Common Ink Cap Identification Guide

The setting and what registers first

The Common Ink Cap announces itself as a crowd. You come across a dense tuft of pale grey-brown eggs pushing up through gravel at the foot of an old stump, through the grass of a park verge above a buried root, or out of soil at the edge of a car park where a tree was felled years ago. Twenty or thirty fruit bodies can be packed shoulder to shoulder, their bases fused into a common lump. Individually they look like closed grey ovals on white stalks; collectively they look like a clutch of eggs on end.

The second thing that registers is the state of decline. In any decent-sized cluster some caps are still egg-shaped and dry, some are opening into bells, and one or two on the outside are already dripping black liquid onto the grass beneath. Coprinopsis atramentaria dissolves itself as it matures, and seeing several stages side by side is a genuinely diagnostic sight.

The cap, stage by stage

Start with a young, unopened one. It is three to seven centimetres tall, ovoid to egg-shaped, and the surface is a soft grey to grey-brown with a warmer ochre-tan flush over the very top. Run a thumb over the apex: you will usually feel and see small, flattened, brownish scales concentrated there, thinning out to nothing before the margin. The rest of the surface is smooth and matt.

Look along the sides. The cap is shallowly grooved with radial furrows, giving it a faintly pleated, melon-like appearance that deepens as it expands. As the cap opens, it goes from egg to bell (campanulate) and rarely gets past that, spreading to five or seven centimetres at most before the margin begins to split, curl upward and blacken. Do not expect a flat cap; a fully expanded plane cap belongs to something else.

Gills and deliquescence

Turn one over early, while the underside is still intact. The gills are extremely crowded, thin, broad in the middle and free from the stipe, and in a very young specimen they are white to greyish white. Watch them progress: pale grey, then dark grey, then black from the margin inward as spores ripen.

Then comes the character the group is named for. The gill tissue autodigests, liquefying from the cap edge upward and inward, so the mushroom turns itself into black ink that drips away and carries spores with it. In practice this means you must collect and examine young specimens. If every cap in the tuft is already a black smear on a bare stalk, look for the youngest egg at the centre of the cluster and cut that one instead.

The stipe and its buried base

The stipe is a clean white, four to seventeen centimetres long, six to fifteen millimetres thick, smooth or very finely silky, and hollow throughout. It tapers slightly upward and is often slightly swollen where it meets the ground.

The base is where a key detail sits. Scrape the soil away and look for a narrow, oblique, ridge-like ring zone low on the stipe, sometimes only a millimetre or two high and easily rubbed off. It is the remnant of a thin veil and can look like a rough collar or a fine line. There is no membranous, movable ring higher up the stalk and no cup-like sac. Below this the stipe continues down as a rooting extension into buried wood, often for several centimetres, ending in a fused mass shared with its neighbours.

Flesh and cut surfaces

Split a fresh specimen from top to bottom. The flesh is thin, soft, white and watery, hollow in the stalk, and there is no colour change on cutting beyond the natural greying and blackening of the gill tissue as it matures. There is no latex, no blueing, and no reddening. Older flesh simply darkens and softens on its way to ink.

Taking a spore print

Printing a self-dissolving mushroom is a race, so pick your specimen deliberately. Choose a cap that has just begun to open into a bell but whose gills are still grey rather than black. Cut it free, set it gills-down on white paper or card, cover it with a glass, and check after thirty to sixty minutes. Leave it too long and you will not get a print, you will get a puddle of ink that soaks into the paper.

The deposit is dense black. That result, plus free crowded gills and a liquefying cap, places the collection in the inky caps immediately. If the deposit comes out dark purple-brown or chocolate rather than true black, or if the cap stays intact overnight, reconsider the identification.

Substrate, season and habit

This is a saprotroph on buried and decaying hardwood: stumps, roots, buried planks, timber offcuts left in fill, and the wood-rich soil of urban plantings. Because the wood is often out of sight, the fungus frequently appears to be growing from bare soil, gravel or lawn, and tracing the cluster down with a trowel to a root or a chunk of rotten wood is a satisfying confirmation.

Habit is caespitose, meaning tufted with fused bases, and this is important. Solitary fruit bodies do occur but the normal presentation is a tight cluster. The season is long, running from spring through to late autumn and into mild winters, with the biggest flushes after rain. Disturbed ground, roadsides, gardens, parks and old orchards are all typical.

Separating it from close relatives on morphology alone

Coprinellus micaceus. Also tufted on buried hardwood, but noticeably smaller and warmer in colour, with tawny to honey-brown caps that are strongly grooved almost to the centre and dusted, at least when young, with glistening mica-like granules that catch the light. It has no basal ring zone.

Coprinus comatus. Much taller and narrower, a cylindrical white column covered in shaggy, upturned, coarse white scales, with a distinct movable ring on the stalk. It grows in grass and disturbed soil rather than in fused tufts on wood.

Coprinopsis lagopus. Slender and fragile, silvery grey and covered in fine woolly fibrils when young, with a cap that expands and recurves into a thin grey parasol before dissolving. It fruits on wood chips and mulch, usually scattered rather than in dense fused tufts.

Coprinellus disseminatus. Tiny, only a centimetre or so tall, in swarms of hundreds over stumps and roots, with strongly pleated pale grey caps that fade rather than dissolving into much ink.

Coprinopsis romagnesiana. Very similar in stature and habit to the Common Ink Cap but with the cap covered in more conspicuous rusty-brown to ochre-brown fibrillose scales spread across the surface rather than confined to the apex.

Psathyrella species. Often clustered on buried wood in the same places, and similar in size and greyish-brown colour, but the caps never dissolve into ink and the spore print is dark purple-brown rather than jet black.

A field checklist

Tufted with fused bases; grey-brown ovoid to bell-shaped cap with fine scales at the apex only; shallow radial grooves; crowded free gills going white to grey to black; a white hollow stipe with a small oblique ring zone near the base; a black print; and a cap that turns to ink within a day. All present together, and you have Coprinopsis atramentaria.