Birch polypore or Chaga

The fruiting body is hoof-shaped or hemispherical and attached to the birch at the side. It measures 5–20 cm across and 3–8 cm thick. The surface is smooth, pale cream to flesh-grey or light brown, often with a thin papery crust that cracks with age. The margin is even and rounded, curving slightly downward in mature brackets.

Chaga has no cap. What you see on the birch is a growth 10–30 cm across, occasionally up to 40 cm, or sometimes a narrow strip up to a metre long running along a crack in the trunk. It's irregular and knobbly, bursts out through the bark and sits firmly on the tree with a broad base.

The surface is black, matt, dry and very hard, broken by deep cracks into small facets, like charcoal or burnt wood. The black colour comes from the pigment melanin. Around the edges, where the growth pulls away from the bark, the rusty brown interior often shows through.

The underside is made of short tubes, 0.3–0.5 cm deep, with round or oval pores, 3–5 per millimetre. They're white to cream, turning yellowish or brownish as they dry, and don't separate easily from the flesh.

The growth has no underside with pores or gills: it's a sterile mass of fungal threads and produces no spores.

The true fruiting body appears once the tree, or part of it, has died, often on the opposite side of the trunk. It spreads under the bark as a thin crust 0.5–1 m long, whitish at first and then brown, made of tubes. Eventually the bark splits open and exposes a brown layer of pores. It is rarely seen.

There's no stem, or only a very short one fused to the tree. The bracket attaches to the birch bark at its side through a knobby outgrowth.

There's no stem. The growth comes straight out of the trunk, usually at an old wound or broken branch, and is fused to the wood so firmly that only an axe will remove it.

The flesh of young brackets is white, springy and soft to the touch, with a distinctive “marshmallowy” edge. With age it turns corky, hard and light. Dried, it's just like cork, and historically it was used for razor strops, for carrying embers and for mounting surgical instruments.

Inside, chaga is dense, corky and woody. The colour is rusty brown, turning yellowish orange towards the wood, often with pale streaks. This bright interior is what sets chaga apart from other black growths on birch. It doesn't change colour when broken.

The smell is faint and mushroomy; fresh brackets smell slightly pleasant, with a sourish note. The flesh tastes bitter and astringent, unpleasant, with a quinine-like aftertaste, which is one reason it's inedible.

Chaga has no distinctive smell, and descriptions of the species don't mention one. Identify it by appearance instead: a black, cracked surface and a rusty yellow interior on a living birch.