Hedgehog Mushroom

Hedgehog Mushroom (Hydnum repandum) is the best-known member of the small group of toothed fungi that grow from soil rather than wood. Turn one over and the underside is not bladed or perforated but bristling with hundreds of soft, crowded, tapering spines, giving the species its common name and making it one of the easiest woodland mushrooms to place to genus at a glance.

Hydnum belongs to the Cantharellales, and molecular work has confirmed the surprising conclusion that hedgehog fungi are close relatives of the chanterelles. The resemblance is real once you look past the hymenium: the same pale apricot to buff colouring, the same thick, brittle, slightly waxy flesh, the same tendency to grow in troops and rings on mossy ground, and the same irregular, wavy, off-centre growth form.

What was long regarded as a single variable species is now understood as a complex of several closely related taxa separated on cap colour, size, stipe position and spore dimensions. H. repandum in the strict sense is the large, pale, robust form; smaller, deeper orange, navel-capped forms are usually assigned to H. rufescens or H. umbilicatum.

Cap

3-17 cm across, at first convex with an inrolled margin, then flattening and often becoming irregular, lobed, wavy-edged or even fused with neighbouring caps. The surface is dry, matt and suede-like, sometimes finely felty, and coloured pale cream, buff, pale apricot or a soft peach-orange; it bruises and ages to a deeper orange-brown, and older caps frequently develop rust-coloured blotches.

Spines

The defining structure. The underside carries dense, crowded, needle-like spines 2-8 mm long, whitish to cream or pale pinkish-buff, tapering to fine points. They are extremely brittle and rub off easily under a thumb, leaving a bare surface. The spines run a short distance down the stipe (decurrent), and are shorter and less developed near the cap margin.

Stipe

3-8 cm tall and 1-3 cm thick, stout, firm, often distinctly off-centre or eccentric, usually white to cream and sometimes swollen toward the base. Handling or bruising turns the surface a dull orange-yellow. There is no ring and no volva.

Flesh & spore print

The flesh is thick, white, firm but brittle, and snaps cleanly like chalk rather than tearing fibrously. Cut surfaces slowly discolour yellowish to pale orange. The spore print is white to cream.

Hydnum repandum is ectomycorrhizal and grows on the ground in both broadleaved and coniferous woodland. It is most frequent under beech, oak, birch, spruce and pine, particularly where there is a good moss layer, and it appears on a range of soils from acidic sands to calcareous loams. It commonly forms long troops, arcs and partial fairy rings, and caps growing side by side often fuse into irregular composite masses.

The season is long: fruiting typically begins in late summer, peaks through mid to late autumn, and can continue into early winter in mild maritime climates, since the firm flesh tolerates light frost better than most soft-fleshed woodland mushrooms. The species is widespread and often common across Europe, temperate Asia and North America, with related members of the complex occurring in Central America and Australasia.

As an ectomycorrhizal partner, Hydnum repandum forms a sheath of fungal tissue around the fine root tips of its host trees and an internal network between the outer root cells. Through this interface it supplies the tree with water, phosphorus and nitrogen scavenged from a soil volume far larger than the roots alone could reach, receiving photosynthetic sugars in return. It is a generalist, capable of partnering with a wide range of both broadleaved and coniferous hosts, which explains its broad habitat tolerance.

The underground mycelium is perennial and can occupy substantial patches of woodland floor, producing fruiting bodies in the same arcs season after season. The spines increase the spore-bearing surface area relative to a smooth underside while remaining simpler to build than gills or tubes, and spores are shed passively into air currents moving beneath the cap.

The Terracotta Hedgehog (Hydnum rufescens) and Hydnum umbilicatum are the closest matches: both are smaller, more deeply orange to reddish-tan, with a more regularly round and often navel-depressed cap, a more central and slender stipe, and spines that are less strongly decurrent. Chanterelles (Cantharellus species) share the colour, brittle flesh and growth habit but carry blunt forking ridges instead of spines.

Scaly Tooth (Sarcodon imbricatus) is much larger and darker, with a brown cap broken into coarse upturned scales and grey-brown teeth beneath, and it drops a brown spore print rather than a white one. The Hericium toothed fungi share the spiny hymenium but grow directly on standing trunks and fallen logs as cascading white masses with no cap or stipe at all.