Domicile Cup Fungus

The Domicile Cup Fungus is the classic 'mushroom growing in the basement'. It is a Peziza that has become closely tied to human structures, fruiting on wet plaster, concrete, tile grout, sand, plaster-board, and the damp backing of flooring and carpet after leaks or flooding. Outdoors it turns up on rubble, gravel and disturbed calcareous ground.

Morphologically it is pale and unassuming: a whitish, cream or faintly buff cup that starts almost spherical and closed, then splits open into a bowl, and finally sprawls into an irregular, often lopsided disc. Because it frequently pushes up through cracks or from beneath carpet edges, the fruiting bodies are commonly distorted, folded or elongated by the shape of the space they emerge from.

It belongs to the Pezizaceae, and shares the family's brittle, fleshy texture, absent or vestigial stalk, and smooth inner spore-bearing surface. Its unusually large spores are the most reliable technical character separating it from the other indoor pezizas.

Fruiting body

Initially a small closed white ball, expanding to a cup 2–10 cm across, and eventually flattening or becoming irregularly wavy, lobed and contorted. Specimens forced through narrow gaps are frequently one-sided or trumpet-like.

Surfaces

The inner spore-bearing surface is smooth, whitish at first, becoming pale buff, tan or faintly greyish-brown as spores mature. The outer surface is whitish and finely mealy or minutely granular, sometimes with faint darker flecking near the base.

Stipe

Usually none; the cup is sessile, or narrows abruptly to a small pale attachment point that may be buried in sand, grout or fibrous debris.

Flesh & spore print

The flesh is thin, white, soft and very brittle, with no latex and no colour change when cut. The spore print is white. Microscopically the spores are smooth, elliptical and comparatively large, typically around 20–25 by 11–14 micrometres, usually with one or two small oil droplets — the sharpest separator from the Cellar Cup.

This is the most consistently indoor cup fungus of temperate regions. It fruits on damp plaster and concrete, on sand and gravel beneath floors, on wet insulation and plasterboard, on grouted tile, and on the damp underside of carpet and matting, especially in basements, crawl spaces, bathrooms and buildings that have suffered a leak or flood. Greenhouses and glasshouse benches are another regular site.

Because interior temperatures are stable, it can fruit in any month, appearing within days or weeks of a water incident. Outdoors it grows on builders' rubble, gravel paths, sand piles and disturbed lime-rich soil. It is widespread across North America and Europe and has been carried more or less everywhere buildings and greenhouses are found.

The Domicile Cup Fungus is a saprophyte of scattered cellulose and other organic material within damp mineral substrates — paper facing on plasterboard, wood fragments, jute and cotton carpet backing, plant debris in sand. It does not parasitise anything living, and the mineral component of its substrate is simply a moist scaffold rather than a nutrient source.

Spores are produced in asci lining the inner cup and are released explosively into still indoor air, which allows a single colony to seed other damp corners of the same building. Fruiting collapses quickly once the substrate dries, so colonies tend to be short-lived episodes tied directly to a moisture event.

The Cellar Cup (Peziza cerea) is the main confusion: it shares the mortar-and-cellar habit but is warmer ochre-buff, thicker and waxier in the flesh, keeps a more regular bowl shape, and has distinctly smaller spores. The Palamino Cup (Peziza repanda) is larger, tan-brown, thin-fleshed, and grows on wood, sawdust and bark mulch, with a strongly recurved wavy margin at maturity.

Outdoor pale cups on rubble may also be members of the Peziza varia complex, which are generally browner inside and more strongly tied to woody debris. Any dark brown cup on woodland humus is far more likely the Common Brown Cup (Peziza phyllogena), and any cup with a violet flush on burnt ground points to Peziza violacea.