Cap 5-15 cm broad, convex, the margin incurved, expanding to nearly plane; surface dry, medium to dark-brown, appressed fibrillose at the disc, appressed squamulose towards the margin; flesh thick, pallid, discoloring vinaceous-brown when injured; odor, of "mushrooms;" taste, mild.
Gills free, moderately broad, close to crowded, drab pinkish-brown becoming blackish-brown.
Stipe 6-15 cm long, 2.5-3.5 cm thick, tapering to an enlarged base, stout, solid at first, the upper portion stuffed at maturity; surface dry, white at the apex, finely striate, pallid below, discoloring pale brown in age or medium-brown where handled, the base typically with pallid to brown scales, sometimes arranged in partial or complete rings; veil membranous, two layered, the upper surface white, striate, the lower surface with cottony, pallid scales, becoming pale brown before rupturing, forming a pendulous, superior ring; flesh white, staining pinkish to vinaceous-brown when cut.
Spores 7-9 x 6-6.5 µm, elliptical, smooth; spore print dark chocolate-brown.
Scattered, gregarious, to clustered in duff under Monterey Cypress (Cupressus macrocarpus); fruiting from mid-winter to early spring.
Edible and excellent, but prone to insect damage.
This large, robust, red-staining Agaricus typically has a dark-brown appressed squamulose cap and enlarged stipe base. Though not widely distributed, it can be locally abundant in cypress groves near the ocean (within the reach of summer fogs). Considerably more common and often confused with it is Agaricus fusco-fibrillosus, also a red-staining cypress dweller. It, however, is seldom as large or robust, and has an innately fibrillose, rufescent-brown cap rather than an appressed squamulose dark-brown cap. Both species frequently have thin circular bands of tissue near the base of the stipe, lilac-brown to pale brown in the case of Agaricus fusco-fibrillosus and medium to dark-brown in A. pattersonae. A third red-staining, scaly, brown-capped Agaricus is A. fuscovelatus. It differs by being smaller, less robust, having a conic cap when young, a coco-colored partial veil at all stages of development, and tending to fruit in caespitose clusters.