Common Inkcap Identification Guide

Standing over the cluster

Picture the moment of the find. A gravelled path edge in a park, a lawn beside a long-gone tree, or the crumbling remains of a stump under a hedge, and rising from it a compact bouquet of grey ovals on white stems, jammed so tightly together that their bases have merged into one woody-looking knuckle. Some are still closed like eggs, others have parted into bells, and at least one at the outside of the group is drooping and black-edged with liquid gathering along its rim.

That mixture of ages in a single tuft is worth more than any one specimen. It lets you read the whole life cycle of Coprinopsis atramentaria at a glance: firm grey egg, expanding bell, blackening margin, and finally a bare white stalk with a puddle beneath it.

Working through the cap surface

Select the plumpest unopened specimen and work systematically.

Shape and size: ovoid to egg-shaped when closed, three to seven centimetres high, opening to a broad bell five to seven centimetres across but not flattening out.

Colour: grey to greyish brown across most of the surface, warming to ochre or tan over the crown.

Texture: with a lens, the apex carries small, dark, flattened, scurfy scales. Slide your eye outward from the top and those scales thin and disappear well before the margin, which is bare, greyish and, in expanded specimens, split and recurved.

Sculpture: hold the cap side-on against the light and you will see shallow radial furrows running from the crown toward the rim, giving a pleated melon-like ribbing. On the youngest eggs the ribbing is faint; it deepens as the cap stretches.

Reading the underside before it liquefies

Timing matters more here than anywhere else. In a young cap the gills are packed so tightly that the underside looks almost solid, and they are free of the stipe, leaving a narrow gutter around the top of the stalk. Their colour sequence is a clock: whitish, then pale grey, then leaden grey, then black. Once black, the tissue begins to self-digest, starting at the cap edge and moving up, and the gills convert to a black fluid that drips clear of the fungus, taking spores with it.

For description purposes, cut two specimens: one closed egg to record pale gill colour and gill attachment, and one half-open bell to record the greying and the onset of deliquescence.

The stalk, ring zone and buried base

The stalk is white, straight, hollow, and smooth to slightly silky, four to seventeen centimetres tall and roughly a centimetre thick, sometimes widening a little downward. Nothing on the upper stalk should catch your fingernail.

Low down, however, there is a fine detail that separates this species from most of its relatives: a narrow, sharp-edged, often oblique ridge or collar encircling the stipe near the base, a remnant of a thin veil. It is easy to rub off while collecting, so excavate carefully. Keep digging and the stipe continues underground as a tapering root-like extension, several centimetres long, fusing with its neighbours and running down to rotting wood.

Cutting it open

A vertical section shows a hollow white stalk with a thin wall, and cap flesh so thin and watery that it is barely a separate layer. It is white, soft and unchanging: no reddening, no blueing, no milk. The only colour development is the natural darkening of the gill plates. If your specimen shows a coloured bruise on the cut, it is not this species.

Getting a print from a dissolving mushroom

Because the fungus destroys its own hymenium on schedule, a print has to be started early and read quickly.

Choose a cap at the point where the margin has just lifted away from the stalk and the gills are grey. Cut the cap off flush with the top of the stipe. Set it gills-down on a white card, cover with a tumbler to hold moisture, and inspect within half an hour to an hour. A good print is heavy, matt and true black. Left overnight, the cap will simply melt and the card will be stained rather than printed, which tells you something but not with any precision.

Habitat, wood and calendar

Coprinopsis atramentaria is a wood recycler that specialises in hardwood below ground level. It fruits from and around stumps, dead roots, buried construction timber, filled ground with wood fragments, and the soft rotten cores of old orchard and street trees. Because the substrate is usually hidden, the mushrooms are typically recorded from lawns, verges, gravel, mulch beds and waste ground.

The default habit is caespitose: many fruit bodies from one fused base. The default season is long, spring to late autumn with extensions into mild winter spells, and the flushes follow rain closely. In parks and gardens the same stump will produce a fresh tuft in the same spot for years.

Telling it apart from its relatives

Shaggy Inkcap, Coprinus comatus. Tall and cylindrical rather than egg-shaped, wrapped in coarse shaggy white scales over the whole cap, and carrying a loose movable ring on the stalk. It grows scattered or in loose lines in grass and disturbed soil, not in fused tufts on wood.

Glistening Inkcap, Coprinellus micaceus. Smaller, honey to tawny brown, deeply grooved almost to the crown, and coated when fresh with tiny reflective granules. It clusters on the same buried wood but has no basal ring zone and never reaches the size of the Common Inkcap.

Coprinopsis lagopus. Grey, hairy-fibrillose and extremely fragile, expanding into a thin recurved grey umbrella. It prefers wood chips and mulch and typically appears scattered rather than in dense fused clusters.

Coprinopsis romagnesiana. The closest match in size and habit, but its cap is decorated with obvious rusty-brown to ochre fibrillose scales spread widely over the surface rather than clustered only at the apex.

Fairy Inkcap, Coprinellus disseminatus. Unmistakable in numbers: hundreds of tiny pale grey pleated caps a centimetre or two tall covering a stump, individually far too small to confuse once you look closely.

Psathyrella species. Frequently clustered on the same buried wood, similar in colour, but with caps that wither and dry rather than turning to ink, and with a dark purple-brown rather than black spore print.

Field summary

Fused tufts on hidden hardwood; grey-brown ovoid caps with scales only at the crown; shallow radial grooves; crowded free gills whitening to grey to black; a hollow white stipe with a small oblique ring zone at the base; a jet-black print taken quickly; and self-digestion into ink within a day. Together these characters describe Coprinopsis atramentaria and nothing else in the group.