Cacti of West and Southwest USA

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Cacti are the most distinctive plants of Southwest USA, and the majority occur in the four southernmost states of California, Arizona, New Mexico and Texas, especially in the Sonoran, Mojave and Chihuahuan deserts, where they are very widespread and numerous. Further north, some types like opuntia and echinocereus are still quite abundant, but there are also a number of smaller plants with much more limited distribution, such as sclerocactus and pediocactus.

The six cactus genera with the largest number of plants, and hence most likely to be encountered, are cereus, cylindropuntia, echinocereus, ferocactus, mammillaria and opuntia.

Less common types include ariocarpus, coryphantha, echinocactus, echinomastus, escobaria, grusonia, pediocactus, sclerocactus and thelocactus, while other rare genera (with the number of US species), are acanthocereus (1), ancistrocactus (2), astrophytum (1), epithelantha (2), glandulicactus (1), hamatocactus (1), lophophora (1) and neolloydia (1), plus a few types that grow only in Florida.

State species lists: Arizona, California, Colorado, Nevada, New Mexico, Texas, Utah.

Common Southwest Cactus Genera
Cereus and related species are tall, often tree-like cacti, and only three are common in the Southwest: the saguaro (carnegia genus), senita (pachycereus genus) and organ pipe (stenocereus genus). The latter two occur most visibly in Organ Pipe Cactus National Monument but the saguaro is widespread, one of the universally recognized symbols of this region. The only other cereus-like plants in the US include bergerocactus emoryi in south California and peniocereus greggii in south Arizona, New Mexico and Texas. Many more varieties of columnar cacti are found in Mexico, Baja California and South America.

Cylindropuntia cacti are a variety of opuntia that have thin, cylindrical stem segments instead of flat, rounded pads, but share other traits including branching profusely to form large clumps, and having small bristles (glochids) in addition to the regular spines. Most species have a very dense covering of spines, sometimes completely obscuring the green stems beneath, and cholla are a familiar sight across a variety of Southwest habitats, from the hottest deserts of Arizona and California to the grassy plains of New Mexico. Most species flower profusely. Spines are covered by a thin, papery sheath, a feature not present in the opuntia species.

Known commonly as the hedgehog cactus, this common, widespread variety is characterised by low clumps of cylindrical or conical stems, sometimes containing a hundred or more plants, and conspicuous red or pink flowers. They grow over a wide range of environments, from the low, hot deserts to cool mountain slopes, but most prefer unshaded conditions. Spines are arranged along vertical ribs, and are always straight, never hooked. There is quite a wide variation in appearance even within individual species, with various subspecies recognized, and it can be difficult to identify a particular plant.

Ferocactus plants are characterized by many heavy spines, growing along prominent ribs, and they include the large, common and widespread barrel cacti found in the hottest parts of the deserts of Arizona and California, some becoming up to ten feet high. There are also two less common US species - one in far south California, and ferocactus hamatacanthus in Texas and New Mexico. Like cereus, far more varieties are found further south in Baja California and Mexico.

Mammillaria are generally small, often delicate plants, usually forming clusters, with a wide variety of color, spination and flowers. Spines grow at the end of small tubercles rather than ribs - a characteristic shared with certain other, less numerous genera such as coryphantha and escobaria - and often include one of more longer central spines, which may be straight (pincushion cacti) or curved (fishhook cacti), surrounded by smaller radial spines. Flowers come not from the apex but lower down, in a ring around the upper part of the stem, and this helps to distinguish them from similar species, as these tend to flower from the tip.

Opuntia ('prickly pears') are branched, joined cacti, usually densely spined, though a few species have no spines. They are characterized by flattened pads, unlike the related cylindropuntia genus (cholla), where the pads are cylindrical. Some opuntia become large and tree-like while others stay at ground level, often forming extensive mats. There are many Southwest varieties, often similar in appearance, a few hybridized and many difficult to identify. The regular spines are surrounded by tiny bristles known as glochids, which are very irritating to the skin if touched.

Other Southwest Cacti
Ariocarpus, the living rock cactus, has just one US representative, found only in the Big Bend region of west Texas. Another half dozen species occur in Mexico, and all are quite rare.

Coryphantha cacti look like mammillaria in that they are small, have tubercles rather than ribs, and are solitary or form closely-spaced clusters; the main difference is that flowers are borne at the tip, rather than a little way down the stem. They are commonly known as beehive cacti, or topflower cacti.

Echinocactus plants resemble some of the ferocactus species; they are spherical or barrel shaped, have dense and/or thick spines arranged along ribs, and may be single or clustered, though they do not reach the great heights (up to ten feet) of the larger ferocacti.

Echinomastus, or pineapple cacti are usually single, forming small globes or cylinders characterised by tubercles arranged in rows, all quite spiny. There are six US species, all with limited distribution, mostly in Arizona and Texas.

Grusonia cacti are a type of cholla, characterized by short, club-shaped stem segments with pronounced tubercles bearing especially strong, sharp, dense spines, usually flattened and tapered. They form low mats, often covering a wide area.
Known variously as foxtail cactus, pincushion cactus or spiny star, escobaria is another group of small, tubercular cacti, similar to mammillaria and coryphantha. The US has over a dozen species, most relatively uncommon.

The pediocactus genus include some of the smallest species in the US, growing mostly in north Arizona and Utah. All are rare, and most occur in very localized areas, sometimes of just a few square miles.

Sclerocactus cacti resemble the echinocereus species but are usually single and have central spines that are hooked rather than straight, hence their common name of fishhook cactus. Upwards of 20 different types have been identified, found in higher elevation desert regions. Spines grow from tubercles arranged in prominent ribs.

Most thelocactus cacti are found only in Mexico; just one species grows in the USA, primarily in the Big Bend area of west Texas. The plants are small, globular or cylindrical, usually solitary, and produce large showy flowers right from the growing tip.
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