| Mammillaria
With Mammillaria, once again a genus of cactus takes the spotlight, and quite a genus it is. There are well over a hundred valid species contained within the genus, and although we might generally think of the species of any given genus as variations on a theme, in the case of Mammillaria the theme would have to be remarkably expansive and diverse. Over the years these widely varying forms have been divided into a number of separate genera, and though most authorities now agree on taxonomy as far as genus, there still are a few areas of fuzzy nomenclature lurking around the edges of the group.
Although the center of distribution for the genus is Mexico, some species range into Central America and the West Indies, and a surprisingly large number of different Mammillaria reside right here in the U.S. A. In addition, the Mexican state of Baja California is the home of a number of related species quite distinct from those from other regions. With so much variety it’s impossible to describe all the kinds of Mammillaria in a few paragraphs, but I’ll try to give a quick overview along with some cultural guidelines.Most Mammillaria are small to medium sized, with more-or-less globular, tubercled, bodies. The tubercles, in fact, give the plants their name (owing to a somewhat fanciful resemblance to breasts—mammillia). These tubercles may be extremely prominent, with sharply defined, angular dimensions, or may be much smaller. The spines emerge from the tips of the tubercles, and the number and length of spines may obscure the tubercles, but they’re always there. The spines themselves, with often neatly organized arrangements, consist of radial and central spines, of varying quantities depending on the plant. The gaps between the tubercles may display bristles or wooly hairs or both. Spines, particularly centrals, may be either straight or hooked, and the central spines themselves may be present or absent. Although differences in spination, wooliness and size of tubercles can be among the most obvious visual features of the plants, cultural conditions have a great impact on the way all these characteristics develop, and plants that look very different from each other may simply represent variations within a single species. Consequently the number of species considered valid within the genus has varied over the years, reaching over three hundred at its peak. Field work and other study has shown that many of these distinct species merely represent points along a continuum. As an example, I’ve seen individual clumping plants that on the side more exposed to sunlight look like what was once called Mammillaria microthele, while on the shady side, with tubercles less densely arranged and less wool between the tubercles, the plant would be considered M. formosa. Mammillaria flower from near their apices, often displaying a ring of flowers arranged around the growing point. Most species have small flowers, (though a few species have flowers proportionately huge for the size of the plant) usually either light yellow or pink to purple. As if to make up for the small size of the flowers the plants often bear them profusely and they may be followed by elongated little bright red fruits. Some Mammillaria remain solitary, but most gradually cluster, either by forming offsets at the base or by dividing themselves in two at the top (called dichotomous branching). Though some short spined plants show off their green epidermis, in many species the clusters of spines are so dense as to obscure the body. These plants may appear largely white, golden-yellow, silvery-gray or even black, depending on their spine colors. Many Mammillaria have hooked central spines, and repotting these can be fairly challenging and may explain why the common name of some species is fishhook cactus. Within these generalities are many exceptions, and there are Mammillaria with spines soft as feathers, others that grow quite tall and almost columnar, others that have one inch bodies and two inch flowers, and all-in-all about as many varied forms as can be imagined for a small to medium sized cactus.The more popular species include plants such as M. plumosa, with spines converted into soft white feathers, M. schiediana, with clusters of only slightly harsher, golden spines. M perbella, (with small purple-red flowers) and the similar M. formosa ssp. microthele (with yellow-white flowers), are both plants that slowly cluster by dichotomous branching and that have extremely reduced or absent central spines, while M. crucigera, from a tiny region of gypsum cliffs in the Mexican state of Oaxaca, has spine clusters reduced to a little cross of four tiny, harmless radial spines. Proportionately large-flowered species include several without central spines that produce one or two inch flowers that completely dwarf their tiny bodies, plants such as M. theresae, M. goldii and M. saboae, all from northwest Mexico, along with the fairly recently discovered M. hernandezii from hundreds of miles to the south. Other mammillarias are appreciated for the intricate geometrical arrangement of their spines, plants such as M. elegans, M. microhelia, and M. bombycina. This last has hooked, but not too fierce central spines, while other species, such as the Baja California plants M. blossfeldiana (very rare in the wild) and the omnipresent (in Baja) M. dioica, along with numerous species from the Mexican mainland, possess fiercely curving centrals that make their repotting a sometimes unpleasant adventure. Among the best of these are the high altitude, red tubular-flowered and generally beautiful Mammillaria senilis, covered with hooked, glassy white spines, and M. guelzowiana, with dark hooked centrals, lots of white wool, and huge, glowing cerise blossoms.Another group of Mammillaria lack central spines but make up for this by having such dense covering of white radial spines as to completely obscure the green body. These plants, looking like snowballs growing in barren deserts, often in white limestone soil, include a number of Chihuahuan desert plants related to M. lasiacantha, (M. egregia, for example) with other plants, such as M. klissingiana, almost as pure white. The most popular of these white plants, because of its relative ease in cultivation, was called M. candida for years, but although it certainly looks, grows and acts like a Mammillaria, differences in its seed structure have led to its current separation into its own awkwardly named genus, Mammilloydia.Another batch of species includes those with shorter spines, and thus exposed green bodies. Some of these, such as M. compressa, form enormous clumps, easily a yard across, composed of dozens if not hundreds of three inch spheres. A more interesting, largely solitary plant is M. sempervivi, with extremely elongated, pyramidal tubercles surmounted by short spines. Many of these solitary green mammillarias grow largely buried in the wild, with just their flat tops protruding a bit above the soil. In cultivation it can be a challenge to keep them flat on top, although others with this general habit form small domes in nature as well as in cultivation.I could go on and on with brief descriptions of Mammillaria for pages. Let it suffice to say that I have just scratched the surface on the individual types within this most interesting cactus genus. Those plants with less dense spination and obviously green bodies can get by with somewhat less light than their more densely spined cousins; however, the attractive, distinctly geometrically arranged spine clusters of the densely spined species provide much of the appeal of Mammillaria, and these plants do require very bright light. Most Mammillaria thrive in an average, quick-draining cactus mix, but some of the more arid growing ones (including some plants totally covered by tight clusters of white spines, some of the hooked spined ones, and those plants with tuberous roots) will do better in a soil mix even better draining than normal. Plants of these types should be watered a little more cautiously during the growing season, and be left very dry during the winter. To complicate things even further, despite their extremely arid habitats, Mammillaria from Baja California, with its winter rains, should not be left as dry at that time of year as other species that actually grow in far less arid circumstances.
All these cautions not withstanding, many Mammillaria will thrive on a sunny windowsill and the genus contains some of the prettiest as well as most satisfying cacti in cultivation. The Garden has an outstanding collection of Mammillaria, and we frequently have some interesting species to sell.
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