Cap 4.5-11.0 cm broad, deeply convex, expanding to plano-convex; margin when young, ornamented with fine cottony scales, incurved, later decurved, often wavy; surface subviscid, color variable: in one form, dingy dark reddish-brown to mahogany-brown, shading to a pinkish or cinnamon-brown margin, overlain by a whitish bloom which becomes inconspicuous with age; in a second form, the color dull, dark orchraceous-brown at the disc, cream-buff at the margin; context white, soft, moderately thick, up to 1.5 cm at the disc, darkening somewhat when cut; odor pungent, slightly of radish; taste, mild.
Gills adnexed in youth, notched to emarginate at maturity, moderately broad, up to 8 mm in width, edges minutely fringed, in old specimens sometimes appearing marginate, i.e. colored brown from maturing spores; gill color at first cream-buff, becoming tan-brown in age; lamellulae 3-4 seried.
Stipe 4.0-8.0 cm long, 1.5 to 2.5 cm thick, hollow to stuffed, the base narrowed or enlarged; surface white, fibrillose-squamulose, discoloring dull light-brown where handled; partial veil absent.
Spores 9.5-11.0 x 5.0-6.0 µm, ellipsoid, thin-walled, roughened, hilar appendage conspicuous, germ pore lacking; spore print dull brown.
In small groups under both conifer and hardwoods; fruiting from late fall to mid-winter; occasional.
It is not clear whether Hebeloma sinapizans is a species complex or simply a highly variable mushroom. Two color forms appear to occur in California. The form commonly seen in the S.F. Bay Area has a dull reddish-brown to almost mahogany-brown cap with a distinctive whitish to pale greyish bloom when young. In age the cap color fades to pinkish-brown and much of the bloom disappears, but it is still recognizable by a pungent, radish-like odor, often wavy cap, and a fibrillose-squamulose stipe. A second form collected regularly in the Sierra typically has an ochre-brown cap with a pale margin. This is similar to the concept of many European mycologists. Of necessity, the above description has been written broadly to encompass both forms. In coastal areas, Hebeloma sinapizans is most likely to be confused with reddish-brown Tricholomas, notably Tricholoma muricatum and Tricholoma fracticum with which it sometimes fruits under Monterey pine (Pinus radiata). These species, like Hebeloma sinapizans, are thought to be toxic and should be avoided.