Firerug Inkcap

The Firerug Inkcap, Coprinellus domesticus, takes its English name from the most striking thing about it, which is not the mushroom at all but the substrate. Fruit bodies arise from a spreading felty mat of rusty-orange to tawny mycelium called an ozonium, which can carpet the surface of a damp log, a rotting board, or a piece of buried timber like a small woven rug. The ozonium is frequently seen without any mushrooms on it and is often the first thing that betrays the fungus's presence.

Coprinellus holds the small, granular-veiled inkcaps formerly lumped into Coprinus, including the Glistening Inkcap and the Fairy Inkcap. C. domesticus is one of the larger members, with a tawny ochre bell-shaped cap that develops deep radiating grooves as it expands and dusted with the pale mealy granules of its veil.

The epithet domesticus reflects a real tendency to turn up in and around buildings — on damp joists, cellar timber, and rotting window frames, as well as on natural dead hardwood outdoors. That habit makes it one of the fungi householders most often encounter fruiting indoors.

Cap

2–5 cm across when expanded, starting as a narrow egg or cylinder, then conical to bell-shaped, and finally flat with a split, upturned margin. It is tawny to ochre-brown, richest over the centre and fading to pale buff-grey at the edge, and becomes strongly radially grooved as it opens. The young cap is covered with fine whitish to pale buff granular or mealy veil particles, densest at the disc, which are gradually shed.

Gills

Crowded and narrow, free to narrowly attached. They pass from white through pale brown to black and then deliquesce, though the collapse is slower and less dramatically inky than in the large Coprinopsis species — often the cap dries and shrivels part-way through.

Stipe

5–12 cm long and 3–8 mm thick, white to cream, hollow, smooth or finely fibrillose, usually with a slightly swollen base that sits directly on the orange ozonium mat. There is no true ring, though a faint fibrillose zone can be present low down.

Flesh and spore print

Flesh is very thin and whitish with no distinctive odour. The spore print is black. The most reliable single field character is not on the mushroom but beneath it: the conspicuous rusty-orange to brownish-orange felted ozonium spreading over the wood surface around the stipe base.

A decomposer of damp dead hardwood. It fruits on fallen logs, stumps, buried roots, wood chips, sawdust piles, and processed timber, and readily colonises structural wood in damp cellars, outbuildings, greenhouses, and behind leaking joinery. Deciduous trees such as beech, ash, elm, and poplar are typical natural hosts; conifer records are rare.

The season runs from spring to late autumn outdoors, and indoors it can fruit at almost any time of year wherever wood stays wet and warm. It is widespread across Europe, North America, and much of the temperate world. Fruit bodies come up singly or in small groups rather than dense tufts, always in association with the orange mycelial mat, which itself may persist for weeks or months without producing mushrooms.

Coprinellus domesticus is a saprotrophic wood decayer, digesting the cellulose and lignin of damp hardwood. The ozonium is a specialised feeding and survival structure: a dense sheet of pigmented mycelium that spreads across and into the wood surface, absorbing nutrients, holding moisture, and persisting through dry spells so the colony can fruit whenever conditions improve.

Because it needs consistently wet wood, the species is a useful natural indicator of persistent damp — indoors, an ozonium on a joist points to a water leak long before it points to a mushroom problem. In woodland it works alongside other early-stage rotters to break down fallen branches and stumps, releasing nutrients into the soil. As with all inkcaps, the deliquescing gills concentrate spore release into a brief window, dispersing spores in dark droplets rather than only as a dry cloud.

Coprinellus radians is the closest match, also arising from a bright orange ozonium; it generally has smaller, more richly orange-brown caps with a more conspicuously granular veil, and the two are separated with confidence only by microscopic examination of veil and cap cells. The Glistening Inkcap (Coprinellus micaceus) grows in dense tufts on stumps, is covered when fresh in sparkling mica-like granules, and has no ozonium mat at its base.

The Common Inkcap (Coprinopsis atramentaria) is much larger and more robust, with a bald pale grey-ochre cap, clustered growth from buried wood, and no orange mat. The Fairy Inkcap (Coprinellus disseminatus) is far smaller and fruits in swarms of hundreds around stumps, with pale buff-grey caps and no ozonium.