berries that ripen over the following season.
Distribution and habitat
The species is native to central and northeastern Mexico, where it grows on rocky limestone hills and in the crevices of arid, well-drained slopes. In habitat it often sits nestled among rocks and low vegetation that give it a little shade and shelter, with its flattened top the main part visible above the substrate during the dry season.
Like the whole cactus family, M. sempervivi is listed under CITES Appendix II, which regulates international trade. Nursery-grown plants are widely available and entirely legal to own and trade; wild collection is not.
Cultivation
Mammillaria sempervivi is a straightforward grower for anyone with experience of desert cacti, with rot from overwatering being the main risk. Plant it in a very free-draining, mostly mineral mix in a pot only a little larger than the plant, and give it bright light with just a little protection from the fiercest afternoon sun to keep the body compact and the wool white.
Water thoroughly only when the mix has dried out completely through the warmer months, then allow it to dry again before the next drink. Keep the plant cool and completely dry over winter; this winter rest both prevents rot and helps trigger the spring ring of flowers. See Watering and Repotting for general technique.
Propagation
Seed is the usual and most reliable method: sow onto a warm, gritty surface kept lightly humid until the seedlings establish, then grow them on hard and bright. Plants that offset can also be increased by removing rooted pups once they are large enough to handle, letting the cut surface callus, and potting them into a dry, gritty mix. See Propagation — seed and Propagation — offsets for full walkthroughs.
Common problems
- Rot — nearly always from overwatering or a slow-draining mix; the body softens and discolours from the base or crown.
- Etiolation — too little light makes the body pale and stretch upward, losing its flat, symmetrical shape.
- Pests — mealybugs love to hide in the dense axillary wool (look for white fluff and cottony masses), and red spider mites can bronze the skin in hot, dry, still air. See Pests and diseases.
See also
- Mammillaria — the genus overview
- Soil and potting mix · Watering · Repotting · Propagation — seed · Propagation — offsets · Pests and diseases