Along with the genus Ariocarpus, discussed last month, Mexico , and northeast Mexico in particular, is home to a number of small cactus genera, plants that in both looks and choice of habitat seem to defy common sense. In this column I’ll talk about most of these true Mexican oddities and try to do them justice.
Visualize a small gray disc, hard as a rock and attached to sheer rock walls as if cemented in, with its upper surface composed of rhomboidal tubercles patterned in a symmetrically spiraling sunburst, and you have Strombocactus disciformis. The only member of its genus (along with an extraordinarily rare dwarf subspecies from Guanajuato state, esperanzae), Strombocactus grows in an extension of Chihuahuan Desert habitat south of the desert’s “official” border, in the Mexican states of Queretaro and Hidalgo . The plants form low gray cylinders, up to about four inches tall and the same in diameter, and completely round. Up to four short, somewhat ephemeral and not very sharp spines surmount slightly protruding tubercles. Looking like oversized spiraling aspirin tablets half encased in gypsum and limestone soil, strombocacti dot steep, crumbly slopes of small canyons and ridges, in some cases packing absolutely vertical walls in mini-colonies so dense that they look like cactus-patterned wallpaper. After pollination their white flowers produce tiny, difficult to germinate seeds. Though Ariocarpus are slow growing, Strombocactus are even slower, taking perhaps a dozen years to reach an inch in diameter. Very occasionally they may split into two bodies in the process known as dichotomous branching.
Though fairly rare, small colonies of Strombocactus occur here and there over a wide area, generally in the vicinity of normally dry water courses and canyons. The massive floods that fill these gorges occur infrequently enough that occasional good sized trees grow right in the middle of the channels. From time to time, though, flash floods undercut and finally wash away sections of the canyon walls where the cacti grow. The same floods that wipe out pockets of plants can carry viable seeds many miles downstream, to deposit the fortunate ones in acceptable settings where a new colony may begin. Always coveted by collectors, strombocacti have generally been rare in cultivation. As collected plants have disappeared from the trade, a few growers have started growing the plants from seed, and though seedlings take years to reach mature size, young plants look exactly like miniature versions of larger ones, and they flower at an inch or less in diameter.
Each a variation on the theme of a collection of tiny domes three quarters buried in soil, and armored either with highly modified tubercles or even more fancifully transformed spine clusters, the two Pelecyphora species are among the weirdest and yet most appealing of cacti. P. strobiliformis somewhat resembles a Strombocactus with elongated armor-like tubercles that end in residual spines now diminished into harmless little bristles. The tubercles, flattened and triangular in shape, with the apex pointing upward, adhere closely to the body of the plant like the scales of a fish or decorative shingles on an ornate old Victorian house. At first glance P. asseliformis, the second member of the genus, with the same general shape, seems devoid of spines, but the minutely toothed rows of thin, sharp-angled, flat-faced ridges that cover the plant are actually highly modified spine clusters. The tiny linear markings that make the ridges look like miniature mammoth teeth are actually the plant’s extremely reduced, almost unrecognizable spines.
In contrast to the white flowers of Strombocactus, Pelecyphora have red-purple blossoms; both species, though exceedingly slow growing, eventually form small clusters of separate heads closely attached to the main stem. Under the weaker sunlight of cultivation pelecyphoras often begin to taper at their tops, but in the wild their apices remain almost flat, the plants mostly underground, protruding a half inch or less above the surface. Their armored bodies seem as hard as rocks, and in combination with their gray coloration, neither species looks much like what we commonly think of as plants, not even cactus plants. Consequently, Pelecyphora is one of those genera that cactus collectors, who generally crave the unusual, have always coveted.
Compared to many Ariocarpus species, or Strombocactus, Pelecyphora grow in less bizarre looking environments. P. asseliformis lives on low, scrubby, limestone hills (some dangerously close to the expanding city of San Luis Potosi ) where it stays mostly hidden among a fairly dense growth of grasses and small shrubs. Several other small cactus species, some quite rare themselves, live on these hills, which present an aspect less barren than other desert areas. Providing a hint as to its cultural preferences, however, the Pelecyphora cling to the summits, where the drainage is quickest and the light most intense. P. strobiliformis, growing a good ways farther north, has chosen a more typically arid landscape for its habitat, but both species can tolerate a reasonable amount of water while in active growth.
Though it shares the large Tamaulipan valley of Jaumave with Ariocarpustrigonus, Obregoniadenegrii (the only member of its genus) usually occurs in habitats that contrast with the fully exposed, sun-drenched flats favored by the Ariocarpus. Obregonia often grow on low hills, around the bases of large rocks where they frequently form small clumps. As is the case with many cacti that prefer a bit of shade, plants may also cluster around the bases of the thorny shrubs that provide a bit of cover on these gray limestone hills.
Individual Obregonia generally reach about five or six inches in diameter, though they may become considerably larger. They form a low, open rosette that visually lies somewhere between Ariocarpusretusus and Pelecyphorastrobiliformis. With tubercles more numerous, thinner, and more erect than the ariocarpus, the rosulate obregonias look a bit like an opened up version of the essentially globular pelecyphora. Unlike an Ariocarpus, each Obregonia tubercle is tipped with a few thin, curling spines, longer than the stiff little bristles of P. strobiliformis, but still not particularly useful for protection.
Though Obregonia bear a general resemblance to Ariocarpus, Pelecyphora and Strombocactus, DNA testing suggests that the plants are more closely related to the peyote cactus, Lophophora. Perhaps lending support to this observation, in contrast to Ariocarpus, Pelecyphora and, most particularly Strombocactus, which grow painfully slowly from seed, Obregonia grow much more quickly, at a rate similar to Lophophora as well as to more typical cacti. As a result, they’re less difficult to find in cultivation than many of the other extreme cacti. They display their proportionately large white flowers early in the warm months of the year.
Aztekium and Geohintonia
Aztekium ritteri, one of the three final oddities to be discussed, could reasonably lay claim to being the most extreme cactus species of all. The remaining two, Aztekiumhintonii and Geohintoniamexicana, somewhat less peculiar in general appearance, were discovered much more recently. Given the isolation, small size, and geologically odd nature of the Chihuahuan Desert fringe micro-habitats where these plants live, it’s not surprising that they remained undiscovered for so many years. Unfortunately, the flurry of illegal collecting set off by the discovery of these two species lets us know that that avarice is still alive and well.
Aztekiumritteri, first found during the height of the Art Deco movement in the late 1920s, received its name because of the plant’s fancied resemblance to Meso-American pyramids (a popular Art Deco motif). The plants grow on a few patches along the walls of a single small canyon system that terminates near the entrance to the valley home of Ariocarpusscaphirostris. A. ritteri is a small plant, rock hard, with individual heads rarely more than two inches across, rock hard and dusty gray-green or yellow-green in color. Though not really resembling architecture, the plants do form a low dome or tent shaped body, divided by a number of slightly meandering, irregular, hump-backed ribs furrowed in turn by rows of rounded, transverse ridges which bear areoles and ephemeral spines. Rudimentary longitudinal secondary ribs line the gaps (or axils) between the primary ribs, and small areoles poke out from the outermost tips of the lateral ridges along the main ribs. As well as spines, the areoles also produce tiny new plant bodies that slowly grow into small clusters.
A. ritteri displays small apical pink-white flowers in summer, and retains its tiny seeds in its dried fruits for years. The many headed clusters cling to absolutely vertical canyon sides, rooting into tiny fissures in the rather crumbly rock walls and localized outcrops of equally steep slopes of crystalline gypsum. Similar to Strombocactus habitat, these steep slopes are almost completely devoid of any other potentially competing vegetation, although beautiful, green discs of Selaginellagypsophila manage to share the gypsum with the aztekiums. Years of collection have reduced the population mostly to seedlings and single headed plants at the canyon tops and bottoms, where the plants can be reached, but the inaccessible middle heights of the walls remain covered with almost solid masses of multi-headed plants.
The 1991 discovery of Aztekiumhintonii caused quite a stir in the cactus world. Occurring somewhat farther to the south than A. ritteri, very far from anything remotely resembling a beaten path, A. hintonii actually has a wider—though still very localized—distribution, than its relative. It grows in a similar landscape of sharply incised canyons where it clings in great numbers to almost vertical walls. A much larger plant than A. ritteri, A. hintonii actually is somewhat less strange looking. The deeply indented axils between its ribs lack the secondary ribs of A. ritteri, and though it shares the rows of transverse grooves, they are smaller and more symmetrically arranged. Individual plants can reach over four inches in diameter and three inches in height, and they sometimes form clumps of a few heads. A.hintonii grows more fully exposed to the sun than A. ritteri, which tends to sink down into its matrix, and the larger plant also produces larger, deep pink to magenta flowers in contrast to the small, pale blooms of A. ritteri.
Discovered a matter of days after A. hintonii, Geohintoniamexicana grows farther downstream in the same general habitat of small, steep canyons and gypsum outcrops (at some locations both species are present). Solitary, typically squat and globular, the plants sometimes grow a little taller, four inches across and up to five inches high. Geohintonias have more ribs—eighteen to twenty—than aztekiums, and lack the transverse grooves. The tops of their steep, narrow rib bear ephemeral, upward pointing spines, and develop a linear corky covering after the spines fall. A faint gray coloration, like a translucent glaze, overlays their light green bodies. Geohintonia looks more like a member one of the odd South American cactus genera such as Uebelmannia or Copiapoa than it does any other Mexican cactus. In spite of morphological differences, however, it’s obviously closely related to Aztekium, and its been proposed, though not widely accepted, that the plant represents an old natural hybrid with A. hintonii as one of the parents. Regardless of its ultimate botanical status, Geohintoniamexicana, along with other inhabitants of precipitous gypsum canyon walls, represents the ultimate among the extreme cacti.
In cultivation aztekiums grow excruciatingly slowly, even more slowly than strombocactus. Although grafting and experiments with unconventional methods of cultivation have resulted in flowering plants after only five or six years, left on their own roots four year old A. ritteri seedlings are no bigger than the head of a pin. It’s difficult to guess how long it would take for such plant to grow into a five or six inch wide, fifteen or twenty-headed cluster, very possibly several human lifetimes.
Though far from the easiest cacti to grow, all these extremely highly specialized plants will adapt to cultivation when given the correct treatment. They need very bright light, for most of them the brightest light possible, though Obregonia thrives best in just a bit less. In the northern parts of the United States providing adequate light may be a problem, but the sunlight in California and the southwest is sufficient. Potting soil for these plants should, above all, be very fast draining, and they do best with little if any organic matter in their soil mix. Many of these cacti grow in alkaline soils or limestone rocks, and, though not really necessary, a little lime in the mix won’t hurt.
Summers in the Chihuahuan Desert can bring sudden downpours, and when in full growth Ariocarpus and its relatives do need water. Aztekiums and Pelecyphora strobiliformis, in particular, can take a surprising amount of water, but to prevent rot, all of these can be given less water than average. For coastal central California, water every ten days to two weeks is about right; adjust accordingly for climates with more or less sunshine and heat. In winter the plants won’t mind total dryness. In areas with much rain, humidity and low light, leaving them completely dry all winter probably is best; in the southwest or southern California , a light watering once or even twice in winter shouldn’t hurt. As with most cacti, the plants can withstand considerable cold if dry. Ariocarpus agavoides tends to begin its growth a bit later than most of these plants, and may rot if given water too early, and Obregonia also can rot if watered too early in the year.
In the past plants of this group offered for sale were almost certainly collected from the wild. Even today, despite laws intended to protect them, some of the species with extremely limited ranges remain in real danger of extinction. Fortunately, a number of growers have accepted the challenge of raising them from seed, often grafting them to increase their rate of growth, and these seed grown and artificially propagated plants are making their way into the trade. At five or six year an Ariocarpus will be a reasonable looking, if fairly small plant, and though Strombocactus and Aztekium in particular, mature extremely slowly, even they are offered by specialist growers from time to time.
The Garden has a fairly good collection of these rare and fascinating plants. We have offered seedlings of several of these plants at sales in the past, and may eventually be lucky (or patient) enough to offer them again.