Summary:
The outstanding feature is growth from a large underground sclerotium. Other features include a moist to subviscid orange cap, subdistant gills that are dingy yellowish brown with whitish edges, a tough, subviscid to dry stem that is orange yellow becoming ochreous where bruised, and habitat in compost piles, rank grass, mulched gardens and loose soil burrowed by mammals, or in wood chips. This is not the same as Stropharia tuberosa Beardslee. The description is derived from Redhead(16) except where specified.

Cap:
1.5-6cm across, convex and subumbonate [more or less umbonate], becoming convex, margin incurved when young, 'brown orange' to 'deep orange' centrally, paler marginally ('light orange' to 'orange yellow'); moist to subviscid, slightly striate on overly mature specimens at margin

Flesh:
fleshy in cap, tough in stem; pale yellow brown in cap, in stem colored as surface or paler

Gills:
adnexed, with or without short decurrent tooth, subdistant; ''light grey yellow brown'' becoming ''grey yellow brown'' with whitish edges, (Redhead), the gill color when young is a pale gray (Paul Kroeger, pers. comm.)

Stem:
4-5.5cm x 0.25-0.4cm, narrowing slightly downwards, stuffed, sometimes abruptly swollen at soil surface, "tapered below into a short to long pseudorrhizae [sic] leading to a subterranean sclerotium", pseudorhiza up to twice length of stem, or directly arising from a sclerotium, stem tough, nearly woody near soil surface but root soft, stem ''orange yellow'' to ''light orange yellow'', becoming ochreous where bruised or handled; below ring zone sometimes subviscid and then appearing somewhat varnished when dry, "overlaid by scattered yellowish fibrils or fibrillose patches or bands", above ring zone minutely fibrillose; root is ''light grey yellow brown''; sclerotium irregular oblong to nearly round, up to 8.5cm x 5cm, often lobed, ''brown'' on exterior, the interior ''grey yellow brown'' to ''dark gray yellow'', with ochraceous areas

Veil:
whitish arachnoid [cobwebby] to fibrillose, whitish partial veil ornaments the upper stem, soon darkened by spores

Odor:
faintly farinaceous, sclerotium earthy to somewhat like green corn

Taste:
slightly bitter; sclerotium slightly radish-like or mild

Microscopic spores:
spores 9.5-11.5(12.8) x 5.2-7(7.8) microns, narrowly oval to elliptic, smooth and with faint germ pore; basidia 4-spored, 27-30 x 8-9 microns, clamped, colorless, suburniform, prominently projecting when mature; pleurocystidia (chrysocystidia) scattered, abundant, 40-49 x 10-15 microns, fusoid to fusoid-ventricose, thin-walled, colorless or occasionally yellowish, mostly embedded in hymenium, cheilocystidia 30-38 x 5.5-8.5 microns, obtusely ventricose to cylindric, often with an undulate neck, thin-walled, smooth, colorless

Spore deposit:
brown vinaceous (''deep grey brown'')

Notes:
Hypholoma tuberosum has been found in Vancouver BC, in arboreta in Seattle WA, and in Portland OR. It has also been found in a peat bog outside Sydney Australia, and is likely to have been imported to North America in materials used for cultivation (Paul Kroeger, pers. comm.).

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