Cellar Cup Identification Guide

The Setting and the First Impression

The Cellar Cup is one of the few larger fungi that people meet indoors. The classic encounter is in a damp basement, a coal store, a crawl space, an old outbuilding or the shaded corner of a garage, where water has been seeping through masonry for a long time. It fruits on damp mortar joints, on the face of old plaster, on sand-cement screed, on the surface of builders' sand and gravel, and on sacking, matting or soil contaminated with lime-rich rubble. Outdoors it turns up on damp calcareous ground, gravel paths, mortar rubble heaps and old fire sites.

The first impression is of pale, thin, waxy blobs. Fruit bodies are commonly clustered so tightly that they deform each other, and a group can look like spilled candle wax or a scatter of dropped pastry. Because indoor conditions are stable, they can appear at any time of year, which is itself a hint: a substantial pale cup fungus growing on masonry in February is already a short list.

Reading the Cup

Measure across the widest point. Fruit bodies are 2-5 cm across, sometimes reaching 8-10 cm on rich substrates. Young ones are deeply cup-shaped with an incurved margin, almost closed at the very earliest stage. With age they expand to a shallow cup or a saucer, and where several are crowded together they split, buckle and become irregularly lobed, so a colony may contain no two specimens of the same outline.

The margin is thin and often wavy, sometimes with small tears. Note whether it is incurved or spreading, and whether it is thickened or knife-thin, because these are among the few characters that separate the pale Peziza species.

The Two Surfaces

The inner surface, which is the hymenium, is smooth and matt to slightly shining when moist, sometimes with faint wrinkles or shallow veins towards the centre. Its colour is pale ochre-yellow, cream, buff or pale tan, deepening slightly to a warmer ochre with age or as the fruit body dries. It should not be pinkish, not olive and not deep red-brown.

The outer surface is paler than the inner one, whitish to pale cream, and finely scurfy or mealy, a furfuraceous texture that is easiest to see with a lens near the base. A cushion of fine white mycelium is often present at the point of attachment, binding sand grains or mortar dust.

That pale exterior against a slightly darker ochre interior is worth recording carefully, since several relatives reverse or flatten the contrast.

Base, Flesh and Colour Change

Most specimens are sessile, or have only a short, thick, tapering stalk-like base rooted into the substrate. Do not judge this from a specimen snapped off at the surface; ease a knife under the base and lift it whole, because the presence or absence of a distinct stalk is one of the few macroscopic characters that helps within the genus.

The flesh is thin, 1-2 mm, pale, watery and brittle. It snaps cleanly when bent and has no elasticity. There is no latex, no milky exudate and no colour change on cutting; the cut edge simply looks watery. The odour is faint and indistinct. As the fruit body dries it collapses to a stiff papery film, and a dried colony on a cellar wall can look like flakes of old paint.

Getting a Spore Deposit

Peziza is an operculate ascomycete: spores are fired upwards from asci lining the inner surface, so a print is taken by inverting the cup rather than by standing it upright. Prop a fresh, mature cup hymenium-downwards a few millimetres above a glass slide in a closed container with a little moisture and leave it for a few hours. A faint whitish deposit will accumulate. On a ripe specimen a puff of spores may be released visibly if you breathe warm air across the hymenium in the field.

The microscope is where this species is actually settled, because the pale Peziza species overlap heavily to the eye. Look for smooth, colourless, ellipsoid spores measuring roughly 13-17 by 8-10 micrometres, generally without oil drops or with only very small ones. The asci contain eight spores in a single row, and their tips turn blue in Melzer's iodine, an amyloid reaction that is standard for the genus and easily seen in a fresh mount. The paraphyses are slender and septate, with slightly swollen tips.

Record spore length, spore width, the presence or absence of oil drops, and whether the spore wall is smooth or ornamented. Those four numbers do more work than any field character in this genus.

Substrate and Season

This is a saprotroph of damp, lime-rich, largely mineral substrates rather than of wood or dung. Mortar, cement, plaster, brick dust, builders' sand, ash and gravel are all typical, along with sacking, coir matting or soil in flowerpots that has been standing on such material. Constant humidity matters more than temperature, which is why cellars, unheated outbuildings, greenhouses and covered service ducts are so productive.

Indoors it can fruit in any month, and successive flushes may appear on the same wall over years as long as the seepage continues. Outdoors it is chiefly a late spring to autumn species, appearing after prolonged wet weather on bare mineral ground.

The association with lime-rich, disturbed, man-made substrates is genuinely diagnostic. Substrate is the first thing to record on the label.

Separating the Look-alikes on Morphology

Peziza domiciliana is the closest relative and shares the indoor habitat exactly, so this pair cannot be split on where they grow. In general P. domiciliana is paler and greyer, whitish to pale grey-buff rather than warm ochre, is more distinctly stalked, and stays more deeply cup-shaped for longer. Its spores are of similar size but usually contain two distinct oil drops. Honest determination of this pair requires a microscope, and a collection recorded without spore detail should be left as a Peziza of the cerea group.

Peziza vesiculosa grows on straw, manure heaps, rotted compost and heavily enriched soil. It is deeper and bladder-like, with a strongly and persistently incurved margin, and its inner surface is characteristically blistered or vesiculose rather than smooth. It is usually a warmer ochre-brown and grows in dense tufts.

Peziza repanda and Peziza varia in the broad sense grow on rotting wood, sawdust, wood-based board and wood-rich debris. They are typically larger, thinner-fleshed and a warmer mid-brown, with a margin that splits as it expands; P. varia often has a short stout stalk. Substrate plus colour separates them from P. cerea in most cases.

Peziza badia and its relatives are much darker, red-brown to olive-brown, grow on bare soil in woodland and on banks, and have warted spores with two oil drops.

Tarzetta catinus and T. cupularis are whitish-buff, distinctly stalked, and have a toothed or crenate margin, a feature Peziza lacks.

Aleuria aurantia is unmistakable by colour alone, a vivid orange disc on disturbed soil and gravel paths.

The small dung discomycetes, Ascobolus, Cheilymenia and Lasiobolus, are an order of magnitude smaller, often hairy, and grow on dung rather than mortar.

Finally, do not confuse a young fruit body with efflorescence or mould on the same wall. Salt efflorescence is crystalline, powdery and structureless; mould colonies are cottony mats. The Cellar Cup is fleshy, has a defined margin, and has two clearly different surfaces.

Working Checklist

Pale ochre to buff cups 2-5 cm across, clustered and often distorted, on damp mortar, plaster, cement, sand or lime-rich rubble indoors or on bare calcareous ground outdoors; smooth ochre hymenium against a paler, finely scurfy whitish exterior; sessile or with a very short stalk-like base; thin, brittle, watery flesh with no colour change; smooth ellipsoid spores about 13-17 by 8-10 micrometres, mostly without oil drops, in asci with amyloid tips.