Grey Shag Identification Guide

Where You Will Meet It

Grey Shag is a fungus of hot, rich, disturbed organic matter. You find it beside a stable muck heap, on a rotting straw bale, in a garden compost bin, on dunged fields, in mushroom-farm casing, in the corner of a greenhouse bed or even in a heavily manured pot. The setting is usually a bit unglamorous, and that is a genuine identification clue: this is not a woodland or grassland mushroom.

The first impression is of a small crowd of pale, woolly, egg-shaped or cigar-shaped caps carried on long slender white stems, standing up out of the substrate. They appear overnight, often after a warm, wet spell, and by the middle of the following day they have unfurled, split and dissolved into black drips. If you want to identify one properly, get to it early in the morning.

Reading the Cap

In the closed stage the cap is 1 to 4 cm tall and roughly 1 to 2 cm wide, ovoid to cylindrical-conic, sitting like a thimble on the stem. The ground colour is pale grey to grey-brown, darkest over the disc.

The covering is the character to fix on. A universal veil coats the young cap in whitish to pale grey, floccose, woolly or mealy tissue, which as the cap expands breaks up into loose fibrous patches, tufts and small scales, densest at the apex and thinning to almost nothing at the margin. Under a lens the veil looks like matted cotton wool rather than granular sugar or glistening mica.

As the cap opens it becomes bell-shaped, then splits radially, then turns its margin upward and finally auto-digests. Radial grooves develop and deepen with expansion, so a half-open cap looks pleated. Because the flesh is only a fraction of a millimetre thick, the outline of the gills shows through the cap surface as fine dark lines.

Gills, Stipe and the Deliquescence

Turn a half-opened cap over. The gills are very crowded, narrow, free or only just reaching the stipe, and they run through a rapid colour sequence: white, then pale grey, then pinkish grey, then black. The blackening starts at the cap margin and works inwards.

Then comes the character that names the whole group. The gill tissue self-digests, liquefying into a black or inky fluid that drips from the retreating cap edge. Within a few hours the entire cap can be reduced to a shrivelled black rag on top of a stem. If your specimen is dissolving from the margin inward, you are in the inky caps and you can discard a great many other grey mushrooms immediately.

The stipe is 3 to 10 cm long and only 2 to 5 mm thick, white, hollow, brittle and silky-fibrillose, sometimes finely downy near the base when young. It is slightly swollen at the base and may end in a short rooting point pushed into the dung or compost. There is no ring: the veil in this species stays on the cap and never forms a movable collar on the stem. Splitting the stipe lengthwise shows a clean hollow tube.

Flesh and Colour Change

The flesh is white, extremely thin, and watery. Cut it and nothing dramatic happens; there is no latex, no blueing, no reddening, and no stable bruising reaction. The only colour change of note is the general blackening of the gills as the cap self-digests, which is a developmental process rather than a bruising response. The odour is faint at best and is not distinctive enough to build an identification on.

Taking a Spore Print

This needs tactical thinking, because the cap is racing you. Choose a specimen whose cap has just begun to open but has not started dripping. Cut the stipe flush, set the cap gills down on white paper or on a glass slide, cover it with a cup and check after only one to two hours; leave it overnight and you will find a puddle of ink instead of a print.

The deposit is black. Alternatively, let a drop of the black liquid fall onto a slide and examine it directly, since the ink is largely a suspension of mature spores.

Under the microscope the spores are smooth, ellipsoid, dark red-brown to almost black in transmitted light, roughly 8 to 14 by 6 to 9 micrometres, and each has a distinct central germ pore at the apex. The gill faces and edges carry large balloon-shaped cystidia, and the cap veil is composed of chains of inflated cells rather than the thick-walled elements found in some relatives.

Substrate, Season and Ecology

Grey Shag is coprophilous and compost-loving. Horse dung is the classic substrate, along with cow dung, dunged straw, spent mushroom compost, hot garden compost, manured greenhouse beds and rotting hay. Because these substrates generate their own heat and are kept damp by the grower, fruiting is not tied to the ordinary autumn season: it can appear in any month, including midwinter under cover, which is unusual among gilled fungi and is itself a hint.

Ecologically it is a fast, early coloniser of nutrient-rich, nitrogen-heavy material, racing through its life cycle in days before slower competitors establish. Its rapid, reliable development from mycelium to mature cap has also made it a standard laboratory model for research into how mushrooms form.

Separating the Look-Alikes on Morphology

Coprinopsis lagopus. Very similar young stage, but it grows on woodchip mulch, leaf litter and garden bark rather than dung. Its young fruit bodies are more densely and shaggily white-hairy, like a hare foot, the stem is longer and more conspicuously woolly at the base, and the expanded cap flattens out into a thin, splitting, upturned umbrella.

Coprinopsis atramentaria. Much larger and stouter, with a 4 to 8 cm grey-brown cap that is nearly bald apart from fine scurf at the disc. It grows in dense clusters from buried wood and roots at tree bases, on lawns and in gardens, never on dung.

Coprinus comatus. Large, tall and white, with a shaggy, upcurved-scaly cylindrical cap and a movable ring on the stem.

Coprinellus micaceus. Tawny to honey-brown, with a veil of glistening mica-like granules rather than woolly patches, in dense tufts on stumps and buried wood.

Coprinopsis stercorea and other tiny dung inky caps. Much smaller, typically under 1 cm, with a granular or crystalline rather than cottony veil, and some species sit on a conspicuous orange-brown mycelial mat.

Panaeolus semiovatus and other dung Panaeolus species. Same dung substrate, but the cap does not deliquesce, the gills go mottled black rather than liquefying, and Panaeolus semiovatus has a distinct ring on a stout stem.

Bolbitius titubans. Also on dunged straw and compost, but the cap is bright yellow when young and collapses without inking, and the gills mature rusty brown.

Field Checklist

Dung, straw or compost. Small ovoid grey cap with woolly whitish veil patches. Deep radial grooving as it expands. Crowded free gills turning black and liquefying from the margin. Slender hollow white ringless stipe. Black spore deposit with large spores bearing a central germ pore. Fruiting possible in any month on heated substrates.