Onion-stalk Parasol

Leucocoprinus cepistipes is the white counterpart of the familiar yellow flowerpot fungus, sharing the same fragile pleated architecture but on a slightly larger and sturdier scale. The species epithet refers to the stem: cepa is Latin for onion, and the base of the stipe swells into a rounded bulb that narrows abruptly below, exactly like a spring onion pulled from the soil.

As a member of Agaricaceae it has free gills, a movable but flimsy ring, and a white spore print. The cap surface is coated in a loose mealy granulation that can be brushed off with a fingertip, and the outer part of the cap is grooved along the lines of the gills, giving the margin a pleated, fluted edge that becomes more pronounced as the mushroom flattens.

It is a fungus of enriched, disturbed ground rather than natural woodland, and modern landscaping has made it common. Fresh wood chip mulch, sawdust heaps and compost regularly produce sudden dense flushes, sometimes carpeting a bed, and the same substrate may fruit repeatedly through a warm damp summer.

Cap

2-8 cm across, ovoid or almost cylindrical when young, then bell-shaped and finally broadly convex to nearly flat with a low umbo. The ground colour is white to cream with a tan or brownish disc, and the whole surface is dusted with soft mealy granules that darken slightly toward the centre and rub away easily. The outer half is radially grooved and the margin becomes wavy and pleated with age; specimens may discolour dingy brownish where bruised or waterlogged.

Gills

Free from the stem, crowded, thin and soft, white at first, ageing to cream and sometimes staining yellowish then brownish where handled. The gill edges are minutely powdery.

Stipe

4-12 cm tall, 3-10 mm thick at the apex, hollow, white and smooth to slightly mealy, and characteristically swollen at or just above the base into a rounded onion-like bulb that tapers again into a narrow rooting point. Handling or age brings out yellowish then brown discolouration. The ring is thin, white and membranous, sits high on the stem, and is often torn or shed altogether.

Flesh & spore print

Flesh thin, soft and white, sometimes slowly turning dingy brownish in the stem base but with no rapid colour change. The spore print is white; spores are smooth, thick-walled ellipsoids with an apical germ pore.

This is a fungus of nutrient-rich, disturbed substrates. It fruits on hardwood chips and bark mulch, sawdust, compost heaps, manured garden soil, greenhouse beds and the rich litter of parks and gardens, usually in crowded clusters or troops with several stems arising close together. It is only occasionally found in unmanaged woodland, and then on accumulations of rotting plant debris.

The species has a broad distribution across Europe, North America and warmer regions worldwide, spread further by the movement of compost and mulch. Outdoor fruiting runs from early summer through autumn during warm humid weather, while heated greenhouses and interior plantings can produce flushes at any time of year.

Leucocoprinus cepistipes is a saprotroph specialising in soft, freshly decaying plant material. Its mycelium colonises the fine fraction of wood chips, bark and compost, breaking down cellulose and releasing nutrients into the surrounding soil, and it can produce visible white mycelial webbing through the top few centimetres of a mulch bed.

The clustered fruiting habit reflects a dense mycelium exploiting a concentrated nutrient supply: when conditions are right, many fruiting bodies are initiated at once, discharge their spores over a day or two, and collapse. Repeated flushes on the same bed gradually consume the substrate until the mulch is reduced to humus.

Leucocoprinus birnbaumii is built on the same plan but is bright sulphur-yellow throughout rather than white, and its base is only slightly swollen. Leucocoprinus fragilissimus is far smaller and thinner, with a greenish-yellow translucent cap and a wire-thin stem lacking the onion bulb.

Leucoagaricus leucothites is a firmer, fleshier mushroom with a smooth silky white cap that is never mealy or pleated, and its gills take on a pinkish tinge before turning cream. Chlorophyllum molybdites is much larger, grows in lawns rather than mulch, has coarse cap scales, and drops a green spore print, while young Coprinus and Coprinellus species in the same mulch have gills that darken and dissolve to black ink.