Gymnocalycium mihanovichii
| Light | Bright, indirect light to gentle sun; more shade than most cacti |
|---|---|
| Water | Moderate in growth; let the mix dry between waterings, keep dry and cool in winter |
| Soil | Fast-draining mineral mix with a little added humus (see Soil and potting mix) |
| Temperature | Keep above freezing; USDA zones 9b–11 |
| Propagation | Seed; offsets; frequently used as the scion in grafts |
| Toxicity | Non-toxic to cats and dogs |
Gymnocalycium mihanovichii is a small, low-growing cactus from Paraguay and neighbouring parts of the Gran Chaco, best known as the green parent stock from which the chlorophyll-free moon cactus or Hibotan grafts are selected. Ordinary green plants form a modest, ribbed globe with a banded, patterned body, but the species readily throws colour mutations — red, pink, orange and yellow seedlings that cannot photosynthesise and so are grafted onto a green rootstock to survive.
Description
Gymnocalycium mihanovichii forms a solitary, somewhat flattened globe usually only a few centimetres across, offsetting with age. The body has a small number of sharp-edged ribs — commonly around eight — broken into low, chin-like bumps beneath the areoles, a feature that gives the genus its common name of chin cactus. Body colour ranges from grey-green to reddish-brown, often marked with faint horizontal bands that lend the plant a subtly striped look.
Spines are short, curved and relatively few, sitting close to the body. Flowers appear from near the crown in spring and summer: broad, funnel-shaped and typically greenish-yellow to olive, sometimes flushed pink, opening over several days. As with other Gymnocalycium, the flower buds are smooth and scaly rather than woolly or spiny.
The form long grown as Gymnocalycium friedrichii is treated here as the same species; it is often more strongly banded and reddish, and much of the coloured moon-cactus breeding traces back to plants under that name.
Distribution and habitat
The species is native to Paraguay and adjacent parts of northern Argentina and eastern Bolivia, in the hot, seasonally dry scrub and grassland of the Gran Chaco. There it typically grows in the light shade of grasses and low shrubs rather than in full exposure, rooting in soils that are often heavier and more organic than the pure grit many desert cacti prefer.
Cultivation
Gymnocalycium mihanovichii is one of the easier and more forgiving cacti, which is part of why it is so widely grown. Give it bright light but, unlike most cacti, a little shade from the harshest summer sun — strong direct light can scorch or bronze the body. Grow it in a fast-draining but not purely mineral mix; a modest amount of humus suits it.
Water moderately through the warm growing season, letting the mix dry out between waterings, then keep the plant dry and cool over winter to encourage flowering and prevent rot. See Watering and Repotting for general technique. Overwatering and cold, wet soil are the main killers.
Note that the colourful moon cactus grafts sold in garden centres are Gymnocalycium mihanovichii scions grown on a green rootstock such as Hylocereus. Because the coloured top has no chlorophyll of its own, it depends entirely on the stock and is often shorter-lived than a normal green plant; see Grafting.
Propagation
Ordinary green plants grow readily from seed, which germinates easily on a warm, moist mineral surface, and clustering plants can be divided by removing rooted offsets. Coloured, chlorophyll-free forms cannot be grown on their own roots and are propagated by grafting the coloured scion onto a vigorous green rootstock. See Propagation — seed for the seed-raising method.
Cultivars
The species is the source of the familiar Hibotan (moon cactus) colour selections — brilliant red, pink, orange and yellow variants first developed in Japan — as well as variegated forms in which patches of coloured and green tissue coexist on the same body. Variegates and partly pigmented plants can sometimes hold on their own roots; fully colourless forms cannot.
Common problems
- Rot — from overwatering, a mix that stays wet, or cold winter conditions; the base or body softens and browns.
- Scorch — too much fierce direct sun bleaches or bronzes the body; this species wants more shade than typical desert cacti.
- Graft failure — on moon cacti, the coloured scion may shrivel or drop if the rootstock is unhealthy, chilled or the graft union weakens.
- Pests — mealybugs (white fluff in the areoles and roots) and red spider mites are the usual offenders; see Pests and diseases.
See also
- Gymnocalycium — the genus overview
- Grafting · Soil and potting mix · Watering · Propagation — seed · Propagation — offsets