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Plant Highlight: Echinocactus platyacanthus
In the early days of cactus classification, the genus Echinocactus was conceived of as a very broad category taking in many diverse globular cacti from both North America and South America. Now, however, the name is restricted to 5 species native to Mexico and the southwestern U.S. These plants, along with members of the genus Ferocactus, are popularly known as barrel cacti. Although there are not many species of Echinocactus, plants are abundant in nature over a wide area from the California deserts in the north down to northern Oaxaca. The most widely-distributed of all, and also the largest, is Echinocactus platyacanthus. It occurs from southern Coahuila southward through the dry areas of eastern Mexico to the Oaxaca-Puebla border area. Plants vary somewhat through its range, and different forms have been given separate species names. These are no longer recognized, but nurseries, collectors and gardens still often feature plants labeled as E. ingens, E. palmeri, and E. grandis, all of which are now considered forms of E. platyacanthus.
Echinocactus platyacanthus is not a fast-growing plant. After over 30 years in the ground at RBG, our largest specimen is about a foot and a half in height (½ meter), as compared to the giants found in some parts of its range, which may be over 2 meters in height. This cactus has an unusual progression from seedling to adult. When young, plants are quite blue in color, followed by a stage in which they become green with striking red or purple stripes. After this, they settle into their solid-green mature appearance. In some of the wild populations, plants remain globular throughout their lives, even when they become huge specimens 4 feet tall (over a meter). In the southern part of the species’ range, however, plants go on to become stout-columnar and may attain heights of 7 or 8 feet (2½ meters). The name E. grandis was coined for this tall-growing form, but in other respects these plants do not differ significantly from other forms, so modern taxonomists do not consider the difference sufficient to warrant even a separate variety.
The spines on E. platyacanthus are stout, but they are shorter and less numerous than those of their close relative E. grusonii (the golden barrel cactus), so the plant body is less obscured by them. In both species, there is a large pool of buff-colored wool covering the central growing point, and the flowers are produced at the rim of this area. Both species have bright yellow flowers, but those of E. platyacanthus are larger. Many other kinds of cacti have a particular time of year when they produce a concentrated burst of flowering, but E. platyacanthus flowers on-and-off throughout the summer (this is true of E. grusonii as well).