Coprinellus radians Identification Guide

The Orange Mat You See Before the Mushroom

With most fungi you find the fruit bodies first. With Coprinellus radians you very often find the mycelium first, and it is the more striking of the two. Spread across the surface of rotting hardwood - a fallen trunk, a stump, an old fence post, a damp beam in a shed or cellar, sometimes plaster over lath - is a dense felted mat of rusty-orange to tawny-brown threads, several centimetres to a metre across, with the texture of coarse cotton wool or frayed orange rope. That mat is the ozonium, a persistent felt-like state of the mycelium, and finding it is more than half the identification.

Out of the ozonium stand small, pale, egg-shaped caps on thin white stems, singly or in small scattered groups rather than in the packed tufts you get with some inkcaps. The picture is memorable: bright orange felt below, delicate fawn thimbles above.

Reading the Cap

Young caps are ellipsoid to ovoid, roughly 1 to 3 centimetres tall, standing upright like little eggs. As they mature they expand to a conical bell shape and then to a nearly flat disc with an upturned, split and ragged margin, up to about 3 or 4 centimetres wide - though most caps are gone to ink before reaching that stage.

Colour runs from rich ochre-brown, tawny or cinnamon over the centre, paling outwards to buff, ochre-grey or pale greyish-fawn at the margin. Over that ground colour lies the veil, and this is the second character to check: the whole young cap, and especially the disc, is coated in fine granular, mealy or minutely floccose ochre-brown particles. They rub off easily on a finger, and rain washes them from the outer part of the cap first, so an old specimen may keep the granules only as a small tawny cap over the very centre.

The cap is also strongly and radially grooved. The furrows follow the lines of the gills and run from the margin almost to the disc, giving the expanded cap a pleated, parasol-ribbed profile. That plication is why old caps look so thin and paper-like.

Gills That Destroy Themselves

Turn a young cap over. The gills are very crowded, narrow, free or only narrowly attached to the stipe, and white at first. They pass through pale grey to grey-brown as the spores mature, and then blacken.

At maturity the gills autodigest. The blackening begins at the outer edge and works inwards, and the gill tissue liquefies into an inky black fluid that drips off the upturned margin. Within a day the entire cap can dissolve into a black smear on the wood, leaving the bare stipe standing. If you want to see the gill structure at all, you need a specimen collected before the margin has begun to blacken, and you need to look at it within the hour.

Stipe, Base and Flesh

The stipe is 3 to 10 centimetres long and only 2 to 5 millimetres thick, cylindrical or slightly widened at the base, white to very pale cream, hollow and extremely fragile. Its surface is silky-fibrillose to finely downy, especially towards the base, and there is no ring - though on some specimens a faint fugitive veil zone can be seen low on the stem in youth.

Follow the stipe down and confirm the connection. The base sits directly on the orange ozonium mat, often with a few tawny strands running up onto the white stem or a small tuft of orange fibrils at the very foot. That direct anchorage into a bright orange felt is what makes this species and its nearest relative stand apart within the inkcaps.

The flesh is negligible - a thin whitish to pale brown layer in the cap and a hollow tube in the stipe. There is no bruising reaction to speak of, no latex, no reddening or bluing; the only colour change of note is the autodigestion itself, which is a developmental process rather than a response to injury. Odour is faint and unremarkable.

Taking a Spore Print

Inkcaps make you work quickly. Choose a cap that has expanded but whose margin is only just starting to go grey-black, cut it from the stipe and set it gills down on white paper under a cover. You will normally have a usable deposit within an hour or two - much faster than a typical agaric. Wait too long and the cap will simply melt onto the paper and give you a black smear rather than a print, which is still informative but ugly.

The print is black to blackish-brown. Under the microscope the spores are smooth, dark red-brown to almost black, ellipsoid to slightly oval, with a distinct central germ pore, generally in the region of 9 to 13 by 6 to 8 micrometres. The cap surface bears rounded veil cells and the gill faces carry large balloon-shaped cystidia, both of which matter for splitting the closest relatives.

Substrate and Season

This is a wood decomposer that favours damp, sheltered, nutrient-rich situations. Typical substrates are dead hardwood - elm, ash, poplar, willow, beech and old fruit trees - buried wood and root fragments, sawdust and wood chip, and structural timber in damp cellars, outbuildings, greenhouses and neglected corners of buildings. Its ability to run over plaster, brick and soil as an orange mat while feeding on buried timber is characteristic.

Outdoors it fruits mainly from late spring through autumn. Under shelter, where temperature and moisture stay steady, it can produce fruit bodies at almost any time of year, and the ozonium mat may persist long after every cap has dissolved.

Working Through the Look-alikes

The genuinely difficult comparison is Coprinellus domesticus, which also grows from an ozonium mat and shares the same habitats. Separate them on scale and tone. Coprinellus domesticus is the larger species, with caps commonly 2 to 5 centimetres and reaching 6 across when expanded, coloured paler cream-ochre to greyish-buff, with a whiter and more flocculent veil and a proportionally longer stipe; its ozonium tends to be a duller orange-brown. Coprinellus radians is smaller, warmer and rustier in cap colour, more densely granular over the disc, and usually on a brighter, denser orange mat. The two overlap enough that microscopy of the veil elements and cap cystidia is the only fully secure separation, and a collection with intermediate characters is better recorded as the pair than forced into one name.

Coprinellus micaceus is the next candidate. It grows in dense clustered tufts at the base of hardwood stumps, has a tawny date-brown cap covered when fresh in glistening mica-like veil granules that soon wash away, and - decisively - has no ozonium. Coprinellus xanthothrix is similar to micaceus in habit and likewise lacks the orange mat.

Coprinellus disseminatus fruits in hundreds of tiny pale grey pleated caps over a stump, is much smaller, and its gills barely liquefy. Coprinopsis lagopus and its relatives grow on litter and mulch, are grey and covered in loose woolly white hairs rather than fine granules, and again have no ozonium.

Finally, the Psathyrella species that share rotting wood are superficially similar in stature but do not dissolve. Their caps are hygrophanous grey-brown, their stipes brittle and white, and their spore print is dark purple-brown rather than truly black.

The Short Route

Find the orange mat; check that the stipe rises from it; confirm the fine granular ochre veil and the deeply grooved cap; watch the gill margin blacken and liquefy; take a fast black print. If all of that lines up and the fruit bodies are small and rusty rather than large and pale, you are looking at Coprinellus radians.