Common Name: Hygroscopic Earthstar
Fruiting body round to slightly compressed, 2-4 cm broad, partially emergent, becoming epigeous; exoperidium splitting into 7-14 rays with an expanded diameter of 4.5-9 cm; rays thick, leathery, the inner surface cracked, grey to brown, hygroscopic, flattened to the substrate in moist weather, encircling the spore sac when dry; spore sac globose to somewhat flattened, 1.5-3.0 cm broad, sessile, buff-brown, light-grey to medium-brown, the surface roughened or hairy, rupturing via an irregular slit or tear; gleba at first white, brown at maturity, columella absent.
Spores 7.5-11 µm, nearly round, warted, brown in mass.
Scattered to gregarious in a variety of habitats but most common locally in open or disturbed areas in woodlands; fruiting from late fall to mid-winter.
Inedible, too tough.
The tough, leathery fruiting bodies of Astraeus hygrometricus can persist for months but are easily overlooked, especially in dry weather, when the rays fold tightly over the spore sac. A related species Astraeus pteridis is less common but considerably larger, between 12 and 15 cm broad. It is found mostly north of the San Francisco Bay Area. Several species of Geastrum also have hygroscopic rays but can be distinguished by the presence of a peristome, (circular area around the spore opening) a columella (sterile stalk within the gleba), and generally smaller spores.