Firerug Inkcap Identification Guide

The first thing you notice

Firerug Inkcap announces itself twice: once with the mushroom and once with the mat it grows out of. The usual setting is damp shaded hardwood debris, a rotting stump, a buried root under leaf litter, a pile of woodchips going soft, or worked timber in a cellar, greenhouse or crawl space where condensation keeps the wood permanently wet. What you see first is a small tawny egg on a thin white stem, pleated like a folded parasol, standing over a patch of rust-orange felt spread across the wood.

That felt is the ozonium, and it gives the species its English name. It is a mat of orange-brown to tawny, densely woven hyphae that creeps across bark, sawn faces and the undersides of boards, sometimes covering an area far larger than any fruit body. It is an asexual stage of the same fungus, it persists long after the mushrooms have dissolved, and it is often the only sign that the species is present. Turn a log or peel back loose bark before deciding there is nothing there.

Reading the cap

Work fast, because this is a species with a short working life. Fresh caps are 1 to 4 cm tall and roughly as wide when they open, starting egg-shaped or narrowly ovoid, then becoming conical and finally bell-shaped, with the margin flaring upward and splitting as the fruit body ages.

The ground colour is ochre-tan to date-brown over the centre, fading to buff and finally to grey-buff toward the margin. The whole surface is radially grooved, the pleats running almost to the disc and following the gills beneath, which gives an expanded cap the look of a collapsed umbrella. Over the young cap lies a thin scattering of whitish to pale buff floccose-granular veil remnants. These are the remains of the universal veil, and they are matt and powdery rather than glassy. Rain and handling remove them, so an older cap can be entirely bare and noticeably darker.

Gills, stipe and the dissolving act

The gills are free to narrowly attached, crowded, very thin, and they run through a fixed colour sequence: white, then grey, then black. Once black they begin to autolyse from the margin inward, liquefying into a dark inky fluid that drips from the cap edge and can reduce the whole thing to a black smear on the stipe within a day. If you want to read gill spacing and attachment, look at a specimen in the morning before the dissolving front has travelled far.

The stipe is 3 to 10 cm tall and only 2 to 5 mm thick, white, hollow, and finely silky-fibrillose along its length. It is slightly swollen at the base, sometimes with a faint band or rudimentary ring zone low down where the veil broke, and often with orange fibrils at the very bottom where it emerges from the ozonium. Trace the stipe down into the substrate: seeing it rise directly out of the orange felt is the observation that settles the genus and gets you most of the way to the species.

The flesh is almost nonexistent, whitish, thin as tissue, and shows no colour change when cut. The important transformation here is not bruising but the autolysis itself, which is a normal part of the life cycle rather than a reaction to damage.

Taking a spore print

A dissolving cap is not an obstacle if you time it right. Choose a specimen at the moment the cap has just begun to open and the gill edge is greying but has not started to run. Cut the stipe flush, set the cap gill-side down on white paper, cover it with a cup, and check after an hour. Alternatively, hold a fully mature cap over paper and let the ink drip, then let the drop dry, which gives you a smear rather than a clean print but confirms the colour.

The deposit is black to blackish-brown. Under the microscope the spores are smooth, ellipsoid to slightly ovoid, roughly 7 to 10 by 4 to 5.5 micrometres, dark red-brown in potassium hydroxide, and each has a central germ pore. Cystidia are present on both gill edge and cap surface, and their shape is what separates the closest relatives.

Substrate, season and range

Firerug Inkcap is a wood decomposer with a strong preference for hardwood. It appears on stumps and fallen limbs of beech, elm, ash, poplar and willow, on buried roots, on woodchip mulch and sawdust, and on structural timber in damp buildings, which is how it earned the "domestic" half of its scientific name. It is widespread across the temperate Northern Hemisphere.

Outdoors it fruits mainly from late spring through autumn, in flushes that follow rain and mild weather. Indoors, where humidity is constant, it can appear in any month. Fruit bodies are usually scattered or in small loose groups rather than in the dense tufts typical of some relatives.

Separating it from its look-alikes

Coprinellus radians is the genuine problem. It also grows from a conspicuous ozonium, and the two species overlap in habitat and season. In practice the ozonium of C. radians is often denser and a brighter, more saturated orange, and its fruit bodies tend to be smaller and more clustered, but these are tendencies rather than rules. Reliable separation rests on microscopic characters, particularly spore dimensions and the form of the cap cystidia, so treat a bare orange mat as evidence for a pair of species rather than for one.

Coprinellus micaceus grows in dense tufts at the base of hardwood stumps, has a tawny honey-coloured cap dusted with glistening mica-like granules that catch the light, and produces no ozonium. The granules are glassy and sparkling; the veil remnants of Firerug Inkcap are dull and powdery.

Coprinellus disseminatus is far smaller, rarely more than 1.5 cm, appears in swarms of hundreds over a stump, is grey rather than tawny, and its gills only weakly autolyse.

Coprinopsis lagopus is grey and clothed in white woolly fibrils that give it a hare's-foot texture, grows on woodchips and litter, and lacks any orange mat.

Parasola species share the pleated, deeply grooved cap but have no veil granules, grow in grass and soil, and their gills scarcely dissolve.

Psathyrella species can look almost identical in silhouette, but the cap never liquefies and the spore print is dark purple-brown rather than true black.