Panaeolus olivaceus Identification Guide

First Impressions and the Setting

This is a mushroom you find by looking down for a long time. Panaeolus olivaceus fruits in grazed pasture, on dunged and manured ground, along field margins and sometimes in lawns and roadside verges, producing small, conical to bell-shaped caps on thread-like stalks that vanish against the turf. It rarely grows in tight clusters; instead it scatters, a few fruit bodies here and a few metres away another group.

The first impression when you crouch is of a dark, slightly olive or reddish-brown thimble with a paler ring around it, because the cap dries unevenly and leaves a distinct band of colour behind. Handled, it feels dry and papery, and the stipe is springy rather than brittle for its width.

Reading the Cap

The cap is small, usually 1-3.5 cm across, at first conical or bluntly egg-shaped, then campanulate, and eventually broadly convex, often keeping a low umbo. It never becomes broadly funnel-shaped or upturned.

Colour is the crux and it moves. Fully moist, the cap is dark olive-brown, reddish-brown or greyish-brown, sometimes with a distinctly olivaceous cast over the disc that gives the species its name. As it dries, the colour lifts to a paler tan, greyish-buff or dull clay, and the drying proceeds outwards from the centre so that a darker, water-soaked marginal zone lingers as a band. Catching that two-tone banded stage is very helpful, because it is the moment when the mushroom is most recognisable.

The surface is smooth to very finely wrinkled over the disc, with fine radial rugulosity sometimes visible under a lens. When moist the margin is faintly translucent-striate, showing the gill lines through the flesh. There are usually no veil remnants along the edge, though very young caps may show a trace of fine fibrils.

Reading the Gills

The gills are adnate, moderately close, and rather broad relative to the small cap. Immature gills are pale brownish-grey; with maturity they show the characteristic uneven, blotchy blackening of the genus, the gill face darkening in patches rather than uniformly. Mature gills are dark grey-black. Under a lens the edges are whitish and minutely fringed, contrasting against the darkened faces.

Stipe, Base and Colour Change

The stipe is 4-10 cm long and only 1-3 mm thick, straight and equal, often slightly enlarged at the base. It is reddish-brown to greyish-brown, usually darker towards the base and paler at the apex, finely pruinose in the upper part and longitudinally silky-striate below. There is no ring and no ring zone. The stipe is hollow and slightly cartilaginous, bending before it snaps.

Split it and watch. The flesh is thin and pallid to brownish and does not change colour appreciably; bruised areas darken slightly to brownish but do not develop blue or green tones. Note that explicitly, because the absence of that reaction is one of the ways this species is told apart from its blue-staining relatives.

Excavate the base rather than snapping it off. You will find it emerging from dung-enriched soil or from decaying grass and fibrous dung fragments, bound with fine white mycelial threads.

Taking a Spore Print

Take a print from a cap that has just fully expanded, since older caps in the field may already have shed most of their spores. Cap gills-down on white paper, covered, two hours. The deposit is jet black. Under a microscope the spores of Panaeolus olivaceus are distinctive: dark, thick-walled and minutely roughened or wrinkled rather than smooth, and that surface ornamentation is the character that most reliably confirms the species when several small brown grassland Panaeolus are in play. A basic compound microscope at 1000x with a drop of water is enough to see it.

Substrate, Season and Ecology

Panaeolus olivaceus grows on manured and dunged soil, in pasture and grassland, occasionally directly on well-rotted dung, and sometimes on lawns and grassy waste ground. It is widespread and is particularly frequently reported from western North America, and it fruits from spring through autumn, with the heaviest flushes after prolonged rain in mild weather.

Ecologically it is a decomposer of grass litter and dung residue in nutrient-rich turf, part of the same guild of small dark-spored agarics that recycle the organic matter accumulating under grazing animals. It is not associated with trees and does not fruit from wood.

Separating the Look-Alikes on Morphology

Panaeolina foenisecii is the classic confusion, being the same size and shape and growing in the same mown grass. Take a print: Panaeolina foenisecii deposits dark reddish-brown, not black, and its gills darken more evenly with less pronounced mottling. Its spores are also roughened, so the microscope confirms genus placement rather than separating them; the print colour is the deciding character.

Panaeolus papilionaceus is larger and greyer, and its cap margin carries obvious white toothed veil fragments that this species lacks.

Panaeolus cinctulus is stouter, with a broader, more convex cap 2-5 cm across, a browner and thicker stipe, and a very pronounced dark hygrophanous band; it favours compost heaps and manured garden ground.

Panaeolus fimicola is very similar in size and habit and often needs microscopy, but tends to be greyer with less olive in the moist cap and has smooth spores.

Panaeolus cyanescens overlaps in slenderness but has a pale, cracking, off-white dry cap and stains blue-green when handled.

Deconica species of grassland and dung have viscid, separable cap cuticles, evenly coloured gills and purple-brown rather than black prints.

A Compact Field Checklist

Grazed or manured grassland; small conical to bell-shaped cap, olive-brown to reddish-brown when moist, drying paler with a dark marginal band; translucent-striate wet margin; no veil teeth; mottled grey-black gills with whitish edges; slender reddish-brown pruinose hollow stipe; no blue staining; jet-black print; roughened spores under the microscope.