Astrophytum
| Light | Bright light; young plants appreciate some shade from harsh afternoon sun |
|---|---|
| Water | Sparingly in growth; allow to dry fully between waterings, keep dry in winter |
| Soil | Fast-draining mineral mix (see Soil and potting mix) |
| Temperature | Keep above freezing; broadly USDA zones 9b–11 |
| Propagation | Seed (primary); grafting to speed seedling growth |
| Toxicity | Non-toxic to cats and dogs |
Astrophytum is a small genus of ribbed, star-shaped cacti from Mexico and southern Texas, ranging from squat, spineless forms that sit almost flush with the ground to densely spined species. Loved by collectors for their geometric bodies and the sprinkling of white woolly flecks that decorate the skin of most species, Astrophytum is arguably the most intensively cultivar-developed genus in the whole hobby — a breeding tradition carried furthest by Japanese growers.
Description
The name Astrophytum means "star plant", and it fits: seen from above, most species form a neat star of pronounced ribs — commonly five in the classic bishop's cap, but ranging from three to eight or more depending on the species and individual. Bodies are usually globular when young, becoming more columnar with age in some species.
The genus's signature feature is the flecking — tiny tufts and specks of white woolly scales (trichomes) scattered across the green skin, giving many plants a marbled or snow-dusted look. The amount varies enormously: some plants are almost solid white, others nearly bare (a trait breeders have selected for in both directions). Spination is just as variable, from the completely spineless Astrophytum asterias and Astrophytum myriostigma to the long, twisting, papery spines of Astrophytum capricorne.
Flowers appear from the woolly areoles near the crown and are typically funnel-shaped and yellow, often with a red or orange throat in some species. They are relatively large for the size of the plant and open in bright daylight.
Distribution
Astrophytum is native to the arid regions of northern and central Mexico, with Astrophytum asterias extending across the border into southern Texas. Plants grow in limestone and gypsum soils, on rocky slopes and in open desert scrub, often partly buried or nestled among stones and low vegetation that shade them from the fiercest sun. Several species have restricted wild ranges and face pressure from habitat loss and illegal collection, so wild-collected plants should always be avoided in favour of nursery-grown, seed-raised stock.
Notable species
- Astrophytum asterias — the "sand dollar" or star cactus; a flat, spineless disc with eight low ribs, heavily worked by breeders.
- Astrophytum myriostigma — the "bishop's cap"; a spineless, typically five-ribbed species, often densely white-flecked.
- Astrophytum ornatum — the largest and most robust species, columnar with age and armed with stout spines.
- Astrophytum capricorne — the "goat's horn cactus", named for its long, curling, flexible spines.
- Astrophytum caput-medusae — an unusual species with finger-like tubercles rather than conventional ribs.
Cultivation
As a group, Astrophytum are among the more forgiving collector cacti, provided you respect their two firm dislikes: soggy roots and prolonged cold damp.
Give them bright light — full sun for established plants in temperate climates, though young seedlings and the flatter, spineless species benefit from light shade during the harshest afternoon hours to avoid scorching. Grow in a fast-draining mineral mix (see Soil and potting mix); many growers use an especially gritty, low-organic blend for this genus, and some add limestone chips in a nod to their gypsum-rich habitats.
Water generously but infrequently during the warm growing season, always letting the mix dry out completely between drinks (see Watering). Through winter, keep them cool and essentially dry — this rest not only prevents rot but also encourages a good flush of spring flowers. Protect from frost; while brief cold snaps are tolerated when bone-dry, wet cold is the fastest route to a dead plant.
Propagation
Astrophytums are grown overwhelmingly from seed (see Propagation — seed), which germinates readily and is the natural route for a genus that rarely offsets. Seed-raising is also how the endless cultivars are produced and selected.
To speed up slow seedlings — or to sustain the many chlorophyll-deficient variegated and coloured cultivars that cannot survive on their own roots — growers frequently turn to Grafting, mounting seedlings on a vigorous stock. Once established, grafted plants can be de-grafted and rooted, though many prized collector forms are kept permanently on stock.
Cultivars and hobby notes
No other cactus genus has been shaped by breeders quite like Astrophytum. Decades of selection — much of it in Japan — have produced named forms emphasising every conceivable variation: dense white flecking (nudum being the opposite, flecking removed), exaggerated or reduced ribs, wavy and quilted skin, variegation, and unusual colour breaks. Familiar cultivar lines include the heavily patterned V-type and Super Kabuto forms of A. asterias, the densely flecked Onzuka selection of A. myriostigma, and countless interspecific hybrids. Because so many of the showiest forms carry variegation or reduced chlorophyll, grafting is a common companion skill for anyone collecting the fancier cultivars.
See also
- Astrophytum asterias · Astrophytum myriostigma · Astrophytum ornatum · Astrophytum capricorne — species pages
- Grafting · Propagation — seed · Soil and potting mix · Watering