Beech Mushroom

The Beech Mushroom (Hypsizygus tessulatus) is a clustered, firm-fleshed hardwood fungus best known in East Asia, where its cultivated form is grown commercially as shimeji. In the wild it forms dense tufts of dozens of small to medium fruitbodies emerging from a single point on a dead or dying trunk, each cap decorated with the faint, water-soaked marbling that gives the species its epithet tessulatus, meaning tessellated or tiled.

It sits alongside Hypsizygus ulmarius in the Lyophyllaceae, a family of firm, white-spored, often clustered agarics. Both species share cracked or spotted cap surfaces, notched gill attachment, and a white spore deposit, but the Beech Mushroom is markedly smaller, far more gregarious, and tied more closely to beech.

The cultivated strains sold in shops — pale white buna-shimeji and tan brown-capped forms — look considerably more slender and uniform than wild collections, which have broader, flatter caps and thicker, more irregular stipes. Wild material also shows the mottled cap much more clearly, especially in damp weather.

Cap

Caps are 2–8 cm across, convex becoming flat or slightly uplifted at the margin, smooth and dry to slightly tacky. The ground colour is tan, buff or pale grey-brown, characteristically overlaid with paler, roughly circular blotches and streaks that give a marbled or tiled appearance. Older caps often develop shallow radial cracks toward the margin.

Gills

The gills are white to creamy, close, moderately narrow, and attached by a notch or a very short decurrent tooth. They remain pale throughout development and have smooth, even edges. Because the caps in a cluster crowd one another, gill faces are frequently pressed and distorted where neighbouring caps overlap.

Stipe

Stipes are 3–10 cm long, 0.5–2 cm thick, white to pale buff, firm, smooth to finely fibrillose, and typically curved as they turn upward from the wood. They are fused at the base into a shared tough mass, and there is no ring and no volva. Within a single cluster the stipes may be central or somewhat eccentric depending on how each fruitbody grew around its neighbours.

Flesh & spore print

Flesh is white, firm and slightly fibrous, unchanging when cut and not bruising to any distinctive colour. The spore print is white; spores are small, smooth and subglobose, a shape shared with the Elm Oyster and useful for confirming the genus microscopically.

This species fruits on dead and dying hardwood, above all beech, but also elm, maple, cottonwood, willow, birch and other broadleaved trees. It emerges from trunk wounds, dead standing stems, large fallen logs and cut ends, and almost always in dense clumps of many fruitbodies from a common attachment point. In beech-dominated forests it can be one of the more conspicuous late-season cluster-formers on fallen giants.

The season runs from autumn into early winter, with fruiting often continuing after the first frosts because the flesh is firm and cold-tolerant. Its distribution is broadly north temperate, spanning Europe, northern and eastern Asia and North America, and it is generally commonest where mature beech or elm are abundant.

Hypsizygus tessulatus is chiefly a saprotroph and weak parasite that causes a white rot of hardwood. It removes lignin along with cellulose, leaving the decayed wood pale, soft, fibrous and light. Colonisation often begins in the standing trunk of a stressed or wounded tree and continues after the tree falls, so the same mycelium may fruit for several years from one large log as it slowly consumes the woody core.

By breaking down heavy beech and elm timber it returns locked-up carbon and mineral nutrients to the forest floor and softens dead trunks, opening them to colonisation by insects, other fungi and cavity-using vertebrates. Its clustered fruiting habit, with many caps sharing a single fused base, maximises spore output from one investment of mycelial resources — an efficient strategy for a fungus that fruits high on vertical surfaces where wind dispersal is effective.

Hypsizygus ulmarius is the nearest relative and shares the cracked cap and notched gills, but it is much larger, whiter, and fruits singly or in small loose groups from wounds on living trees rather than in dense multi-capped tufts. Pleurotus ostreatus forms overlapping shelves with gills running well down a very short lateral stipe, and its caps are unmarbled grey to brown.

Clustered wood-growing Armillaria species can superficially resemble it in habit but have a persistent ring on the stipe, finely scaly or hairy cap centres, and typically honey-brown colours without pale tessellated blotching. Lyophyllum decastes and relatives also form fused clusters but grow from soil and disturbed ground rather than directly from wood, and have greyer, unmarbled caps. Small clustered Pholiota species differ in their brown spore print, scaly caps and ringed stipes.