Echinocactus is a small genus of barrel cacti in the family Cactaceae (subfamily Cactoideae), order Caryophyllales. Together with the closely related genus Ferocactus, it forms the two genera commonly known as barrel cacti. The genus comprises approximately 3–9 accepted species native to the deserts of southwestern North America, from the southwestern United States south into Mexico.
Plants are characteristically barrel-shaped or spheric to short-cylindric, with unsegmented stems ranging from gray-blue to grass-green, typically 4–40 cm tall and up to 80 cm in diameter, though large old specimens can reach 2 m in height. The apex is conspicuously woolly. The prominent ribs — usually 8 to 27 — are very straight and broadly rounded. Each areole bears 5–19 stiff, rigid spines that are conspicuously ridged in a ring-like (annulate) pattern and range in color from straw to brown, pink, red, or gray. Flowers are diurnal and appear near the stem apex; they are broadly funnel-shaped with inner tepals in yellow or pink to magenta tones. A key character separating Echinocactus from Ferocactus is the fruit: in Echinocactus the fruit is copiously woolly, nearly dry, and whitish-tan to pinkish at maturity, whereas Ferocactus fruits are succulent and colorful. Seeds are reddish-brown to black, 2.4–4.7 mm, and shiny or dull. The base chromosome number is x = 11.
Most species are solitary and will die rather than produce a lateral bud if the growing apex is damaged, making them notably sensitive to physical injury. An exception is Echinocactus polycephalus, which forms compact mounds of 30 or more branches. Propagation throughout the genus is by seed.
Etymology
The name Echinocactus is formed from the Ancient Greek ἐχῖνος (echînos), meaning "spiny" or "hedgehog," and kaktos, referring to a spiny plant. The name thus translates roughly as "hedgehog cactus," an apt description of the genus's heavily armed, spiny stems.
Distribution
Echinocactus is native to the arid and semi-arid regions of southwestern North America, encompassing approximately six species across the southwestern United States (including Arizona, New Mexico, and Texas) and Mexico. The genus is associated with Chihuahuan and Sonoran desert habitats. In the flora of Arizona, the best-documented species are E. horizonthalonius and E. polycephalus.
Ecology
Echinocactus species are deep-desert barrel cacti adapted to harsh, rocky, and well-drained arid soils. Stems are typically solitary and deeply anchored; if the apex is injured most species die rather than sprouting lateral branches, a vulnerability that makes established plants particularly valuable. The exception is E. polycephalus, which forms large clumping mounds. Flowers are produced diurnally near the apex, and the woolly fruits persist on the stem.
Cultivation
Echinocactus is propagated exclusively by seed. In cultivation the genus is valued as a slow-growing ornamental in dry gardens and collections; the copious spination and barrel form make specimen plants striking features in succulent landscapes.
