Verdigris Agaric

Stropharia aeruginosa is the best known of the roundheads, a genus in the family Strophariaceae characterised by a viscid cap, a membranous ring on the stem, gills that darken to purple-brown or purple-grey as the spores ripen, and a dark purple-brown spore print. Roundheads are litter and dung decomposers rather than tree partners, and several species are conspicuous in gardens and on landscaped ground.

The Verdigris Agaric is remarkable for its colour. The young cap is a saturated blue-green, the colour of oxidised copper from which the English name comes, coated in a thick slime layer that carries the pigment. As the fungus ages the slime is washed away by rain and the underlying surface bleaches through yellowish green to dingy ochre, so a single troop can contain caps ranging from vivid turquoise to plain buff.

A closely related species, Stropharia caerulea, shares the blue-green colouring and was long confused with it. Current practice separates the two on the ring, the gill colour and the cap margin, and many field records of blue-green Stropharia in fact refer to S. caerulea, which is the commoner of the pair in many areas.

Cap

3-8 cm across, conical to bell-shaped at first, expanding to convex and finally almost flat with a low central boss. The surface is thickly slimy or glutinous when moist, drying shiny, and coloured a strong verdigris blue-green, fading with age and rain to yellow-green, ochre-yellow or dingy buff, usually keeping more green toward the centre. The margin carries scattered small white veil flecks or fibrils, especially when young.

Gills

Adnate to broadly attached, moderately crowded and fairly broad. They begin whitish to pale grey, become clay-brown, and finally darken to a distinct purple-brown or chocolate-purple as the spores mature, usually with a paler, minutely fringed white edge.

Stipe

4-10 cm tall, 0.4-1.2 cm thick, cylindrical, hollow at maturity, and coloured like the cap in blue-green tones that fade to whitish. Below the ring it is covered in soft, white, cottony scales or fibrillose bands; above the ring it is smoother. The ring itself is membranous, spreading, and usually persistent, often darkened by fallen spores on its upper surface.

Flesh and spore print

The flesh is whitish to pale blue-green in the cap and stem and does not change colour appreciably when cut. The spore print is dark purple-brown, matching the mature gill colour, and is the character that places the species in the Strophariaceae rather than among the many white-spored blue-green fungi.

The Verdigris Agaric grows on rich, damp, organic ground: grassy rides and clearings in deciduous and mixed woodland, leaf litter and needle duff, wood chip mulch and bark beds, hedge bottoms, roadside verges, pastures and garden borders. It also appears on rotting sawdust and among nettles on nutrient-enriched soil. It has no attachment to any particular tree species.

Fruiting runs from late summer through autumn, sometimes continuing into early winter in mild weather. Fruit bodies appear singly, scattered, in troops and occasionally in small tufts, and are conspicuous because of their colour even from a distance. The species is widespread and locally common across Europe and temperate Asia, and it or close relatives are recorded in North America. Its spread has been assisted by the increasing use of wood chip in landscaping.

Stropharia aeruginosa is saprotrophic. Its mycelium colonises leaf litter, buried twigs, grass thatch, bark chips and other accumulated plant debris, secreting enzymes that dismantle cellulose and lignin and returning the resulting nutrients to the soil. Roundheads as a group are efficient early colonisers of freshly laid wood chip, and the white cords of mycelium are often visible when mulch is disturbed.

The blue-green pigment sits mainly in the gluten layer of the cap and stem, and its loss with weathering explains the wide colour range seen in a single population. The species plays a part in the rapid breakdown of mulch and litter in managed landscapes, and it is one of a suite of fungi that establish quickly on newly disturbed, organically enriched ground.

Stropharia caerulea, the Blue Roundhead, is the closest match and is separated by its ring, which is thin, incomplete and usually vanishes early leaving only a fibrous zone, by gills that mature a duller olive-brown to brown rather than a clean purple-brown, and by a cap margin with fewer white flecks. Stropharia pseudocyanea is more slender with a paler, greyish blue-green cap that fades quickly to yellowish and a narrower stem.

Other Strophariaceae such as Stropharia semiglobata have the same ring and purple-brown gills but a plain yellow to straw hemispherical cap with no green whatever. Away from the genus, the small blue-green cup fungus Chlorociboria aeruginascens is sometimes mentioned in the same breath because it stains dead wood blue-green, but it is a tiny stalked disc a few millimetres across, not a gilled mushroom, and the green colour is in the wood itself. Blue-staining boletes and Psilocybe species are separated by pores in the first case and by a papillate, non-slimy, ringless cap in the second.