Gymnocalycium mihanovichii 'Hibotan'
Gymnocalycium mihanovichii 'Hibotan — widely sold as the moon cactus, ruby ball or red cap cactus — is a chlorophyll-free colour mutant of Gymnocalycium mihanovichii. Because it lacks the green pigment needed to photosynthesise, the brilliantly coloured red, orange, yellow or pink ball cannot survive on its own roots and is almost always sold grafted onto a green rootstock, most often a Hylocereus (dragon fruit) column.
Description
'Hibotan' is a small, globular Gymnocalycium whose body has lost most or all of its chlorophyll, allowing the underlying carotenoid and betalain pigments to show through as vivid red, orange, yellow, pink or purplish tones. The body keeps the ribbed, chinned outline typical of G. mihanovichii, with low areoles bearing short, weak spines. Individual plants are usually only a few centimetres across.
Grafted specimens sit as a bright coloured sphere atop a contrasting green stock, which is what gives the "moon cactus" its familiar lollipop look. Well-established plants can flower, producing the pale-pink to greenish, funnel-shaped blooms characteristic of the parent species, and mature bodies often cluster into offsets around the base.
Cultivation
Care follows that of the parent species, Gymnocalycium mihanovichii, but with important differences that come from grafting and the loss of chlorophyll.
Because the colourful top cannot feed itself, its health depends entirely on the rootstock. The most common stock, Hylocereus, is tender and dislikes cold and wet, so keep a moon cactus warm (well above freezing), in bright but not scorching light, and water only when the mix has dried — overwatering rots the stock and takes the whole plant with it. Use a fast-draining, mostly mineral mix and a snug pot. Strong, direct sun can scorch the pigmented top, while too little light weakens the graft, so a bright position with some shade from fierce afternoon sun suits it best. See Watering and Repotting for general technique.
Moon cacti are often treated as short-lived novelties, but a well-matched graft on a vigorous stock can live and grow for many years.
Propagation
Because the coloured body has no chlorophyll, it cannot survive on its own roots and is always propagated by grafting. New coloured plants arise in two ways: chlorophyll-free seedlings appear among seed of the parent species and are grafted while still tiny, before their seed reserves run out; and coloured offsets cut from an established plant are grafted onto a fresh green rootstock. Vigorous plants produce plenty of pups for this purpose. See Grafting and Propagation — offsets for method.
Common problems
- Failing rootstock — the Hylocereus stock is cold- and rot-sensitive; if it yellows, shrivels or softens the coloured top will fail with it. Keep it warm and on the dry side.
- Graft separation — a poorly healed or aged graft union can dry out or pop apart; the coloured top can sometimes be re-grafted onto fresh stock.
- Scorch — the pigmented, chlorophyll-free body burns easily in harsh direct sun, showing pale or corky patches.
- Pests — mealybugs (white fluff around the areoles and graft union) and red spider mites are the usual offenders. See Pests and diseases.
See also
- Gymnocalycium mihanovichii — the parent species
- Gymnocalycium — the genus overview
- Grafting · Soil and potting mix · Watering · Propagation — offsets