September 13, 2019

Living on the Edge: the Cottontop Cactus

image by Zdenek Richter

The Cottontop Cactus, Echinocactus polycephalus, is a moderate-sized barrel cactus that lives in some of the driest areas of North America. There are two similar subspecies: polycephalus, the most widespread, found in parts of California, Nevada, Utah, Arizona and northern Mexico, and xeranthemoides, found near the Grand Canyon in southeastern Nevada and Northern Arizona. “Way over to Hell and gone” is an apt description of the species’ habitat. It occurs from the spectacularly desolate mountains overlooking Death Valley to the wretchedly arid flats of the Cabeza Prieta National Wildlife Refuge.
Cottontop Cactus grow in arid or hyper-arid landscapes that usually receive no more than 5 inches of precipitation per year. In the Mojave Desert the species must survive exclusively on winter rainfall, but plants in the Sonoran Desert occasionally experience summer thunderstorms during the monsoon season. In the hot, stony wastes in which they grow, the sparse rains quickly drain away or evaporate.

The barrels grow to about 2 feet or so, gradually branching from the base to form clumps of up to 50 heads. Thick, fierce spines give the heads the appearance of medieval weapons. Rainfall temporarily turns the spines brilliant red. Given the slow rate of growth, it’s thought that large plants are over 100 years old. New growth tends to occur in Spring, months after the Winter rains have ended, and flowering takes place during the scorching heat of Summer.

The ripening fruit is embedded in a dense mass of tough, white fiber, thus the name "Cottontop".

image by Michael Thomas Bogen

Packrats and other rodents have to chew through the protective spines and fiber to get to the fruit.
Seeds found in ancient packrat middens indicate that the species has been dancing around its current range for the past 30,000 years. Favorable conditions for seed germination are infrequent occurrences, and seedling growth is very slow. With increasing temperatures and the long term prognosis of climatological drying of the Southwest, the survival of this tough, tough species in its exquisite environmental niche is, unfortunately, in question. 

image by Michael Thomas Bogen


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