Domicile Cup Fungus Identification Guide

The Setting, Which Is Half the Identification

Most fungi are found by walking. This one is usually found by opening a door. Domicile Cup Fungus is the classic indoor Peziza, and the typical encounter is in a damp cellar, a flooded basement, a bathroom with a long-standing leak, a garage, a boiler room, a greenhouse bench, a builder's site hut, or the cavity under a lifted carpet or vinyl floor. It appears on plaster, mortar, brickwork, damp concrete, sand, gravel, coal dust, cardboard, plasterboard, old sacking and rotting fabric. It will also turn up outdoors in comparably mineral, disturbed places: the mortar of an old wall, a damp rubble pile, the sand and ash around a demolition site.

The first impression is of a pale, waxy, brittle cup or saucer sitting almost directly on the substrate, one to ten centimetres across, whitish to cream when young and ageing through pale buff to a dull tan or greyish-brown. In a colony they crowd together and deform one another, so the neat round outline of a solitary specimen gives way to lobed, wavy, split, half-collapsed shapes that look almost like crumpled paper. The texture is the thing to fix in memory: it feels like a piece of stiff, damp wax, and it snaps cleanly rather than tearing.

Reading the Cup

Work the fertile surface first. The inner face of the cup is the hymenium, and in this species it is smooth, matt to slightly waxy, and noticeably paler than most soil-dwelling Peziza species: pale grey-brown, greyish-buff, café-au-lait or dull ochre, sometimes almost the colour of milky tea. It is not glossy, not violet-tinged, not deep chestnut. As the fruit body ages and the margin splits, the inner surface can become a little wrinkled or veined towards the base, but it never develops folds, ribs or a network of ridges the way a Discina or a Gyromitra does.

The outer surface is paler still, whitish to cream, and finely scurfy or minutely mealy rather than smooth. Rub it gently with a fingertip and it feels faintly floury. Under a hand lens the scurf resolves into small pale granules and tufts. This whitish, slightly furfuraceous exterior against a greyish-brown interior gives the whole cup a two-tone look that is a good field character.

The margin starts inrolled in very young cups, then flattens, then usually splits into a few coarse radial tears as the cup expands and flares. The edge is even, not toothed or fringed, and there is no ring of hairs.

Stalk, Base and Attachment

Domicile Cup Fungus is essentially stalkless, but it is not always flatly sessile. Many specimens narrow at the bottom into a short, thick, stem-like base, a few millimetres to a centimetre long, that is buried in the substrate or gripping the mortar. Excavate it and you will usually find pale mycelial threads running out into damp plaster, sand or debris. This tapered base is worth checking because several of the closest relatives are strictly sessile and attached over a broad area.

Flesh, Section and Colour Change

Cut a cup vertically with a knife. The flesh is thin, one to three millimetres, whitish to very pale buff, and brittle and waxy in texture, breaking with a clean edge. It is not layered, not gelatinous, and not fibrous. Importantly, there is no colour change: the cut surface does not blue, redden, blacken or exude latex, and bruising the fertile surface with a thumb produces at most a slight darkening from handling moisture. Any species that stains distinctly on handling is something else.

If you leave the cup in a warm room for a few hours and then breathe on it or jolt it, you may see it puff: a faint smoke-like cloud of spores released simultaneously from thousands of asci, sometimes accompanied by a very faint hiss.

Taking and Reading the Spore Deposit

Cup fungi shoot their spores upward and outward rather than dropping them, so the technique differs from a gilled mushroom. Cut the cup free, invert it, and rest it fertile-side-down on a slip of dark card or a glass slide, propped on two matchsticks so the hymenium is a few millimetres clear of the surface. Cover with a tumbler and leave it somewhere cool for four to twelve hours. What collects is a whitish to very pale cream deposit, thin and often streaky rather than dense, and it shows only against a dark background.

Under a microscope the spores are the decisive character. In Peziza domiciliana they are colourless, smooth-walled, ellipsoid, and comparatively large: roughly twenty to twenty-five micrometres long by eleven to fourteen wide. Fresh spores characteristically contain two prominent oil droplets, one towards each end. That combination of large, smooth, two-drop spores is the cleanest way to confirm the species when the field characters are ambiguous.

Season and Conditions

Indoors it has no season at all. A heated, humid cellar or a leaking bathroom will produce fruit bodies in January as readily as in July, and a single wetting event, a burst pipe, a flood, a botched screed, will often be followed by a flush two to four weeks later. Outdoors it is commonest from spring to autumn on damp mineral substrates. The species is a coloniser of freshly disturbed, mineral-rich, water-saturated material with little competition, which is exactly why it thrives on new plaster and wet concrete: it gets there first and fruits before anything else establishes.

Separating It From Its Look-alikes on Morphology

Peziza cerea is the hardest. It shares the pale colouring, the indoor and mortar habitat, and the general shape, but it tends to be more distinctly yellowish or waxy-cream throughout, is more often sessile with a broad attachment, and its spores are smaller, roughly fourteen to eighteen by eight to ten micrometres, and typically lack the paired oil drops.

Peziza vesiculosa is a larger, deeper, more goblet-shaped species whose inner surface is conspicuously blistered and bubbled, whose margin stays strongly inrolled and often remains toothed, and which grows on manured straw, compost heaps and rotted dung rather than plaster. The blistered hymenium alone is enough.

Peziza repanda and Peziza varia are wood dwellers, found on rotting stumps, buried timber and sawdust, and both are distinctly darker: mid to dark brown inner surfaces and a thinner, more flaccid texture with a strongly wavy, splitting margin.

Peziza badia is a soil species of woodland banks with a deep reddish-brown to olive-brown interior, far darker than anything domiciliana produces, and its spores are ornamented with a coarse network rather than smooth.

Tarzetta and Geopyxis species have distinctly toothed or crenate margins and usually a clearer stalk, while Aleuria aurantia is unmistakably bright orange.

A Working Field Sequence

Note the setting: is this mineral, indoor, recently wet? Check the two-tone cup, greyish-buff inside and whitish-scurfy outside. Feel for waxy brittleness. Look for a short tapering stem-like base. Section it and confirm no colour change and no latex. Take an inverted deposit on dark card. If you have a microscope, measure the spores and look for the two oil drops.