Astrophytum myriostigma f. tricostatum
Astrophytum myriostigma f. tricostatum (often just called Tricostatum or the three-rib bishop's cap) is a rare rib-reduced form of the popular bishop's cap cactus in which the body carries only three ribs instead of the four to eight — most often five — seen in the species. The reduced rib count gives the plant a sharp, distinctly triangular cross-section that collectors prize, so that the whole cactus reads almost like a three-sided pyramid or star when viewed from above.
This is a form (forma), not a true cultivar in the horticultural sense — three-ribbed individuals crop up spontaneously among seedlings of Astrophytum myriostigma, and growers select and grow them on for the unusual geometry. Because the trait is not perfectly stable, plants can revert or drift toward four or more ribs as they mature, and seed from a tricostate parent will only sometimes come true.
Description
In every respect other than rib count, tricostatum looks like a typical Astrophytum myriostigma: a spineless, globular-to-columnar body clothed in the fine white flecking of woolly trichomes that gives the whole Astrophytum genus its characteristic marbled, "snow-dusted" appearance. The defining difference is the three broad, sharply angled ribs, which meet at deep, clean furrows and produce the acute triangular silhouette. Young plants show the three-rib geometry most crisply; as the plant gains height it becomes a neat three-sided column. The felted areoles sit along the rib crests, and mature plants bear the familiar yellow, silky, funnel-shaped flowers of the species, sometimes with a reddish throat.
Cultivation
Care is exactly as for the parent species — see Astrophytum myriostigma for the full account. In brief, grow it in bright light with a little shade from the fiercest afternoon sun, in a gritty, fast-draining mineral mix. Water thoroughly once the mix has dried out completely, and keep the plant dry and cool through winter to encourage a proper dormancy and a good flush of spring flowers (see Watering). Keep it above freezing.
One point specific to selected forms like this: because the three-rib trait is not fully fixed, give the plant steady, even light and unhurried growth. Poor light causes etiolation — weak, stretched, pale growth — and vigorous plants may in any case add ribs as they mature, so even, unrushed culture helps preserve the clean three-rib form for as long as possible.
Propagation
Tricostatum is raised from seed, as Astrophytum rarely offsets. Sow seed from tricostate parents and select the seedlings that show only three ribs — a proportion will, but many will revert to the normal four- or five-ribbed form. Choice or reluctant plants are sometimes grafted onto a vigorous stock to speed them along, though a well-grown three-rib plant on its own roots is the more prized result.
See also
- Astrophytum myriostigma — the parent species
- Astrophytum — the genus overview
- Grafting · Soil and potting mix · Watering · Propagation — seed