Gymnocalycium stenopleurum
| Light | Bright, filtered light; protect from harsh midday summer sun |
|---|---|
| Water | Moderately in the growing season, letting the soil dry between waterings; keep dry in winter |
| Soil | Open, fast-draining mineral mix (see Soil and potting mix) |
| Temperature | Keep above freezing; USDA zones 9b–11 |
| Propagation | Offsets and seed; coloured mutants are grafted |
| Toxicity | Non-toxic to cats and dogs |
Gymnocalycium stenopleurum is a small, low-growing South American cactus of the genus Gymnocalycium, closely allied to G. mihanovichii and often treated as a synonym or subspecies of it. It is best known as the taxon behind most of the colourful, chlorophyll-deficient moon cactus selections (the little red, pink, orange and yellow grafted "hibotan" plants sold worldwide), though the wild green form is a handsome, easily grown collector's cactus in its own right.
Description
Gymnocalycium stenopleurum forms a small, flattened-globular body, usually solitary at first but often clustering with age. The stem is grey-green to brownish-green and divided into a modest number of broad, low ribs that are typically notched into rounded chin-like humps beneath the areoles — the "chin" that gives the genus its common name of chin cactus. The scientific name Gymnocalycium (gymno-, naked; -calycium, calyx) instead refers to the naked flower buds, which are covered only in smooth scales rather than wool or spines. The short, slightly curved spines are pale and pressed against the body.
Funnel-shaped flowers appear from near the crown in the warmer months, generally in soft off-white to pale pink tones with a paler or greenish throat. As with other members of the genus, the flower tube and buds are smooth and scaly rather than woolly or spiny.
Distribution and habitat
The species occurs in the arid Gran Chaco of Paraguay, growing among grasses and low scrub in gritty, well-drained ground. In habitat plants are often partly shaded by surrounding vegetation, which suits their preference for bright but not scorching light in cultivation.
Cultivation
Green, normal forms of G. stenopleurum are undemanding and well suited to beginners. Grow them in an open, mostly mineral mix in a pot that drains freely, and give bright light with some shielding from the fiercest summer sun — unlike many desert cacti, Gymnocalycium tends to bronze or scorch in unrelenting full sun. Water moderately once the soil has dried through the growing season, then keep the plant dry and cool over winter to firm it up and encourage flowering. Overwatering and stale, water-retentive compost are the main causes of loss. See Watering and Repotting for general technique.
The famous coloured moon cactus selections are a special case. These are mutant plants that lack most or all chlorophyll — the red, pink, orange or yellow pigments that would normally be masked by green are left exposed. Because they cannot photosynthesise enough to sustain themselves, they are kept alive by grafting the coloured top onto a green, vigorous rootstock (commonly Hylocereus) that feeds it. Grafted moon cacti are best grown in bright but diffuse light and kept notably warmer and drier than ordinary cacti; the rootstock is the weak link and rots easily if kept cold and wet.
Propagation
Normal green plants are readily raised from seed, which germinates well on a warm, humid mineral surface, and clustering plants can be increased by removing rooted offsets. The coloured mutants, having no chlorophyll, cannot be grown on their own roots and are propagated by grafting pups onto fresh rootstock. See Propagation - seed and Propagation - cuttings for fuller walkthroughs.
Cultivars
Selection within this taxon has produced an enormous range of coloured "hibotan" moon cacti — solid reds, pinks, oranges, yellows and multicoloured and variegated forms — nearly all maintained by grafting. Green-bodied and part-variegated ornamental clones also circulate among collectors. Because so much of the trade is grafted onto a temporary rootstock, the same colour selection may be re-grafted many times over its life.
Common problems
- Rot — usually from overwatering or a slow-draining mix; the base softens and browns. Grafted moon cacti most often fail where the cold-sensitive rootstock rots.
- Scorch — exposed to harsh full sun the green skin bronzes and corks; the delicate coloured tops of moon cacti bleach and scar even faster.
- Pests — mealybugs (white fluff in the areoles and around the roots) and red spider mites are the usual offenders. See Pests and diseases.
See also
- Gymnocalycium — the genus overview
- Gymnocalycium mihanovichii — the closely allied species
- Grafting · Soil and potting mix · Watering · Propagation - seed · Propagation - offsets