Gymnocalycium
Gymnocalycium is a genus of small, mostly globular South American cacti, much loved by hobbyists for the smooth, chin-like bumps that sit beneath each areole and for their forgiving, shade-tolerant nature. The genus name comes from the Greek for "naked calyx", a reference to the hairless, scaly flower buds that distinguish the group; the same rounded projections below the spines give rise to the common name chin cactus.
Description
Most Gymnocalycium are solitary, flattened-globular plants ranging from a few centimetres to (in the larger species) 20–30 cm across, though many stay comfortably small in a pot. A characteristic feature is the ribs, which are broken up into rounded tubercles, each with a distinct swelling or "chin" below the areole — the trait that gives the genus its common name. Bodies are usually some shade of green, grey-green or bronze, sometimes flushing purple or red in strong light.
Spines are variable: some species are almost naked, others bristle with stout, curved or comb-like spines pressed against the body. The flowers, borne near the crown, open in a broad funnel and come in white, cream, pink, pale green and shades of red, emerging from smooth, scaly, hairless buds. Blooms are often large relative to the plant and appear readily even on young specimens.
Distribution
The genus is native to South America, centred on Argentina but ranging into Paraguay, Uruguay, Bolivia and southern Brazil. Plants grow across a wide spread of habitats — grassland, thornscrub, rocky slopes and open woodland — frequently tucked among grasses and low shrubs that filter the sun. This habit of growing in light shade is part of why the genus tolerates lower light in cultivation than most desert cacti.
Notable species
- Gymnocalycium mihanovichii — the parent of the colourful, chlorophyll-free Moon Cactus grafts sold worldwide.
- Gymnocalycium baldianum — compact and free-flowering, with striking deep red to magenta blooms.
- Gymnocalycium saglionis — one of the largest species, forming a broad, heavily spined globe with age.
- Gymnocalycium bruchii — a tiny, clustering species that offsets freely and flowers pale pink.
- Gymnocalycium horstii — smooth-bodied Brazilian species with large soft-pink flowers.
- Gymnocalycium pflanzii — robust and variable, with whitish to pink flowers.
Cultivation
Gymnocalycium are among the easiest cacti for beginners, which accounts for much of their popularity. Unlike many desert cacti they resent full, scorching sun, so bright light with some shade from the fiercest afternoon rays suits them best; a plant grown too hard often turns dull bronze or corky, while one grown too dark etiolates and loses its symmetry.
Grow them in a free-draining, mostly mineral mix and water thoroughly through the warm months once the soil has dried, then keep them dry and cool over winter to encourage the following season's flowers. They are generally tolerant of a little more moisture and less light than the average cactus, but standing wet — especially in cold — still leads to rot. See Watering and Repotting for general technique.
Hobby and cultivar notes
The best-known plants in the genus are the grafted Moon Cactus: mutant, chlorophyll-free forms of Gymnocalycium mihanovichii in vivid red, orange, yellow or pink, which cannot photosynthesise and so are grown atop a green rootstock such as Hylocereus. These grafts are widely sold as novelty plants and are best understood as a scion-plus-stock combination rather than a plant that will ever grow on its own roots.
Beyond the Moon Cactus, the genus is popular for its variety of body forms and spination, and many species offset readily, making them easy to share and propagate. Named and selected forms exist, but most collector interest is in the wild species themselves.
Propagation
Seed is the most common and reliable method, germinating readily on a warm, mineral surface kept humid; see Propagation — seed. Many clustering species also produce offsets that can be removed and rooted, a quick way to increase a favourite plant — see Propagation — offsets. Prized or slow forms, and the chlorophyll-free Moon Cactus mutants, are maintained by grafting.
Common problems
- Rot — from overwatering, a slow-draining mix or cold wet conditions; the body softens and discolours from the base.
- Sun scorch — too much direct sun bleaches or bronzes the skin and can leave permanent corky scarring, since these plants naturally grow in light shade.
- Etiolation — too little light makes the body pale and stretch upward, losing its low, rounded form.
- Pests — mealybugs (white fluff in the areoles, and root mealybugs at the roots) and red spider mites are the usual offenders; see Pests and diseases.
See also
- Gymnocalycium mihanovichii — parent of the Moon Cactus
- Grafting · Soil and potting mix · Watering · Propagation — seed · Propagation — offsets