Frosty Webcap Identification Guide

Where You Will Be Standing

Frosty Webcap is a birch specialist, and knowing that shapes the whole search. The typical find is in damp birch woodland on acid ground, along the mossy margin of a heath, in a wet birch carr, or under a scatter of birch on a reclaimed spoil heap. You will be looking down into moss, bilberry, wet leaf litter or bare peaty soil, and the fruit bodies will be small, dark and easy to walk past. September to November is the window across most of the northern temperate range, with the best flushes after cool autumn rain.

The first impression is of a modest brown mushroom on a slim stem, but with something off about the surface: the cap looks as if it has been dusted or spattered with white, and the stem carries pale bands rather than a clean ring. That white overlay on a dark ground is the frostiness in the name, and it is the character that pulls your eye to the species in the first place.

Confirming the Genus Before the Species

Cortinarius is a huge genus, so establish the genus first and the species second. Find a young, unopened fruit body and look at the gap between the cap margin and the stem. In a webcap you will see a cobwebby veil, the cortina, stretched as fine silky threads from the rolled cap edge to the stem. It is ephemeral: within a day or two it collapses onto the stem and leaves a fibrillose band or a faint girdle instead of a membrane.

The second half of the genus test comes later, when the spores start dropping. Maturing webcaps dust their own stems with rusty spore powder, so the collapsed veil zone turns cinnamon or rust-brown. A small brown mushroom with a cobweb veil and a rust-dusted ring zone is a webcap, and no amount of further staring at the cap is needed to establish that.

Reading the Cap

The cap of Cortinarius hemitrichus is two to five centimetres across, conical or bell-shaped when young and expanding to broadly convex with a low blunt umbo. It never becomes flat and dished. The ground colour is dark greyish-brown, date-brown or umber when moist, and the surface is dry, not slimy or greasy.

Over that dark ground lies a coat of whitish to pale grey fibrils and small appressed scales, densest toward the margin and often forming a shaggy fringe on the extreme edge. On very young caps the coating can be almost continuous, giving a hoary grey appearance; on old, weathered caps it wears away first at the centre so you get a dark disc ringed by a pale frosted zone.

The cap is strongly hygrophanous, which means it changes colour as it loses water. Moist caps are dark and the margin may be faintly striate; as they dry they pale outward from the centre to greyish-ochre or dull clay-brown, and a single cap can show both states at once as a sharp irregular boundary. Judge colour on a wet fruit body if you can, and note the drying pattern rather than treating it as damage.

Gills, Stipe and Base

The gills are adnate, moderately spaced, and fairly broad for the size of the mushroom. Colour is the sequence to record: pale clay or greyish-brown at first, then ochre-brown, and finally a warm rusty cinnamon as the spores ripen. There is no violet tint in the gills of typical material, and the gill edges are usually slightly paler and minutely uneven.

The stipe is three to eight centimetres long and only three to six millimetres thick, cylindrical or tapering slightly downward, sometimes with a barely swollen base. Its ground colour is greyish-brown, darkening toward the base with age, and it is wrapped in white silky fibrils that break into two or three distinct transverse girdles or bands below the veil zone. Those white bands on a brown stem are the single most memorable field mark. Above the veil zone the stem is paler and finely silky. The base is often bound into moss or litter by white mycelial threads. Cut the stem: the flesh is thin, brownish, watery, and does not change colour beyond a slight darkening as it dries.

Taking a Spore Print

For a small hygrophanous mushroom the print takes patience. Choose a cap with fully coloured gills, cut the stem flush, set the cap gill-down on white paper, cover it, and leave it overnight in a cool place with the paper protected from draughts. Thin fruit bodies dry out and stop dropping, so a scrap of damp tissue under the cover helps.

The deposit is rust-brown to reddish-cinnamon, obvious against white paper and unmistakably not brown-black, purple-brown or white. Under a microscope the spores are ellipsoid to almond-shaped, roughly seven to nine by four and a half to five and a half microns, and finely to moderately warted rather than smooth. Rough rusty spores are a genus-level confirmation of Cortinarius against most of the other brown-spored genera.

Substrate, Season and Ecology

Cortinarius hemitrichus is mycorrhizal, forming sheathing root partnerships with birch, and it is effectively confined to birch across its range. It fruits on soil, moss and litter, never on wood, singly or in small scattered groups rather than in tight clusters. Damp, acidic, nutrient-poor ground suits it, so heath edges, birch carr, mossy banks and young birch regeneration are the productive places. Season runs from early autumn to the first hard frosts.

Sorting It From the Closest Look-Alikes

The other white-fibrillose small webcaps are the real problem. Cortinarius paleaceus is smaller and more slender, with a sharper umbo and coarser, more upstanding whitish scales on the cap; its most decisive separation from hemitrichus is a strong scented-geranium odour that hemitrichus lacks or shows only faintly. Cortinarius flexipes is similarly scented, often darker, and typically shows violaceous tints in the young gills and stem apex that hemitrichus does not.

Cortinarius species of the same section without a white coating are eliminated by the frosted cap itself. Inocybe species share brown spores and small brown fruit bodies, but they lack a cobwebby cortina, their caps are typically radially fibrillose or splitting rather than white-flecked, and their spore print is duller clay-brown; many also have thick-walled cystidia visible microscopically. Galerina and Pholiotina have smooth, slimy or shiny caps, no white fibril coat, and often a membranous ring rather than a fibrillose band. Anything on wood is not this species.

Quick Field Checklist

Birch present; cap two to five centimetres, dark greyish-brown, conical then umbonate, coated with whitish fibrils and hygrophanous; gills adnate, clay then rusty cinnamon; stem slim, brownish, with two or three white fibrillose girdles and a rust-dusted veil zone; cobwebby cortina in young fruit bodies; rust-brown print with finely warted spores; no strong geranium smell and no violet tones.