Onion-stalk Parasol Identification Guide

First impression, and the setting

You are looking at a fresh bark mulch bed, a compost heap, a glasshouse border, a pile of sawdust behind a shed, or the wood chip on a landscaped path, and after a warm wet spell a crowd of pale, fragile, umbrella-like mushrooms has come up overnight. They are white to cream with a brownish centre, the caps are deeply pleated like a folded paper parasol, and the whole troop looks too delicate to touch.

That impression of fragility is diagnostic in itself. These are soft, thin-fleshed, short-lived mushrooms that collapse within a day or two, and by afternoon the earlier ones are already flattened and browning. If you want good material, look in the morning.

The second thing to register is the crowd. This species is typically gregarious, often clustered with several stems arising close together from the same patch of mulch, sometimes in dozens across a bed.

Reading the cap

Young caps are ovoid to egg-shaped or almost cylindrical, then expand to bell-shaped, then broadly conical, and finally to nearly flat with a low central umbo. The size range is modest: 2 to 8 cm across at full expansion, occasionally a little more in rich compost.

Two surface characters carry the identification.

First, the powder. The entire cap is covered in a fine mealy to granular-floccose coating, whitish to cream, sometimes with a faint ochre or greyish tinge, which rubs off readily onto a fingertip and gathers into small woolly patches or minute scales toward the centre. The central disc is usually darker, a definite brown to greyish brown, where the coating is thicker and less disturbed.

Second, the pleating. From about halfway to the centre outward, the cap is strongly and regularly grooved and furrowed in radiating lines that follow the gills beneath, giving a folded, pleated, parasol-like margin. Under raking light the effect is very obvious. That plicate-striate margin is one of the best characters for the genus.

Reading the gills

The gills are free from the stem, leaving a clear space around the stipe apex, and they are crowded, thin, soft and rather narrow.

Colour begins white and passes to cream or very pale buff. In age or where damaged, they may take on a faint pinkish buff, ochre or brownish cast, but they never go rusty, chocolate or black.

Reading the stipe and the base

The English name comes from the stem, so this is the section to be careful with.

The stipe is 4 to 10 cm tall, slender at 3 to 8 mm thick through most of its length, and hollow. It is white to cream and finely powdery or mealy over the surface, especially in the upper part, and it usually discolours to pale brownish, ochre or dingy tan where it has been handled or bruised, and in age from the base upward.

The base is the character. It swells noticeably into a bulb or a club, often onion-like or spindle-shaped, sometimes markedly so, and that swollen base is frequently embedded in a mat of white mycelial threads that bind the wood chip together. Excavate carefully with a knife rather than pulling, because the stem breaks easily and the base is exactly what you need to see.

The ring sits on the upper third of the stem. It is thin, membranous, white, often with a slightly upturned edge, movable when fresh, and thoroughly fugacious, so on many specimens it has already collapsed onto the stem as a faint fibrous zone or vanished entirely. Its absence on an old fruitbody does not count against the identification.

Reading the flesh

Slice a fruitbody in half from cap centre to stem base.

The flesh is very thin, soft and white, almost membranous in the cap away from the disc. In the stem it is white and the interior is hollow with a narrow canal.

Colour change on cutting is slight and slow. You may see a gradual drift toward pale brownish or dull reddish tones in the stem, particularly in the base, over some minutes, and old fruitbodies discolour overall as they collapse. There is no rapid or dramatic reaction, no blueing, no bright saffron flush.

Taking and reading a spore print

Work fast, because the caps deteriorate quickly.

Choose a freshly expanded cap that has flattened but has not yet begun to droop, cut it free at the stem apex, place it gills down on white paper or, for better contrast, on black card, and cover it. Two to six hours is often enough given how quickly these mushrooms mature; leaving them overnight risks the cap collapsing into a smear.

The print is white. Read that alongside the free gills and the ring, because the combination of free pale gills, a ring, and a white print is what places the specimen in the dapperling group and excludes several superficially similar mulch mushrooms with coloured prints.

Substrate, season and behaviour

This is a saprotroph of rich, decaying plant material, and it is strongly associated with human activity because that is where such material accumulates. The classic substrates are wood chip and bark mulch, compost, leaf mould, sawdust and shavings, spent potting compost, greenhouse and glasshouse beds, and the soil of large indoor planters.

Outdoors in temperate regions it fruits mainly from late spring through autumn, keyed to warmth plus moisture, and it often appears in a flush a few days after heavy rain on a freshly laid mulch bed. Under glass or in heated interiors it can appear at any time of year, which is why it turns up in conservatories and office planters in midwinter.

Separating it from close look-alikes

Leucocoprinus birnbaumii shares the pleated cap, powdery surface, fragile build and plant-pot habitat, but it is bright lemon to golden yellow throughout, cap, gills and stem, at least when young. Colour alone settles it.

Leucocoprinus brebissonii is white with a strongly contrasting dark grey to blackish central disc and distinct dark grey granular scales scattered over the whitish surface, a much bolder speckled appearance, and it is more often found in leaf litter in woodland than on mulch.

Leucocoprinus cretaceus is a larger, chunkier, wholly chalk-white mushroom with a thick, dense, cottony to floccose covering that stands up in soft warts, without the brownish disc, and with a stouter stem.

Leucoagaricus leucothites and related species are stouter and firmer, with a smooth or only finely silky cap that is not pleated, a more robust stem, and much thicker flesh.

Lepiota and Cystolepiota species typically have fibrillose or granular caps that break into definite scales rather than a mealy bloom, and they lack the strongly plicate-striate margin.

Coprinellus and Parasola species also produce pleated fragile caps on mulch and wood, but their gills darken to grey then black, several deliquesce into ink, and their spore prints are dark brown to black.

Field checklist

Fragile, short-lived, in troops on mulch or compost; cap white to cream with a mealy powdery coating and a browner disc; margin deeply pleated and grooved; gills free, white to cream, soft; stem slender, hollow, powdery, browning when handled, with a swollen onion-like base in a white mycelial mat; thin fugacious movable ring; white spore print.