Blue Tooth

Blue Tooth is one of the most distinctive of the Hydnellum species, a genus of tough, ground-dwelling toothed fungi that look more like carved wood than mushrooms. The fruiting bodies are irregular, lumpy and often fused with their neighbours, and instead of gills they carry a bed of fine spines on the underside. Unlike fleshy mushrooms, they persist for weeks or months, drying in place and slowly discolouring rather than collapsing.

Hydnellum sits in the Thelephorales, the order of stipitate hydnoid fungi, and shares the brown, warted spores typical of that group. The genus is defined by its corky to woody, concentrically zoned flesh, a structure best seen by cutting a specimen from cap to stalk base with a knife.

Hydnellum caeruleum earns its name from the blue pigment concentrated in the fresh growing margin and in the upper flesh. Combined with the contrasting orange-brown of the stalk interior, that gives a section through the fungus a striking two-colour banding that no other member of the genus reproduces in quite the same way.

Cap

Typically 3-10 cm across, very irregular in outline, often knobbly, pitted or ridged, and frequently fusing with adjacent fruiting bodies into a compound mass. Young caps are velvety and pale bluish-white to steel blue, particularly at the actively growing margin, and the older central area matures through brown to rusty brown. Old specimens may lose the blue entirely and read as plain brown, so the margin is the place to look.

Spines

The underside carries short, crowded, decurrent spines 2-5 mm long, bluish-white to pale grey when young and darkening steadily to purple-brown and finally dark brown as the spores ripen.

Stipe

Short, stout and often ill-defined, 2-5 cm tall, tapering downward or swollen at the base, orange-brown to rust-brown and felted, usually with soil, moss and needles bound into the base.

Flesh and spore print

Cutting the fungus in half vertically is the decisive test. The flesh is tough and corky, faintly zoned, and shows blue to bluish-grey tones in the cap contrasting with a distinctly orange to orange-brown stalk base. The spore print is brown; spores are irregularly rounded and coarsely warted.

This species grows on the ground in coniferous woodland, favouring mossy, sandy, acidic and nutrient-poor soils under spruce and pine. It appears among moss carpets and needle litter, often on banks, path edges and other places where soils are thin, and frequently forms clustered or fused groups rather than isolated fruiting bodies.

Fruiting bodies develop from late summer through autumn but are tough enough to persist as blackened, weathered remains well into winter, which is why old material is often found long after the season. The species is distributed across northern and montane Europe, boreal Asia and northern North America, and like other stipitate hydnoids is scarce in intensively managed or nutrient-enriched forest.

Hydnellum caeruleum is ectomycorrhizal with conifers, chiefly spruce and pine. Its mycelium encases the fine roots of its partner and extends far into the surrounding soil, taking up phosphorus, nitrogen and water that the tree cannot reach on its own and receiving carbohydrates in exchange. This relationship is what enables conifer stands to persist on the poor sandy substrates the fungus prefers.

Because the mycelium is perennial and spreads slowly, established colonies remain in one place for decades, and their tough fruiting bodies incorporate surrounding litter as they grow, sometimes engulfing grass blades and twigs entirely. The genus is highly sensitive to nitrogen deposition, so its presence signals soils that have stayed unfertilised and undisturbed over long periods.

Hydnellum suaveolens also shows blue tones but concentrates them in a blue-black stalk interior beneath a whitish cap, and it carries a powerful sweet aniseed-like odour. Bleeding Tooth (Hydnellum peckii) exudes bright red droplets on the white surface of young caps and is pinkish-brown throughout in section, without the orange stalk base. Hydnellum aurantiacum has the same lumpy build and orange interior but a white to orange cap margin with no blue at any stage.

Zoned Tooth (Hydnellum concrescens) is thinner, flatter and marked with strong concentric bands of pink and brown, and lacks blue altogether. Phellodon species can look similar in the field but drop a white spore print and have much thinner, leathery, duplex flesh rather than the thick corky interior of Hydnellum.