Black Poplar Mushroom Identification Guide

The first impression

A riverside avenue of black poplars, or an old willow pollard beside a ditch, and out of a split in the bark at chest height comes a tuft of mushrooms, half a dozen or more, their stems curved and crowded where they have pushed through the wound. The caps are two-toned: the youngest are dark chocolate brown, while the expanded ones have paled from the centre outward to buff or cream, so the cluster looks like it contains two different species.

Step closer and two things confirm the impression. Each stem carries a distinct membranous collar high up, and that collar is dusted brown on its upper surface with fallen spores. Below the ring the stem is clean and fibrous, not scaly.

Reading the cap

Caps are 3 to 10 cm across and can reach 15 cm on a vigorous cluster. They begin hemispherical, become convex, and finally flatten and often go irregularly wavy at the margin.

The colour change is central to recognition. Fresh young caps are dark red-brown to chocolate or umber. As the fruitbody expands and loses water the colour fades progressively from the disc outwards to ochre, buff, pale cream or nearly white, frequently leaving a darker zone near the rim. In dry weather the disc commonly cracks into a network of small polygonal patches, a fine crazing that is very characteristic.

The surface is dry, matt to faintly silky, never viscid. Young caps often carry a few white veil fragments hanging from the margin, remnants of the tissue that later forms the ring.

The gills

Turn a cap over. The gills are moderately crowded, fairly broad, and attached adnate with a slight decurrent tooth running a short distance onto the stem. They pass through three colour phases: whitish or pale grey when young, dull clay or pale brown at maturity, and finally a dark tobacco or cigar brown once the spores have ripened.

The edges are usually a little paler than the faces and slightly uneven under a lens. There is no green or olive tone at any stage, which is a useful negative check against a couple of other clustered wood mushrooms.

The ring

This is the character that anchors the identification. The ring is membranous, white, persistent and set high on the stem, well above the middle. It flares outward like a small skirt and it does not disintegrate into a cobwebby haze. Very quickly it collects falling spores, so its upper surface becomes dusted a rich brown while its underside stays white.

Check two related points. There is no cobweb-like cortina joining the cap margin to the stem, and there is no volva or sac at the base. And, importantly, the stem below the ring is smooth to longitudinally fibrous, with no shaggy scales.

Stipe, base and flesh

The stem is 5 to 12 cm long and 1 to 2 cm thick, whitish to cream, becoming brownish and often grubby towards the base. It is tough and fibrous, frequently curved or bent because the whole cluster has had to grow around each other and out of a crack in bark. The base may be slightly swollen and knitted to its neighbours, and white mycelial strands usually run from it into the wood.

Slice the fruitbody lengthwise. The flesh is white and firm, thicker in the cap centre, more fibrous and sometimes hollowing in the stem. It does not change colour on cutting: no blueing, no reddening, no blackening, no latex. The smell is floury and slightly winey, most obvious when the flesh is crushed.

Taking and reading a spore print

Spore colour is essential here because several look-alikes are separated on print alone. Cut a mature cap free of the stem, place it gill side down on white paper, cover it with a bowl and leave it two to four hours; a large cap will drop a heavy print quickly.

The deposit is a dark clay brown to tobacco brown, sometimes described as dull cigar brown. It is not rusty orange and it is not purple-black. If you have no paper handy, the dusting already present on the ring is a decent proxy.

Substrate, ecology and season

The Black Poplar Mushroom is a wood-decay fungus of broadleaves, and it is not confined to dead timber. It behaves as a saprotroph and a weak parasite, entering living trunks through wounds, pruning cuts, broken limbs and frost cracks, and causing a white rot in the heartwood. Once established it fruits from the same tree repeatedly over many years, so a productive poplar becomes a fixed point on a walk.

Its principal hosts are poplars, especially black poplar, along with willow, elder, mulberry, ash and fig. It fruits in dense tufts directly from bark wounds on standing trunks and pollards, from cut stumps and from fallen trunks.

The season is unusually long, with peaks in spring and autumn, and in warm-temperate and Mediterranean regions it can fruit through the winter whenever the weather is wet and frost-free. It is frequent in riverside poplar plantings and old orchards across southern Europe, and has been recorded increasingly further north.

Separating it from look-alikes

Agrocybe praecox group. Similar brown spore print and a ring, but the habit is quite different: these fruit from soil, wood chip mulch, hedge banks and grassy paths in spring, not in tufts on standing trunks. The caps are smaller and a more uniform pale ochre-buff, the ring is thinner and often disappears, and the stem base is usually rooting into the ground.

Cyclocybe erebia. Also ringed, but terrestrial on rich woodland soil, with a darker, somewhat viscid cap and a slighter build.

Kuehneromyces mutabilis, the sheathed woodtuft. Clustered on hardwood stumps and superficially very close. Its caps are smaller, 2 to 6 cm, and strongly hygrophanous with a sharply defined watery brown outer band around a paler dry centre, giving a two-tone effect on a single cap rather than across a cluster. Decisively, its stem below the ring is covered in small recurved scales, whereas the Black Poplar Mushroom has a smooth or merely fibrous stem below the ring.

Galerina marginata. Much smaller and slighter, with a thin tawny hygrophanous cap up to about 4 cm, a fragile stem with a fleeting narrow ring, and a rusty brown spore print rather than a dark tobacco one. Size and print colour separate them cleanly.

Pholiota populnea (P. destruens). Also on poplar, but the cap and stem are clothed in coarse, woolly, whitish to buff scales, and the fruitbodies are heavier and more solitary.

Hypholoma species. Clustered on stumps, but with no membranous ring, only a fibrillose veil zone, with yellow to greenish young gills and a purple-brown spore print.

Pleurotus ostreatus. No ring at all, broad decurrent gills, a lateral or strongly eccentric stem and a white spore print.

Volvariella bombycina. Also on wounds of living broadleaves, but with a silky-hairy white cap, a large sac-like volva at the base, free gills that turn pink, and a pink spore print.

Field checklist

Dense tufts from wounds on poplar, willow or elder; caps fading from chocolate to buff from the centre outward and often finely cracked; adnate gills with a small decurrent tooth turning tobacco brown; high, persistent, brown-dusted membranous ring; no scales below the ring; white unchanging flesh with a floury smell; dark clay-brown spore print; long season with spring and autumn peaks.