Strombocactus
Strombocactus is a small genus of slow-growing, cliff-dwelling cacti native to the arid canyons of central Mexico. Members are prized by collectors for their flattened, disc-shaped bodies, geometric spiral of angular tubercles, and papery, deciduous spines — and for producing some of the tiniest seeds found anywhere in the cactus family. Long treated as a single-species genus, Strombocactus is now generally understood to contain a handful of closely related taxa, all of them strictly protected under CITES Appendix I.
Description
Strombocactus plants are small, solitary and slow, forming a depressed, top-shaped (turbinate) body that in habitat sits almost flush with the rock, tapering below into a stout taproot. The surface is divided into low, rhomboid or diamond-shaped tubercles arranged in an elegant clockwise-and-anticlockwise spiral, giving the crown a tessellated, artichoke-like pattern. Body colour ranges from dull grey-green through blue-green to a pinkish or reddish cast in some populations.
Each tubercle tip bears an areole with a few weak, papery spines that are often shed with age, leaving older plants nearly naked. Flowers open at the woolly crown in spring and summer — funnel-shaped and modest in size, in shades of creamy white, pale yellow, or (in the reddish taxa) soft magenta-pink. The fruit is small and dries to release enormous numbers of dust-fine seeds, among the smallest of any cactus.
Distribution
The genus is endemic to central Mexico, where plants grow as obligate cliff- and rock-dwellers on steep limestone and other calcareous faces, often rooted directly in crevices with almost no soil. The classic species is associated with the canyon country of the states of Querétaro, Hidalgo and Guanajuato, while the reddish-flowered taxa occur further west in the Río Santa María drainage. In habitat the plants endure intense sun, sharp drainage and long dry spells, contracting into their crevices during drought.
Notable species
- Strombocactus disciformis — the type and by far the most widely grown species; grey-green disc-shaped body with creamy to pale-yellow flowers.
- Strombocactus corregidorae — a more recently described taxon from a restricted range, with subtle differences in body and flower.
- Strombocactus pulcherrimus — a distinctive form with a reddish body and magenta-pink flowers, sometimes treated as a variety or subspecies of S. disciformis rather than a separate species.
Botanical opinion on the boundaries within the genus continues to shift, and some of these names are treated by other authors as varieties or synonyms of S. disciformis.
Cultivation
Strombocactus has a reputation as a slow, somewhat demanding genus, but it is far from impossible for a careful grower. The overriding rule is drainage: plant in a very gritty, mostly mineral mix in a deep pot that accommodates the taproot, and err firmly on the side of dryness. Overwatering and heavy soils are the usual cause of loss, since the fleshy root rots easily.
Give bright light — these are full-sun cliff plants — with only light protection from the fiercest afternoon heat. Water sparingly during the warm growing season once the mix has dried completely, and keep the plants cool and bone-dry through winter, both to prevent rot and to encourage spring flowering. Because own-root seedlings grow at a glacial pace, many collectors graft young plants onto a vigorous rootstock to build size quickly, later growing choice specimens on their own roots. See Watering and Repotting for general technique.
Hobby and propagation notes
Seed is essentially the only means of propagation, as Strombocactus plants stay solitary and rarely offset. The dust-fine seeds are sown on the surface of a mineral mix and kept warm and humid; the seedlings are minute and must be grown on patiently for years before reaching flowering size. This slow tempo, combined with strict protection of wild plants, is precisely why grafting is so popular in the hobby. All material in cultivation should be nursery-raised — wild collection is both illegal and ecologically damaging. See Propagation — seed for a full walkthrough.
Legal status
Unusually among ornamental cacti, Strombocactus is listed under CITES Appendix I, the most restrictive category, reflecting its narrow distribution and vulnerability to poaching. This means international trade in wild-collected plants is essentially prohibited, and even artificially propagated specimens require CITES documentation to cross borders. Nursery-grown plants raised from seed are legal to own and trade domestically in most places, but growers moving plants internationally should be aware of the paperwork involved. Collecting from the wild is illegal.
See also
- Strombocactus disciformis — the type species
- CITES · Grafting · Soil and potting mix · Watering · Propagation — seed · Repotting