About Cirsium arizonicum (A.Gray) Petr.
Cirsium arizonicum, common name Arizona thistle, is a North American thistle species belonging to the daisy family Asteraceae. It is native to the southwestern United States and northwestern Mexico, and has been documented growing in Arizona, southeastern California, New Mexico, Nevada, Colorado, Utah, Sonora, and northwestern Chihuahua. Arizona thistle is a herbaceous plant that may be biennial or perennial, and reaches a height between 30 and 150 cm. It has spiny oblong-obovate leaves with pinnate lobes. Basal leaves are larger, ranging from 10 to 40 cm long, while upper stem leaves can be as short as 3 cm. The inflorescence measures 3–4 cm long and 1.5–2 cm in diameter, and ranges in color from red to pink or purple. All florets in each head share the same form, with no division between disc and ray florets. Individual plants vary widely in the number of flower heads produced, ranging from just one head to 100 heads. Flowering occurs from mid spring to mid fall. This species grows in mountain habitats, at elevations between 900 and 3,600 m. Five varieties of Cirsium arizonicum have been formally identified. Cirsium arizonicum var. arizonicum (synonym var. nidulum (M.E.Jones) S.L.Welsh) is found in Arizona, New Mexico, California, Nevada, Utah, Sonora, and Chihuahua. Cirsium arizonicum var. bipinnatum (Eastwood) D.J.Keil, common name Four Corners thistle, occurs in Arizona, New Mexico, Colorado, Utah, and Sonora. Cirsium arizonicum var. chellyense (R.J.Moore & Frankton) D.J.Keil (synonym C. chellyense R.J.Moore & Frankton), common name Navajo thistle, is native to Arizona and New Mexico. Cirsium arizonicum var. rothrockii (A.Gray) D.J.Keil (synonym C. rothrockii (A.Gray) Petrak), common name Rothrock's thistle, grows in Arizona, New Mexico, and Sonora; this variety occurs on both rims of the Grand Canyon, and extends a short distance down into the canyon below the rims. Cirsium arizonicum var. tenuisectum D.J.Keil, common name Desert mountains thistle, is found in California, Nevada, and Sonora.
