Gymnocalycium denudatum
| Light | Bright light with protection from harsh midday sun; tolerates a little more shade than most cacti |
|---|---|
| Water | Regularly in the growing season once the mix dries; keep dry and cool in winter |
| Soil | Free-draining but slightly humus-rich mineral mix (see Soil and potting mix) |
| Temperature | Keep above freezing; USDA zones 9b–11 |
| Propagation | Seed; occasional offsets |
| Toxicity | Non-toxic to cats and dogs |
Gymnocalycium denudatum, the spider cactus, is a glossy dark-green cactus from southern Brazil, Uruguay, northeastern Argentina and possibly Paraguay. It is easily recognised by its smooth, shining body divided into just a few broad, rounded ribs and its very short spines, which curve back and press flat against the plant like the legs of a spider — the trait that gives the species both its common name and much of its charm. It is one of the more forgiving and popular members of the genus Gymnocalycium for beginners.
Description
Gymnocalycium denudatum forms a flattened-globular to shortly cylindrical body, typically glossy deep green and around 6–10 cm across (larger in cultivation), usually solitary but sometimes offsetting to make small clumps with age. The body is divided into a small number of broad, low, rounded ribs — often only five to eight — separated by shallow notched grooves. This low rib count and the polished skin give the plant a distinctly smooth, tidy look compared with its more heavily ribbed relatives.
The spines are the species' signature feature: short, slender and usually pale, they radiate from small areoles and curve down and inward so they lie flat against the body, splayed out like spider legs. There is generally no central spine. As in the rest of the genus, the flower buds are naked (the name Gymnocalycium means "naked calyx"), lacking the wool or bristles seen on many cacti. Flowers appear near the crown from spring into summer and are large, funnel-shaped and cream-white, occasionally flushed very pale pink.
Distribution and habitat
The species is native to subtropical southern South America — southern Brazil (notably Rio Grande do Sul), together with Uruguay, parts of northeastern Argentina, and possibly Paraguay. It grows among grasses and low vegetation on rocky ground and in rock crevices, often where it receives some shade and rather more seasonal moisture than the desert cacti many growers picture. This background explains its tolerance of gentler light and more regular watering in cultivation.
Cultivation
Gymnocalycium denudatum is among the easier cacti to grow and a good choice for newer collectors. It does not demand the fierce, unbroken sun that many desert species need; bright light with some protection from the hottest midday sun keeps the body a healthy dark green and prevents scorching of the glossy skin. Grow it in a free-draining mix that can hold a little humus, and water regularly through the warm months once the mix has dried, then keep it dry and cool over winter to rest the plant and encourage flowering. See Watering and Repotting for general technique.
The main risk, as with all cacti, is rot from a waterlogged mix or cold, wet conditions — but this species is more relaxed about watering than most, which is a large part of its beginner-friendly reputation.
Propagation
Seed is the usual and most reliable method: sown on a warm, mineral surface kept humid, the seed germinates well and produces variable, characterful seedlings. Plants that clump can also be increased by removing rooted offsets, though G. denudatum offsets less freely than some of its relatives. See Propagation — seed and Propagation — offsets for full walkthroughs.
Common problems
- Rot — from overwatering, a slow-draining mix, or cold and damp in winter; the plant softens and discolours from the base.
- Etiolation — too little light stretches and pales the body and can flatten its neat ribbing.
- Pests — mealybugs (white fluff in the areoles and among the roots) and red spider mites are the usual offenders; see Pests and diseases.
See also
- Gymnocalycium — the genus overview
- Soil and potting mix · Watering · Propagation — seed · Propagation — offsets · Repotting