Common Fieldcap

Agrocybe pediades is the common small fieldcap of open grassland, a modest and easily overlooked agaric that nonetheless turns up in vast numbers on lawns, pastures and dunes through the warmer half of the year. It is the grassland counterpart to the larger, ringed, woodchip-loving members of the genus such as A. praecox and A. rivulosa.

The species has carried several names, most often A. semiorbicularis, a reference to the persistently hemispherical cap that only slowly flattens with age. Modern treatments place these under A. pediades, although the group remains variable and several closely similar grassland taxa are recognised by some authors on the basis of spore dimensions and cap colour.

Its value as a field species lies in the combination of characters that are unusual together: a small, smooth, dryish ochre cap, a slender ringless stipe, gills that mature deep brown, and a habitat of closely grazed or mown turf, particularly where the soil has been enriched by grazing animals.

Cap

Small, generally 1-3.5 cm across, hemispherical for a long time and eventually becoming convex or nearly flat. The colour is a clay-yellow, ochre-tan or dull honey, often paler at the margin and drying to a duller buff. The surface is smooth and slightly greasy when damp, drying matt and sometimes finely wrinkled or minutely cracked at the disc. The margin is not grooved.

Gills

Adnate and rather broad, moderately spaced. They start pale clay-buff or greyish, then darken steadily through cinnamon to a deep dark brown as the spores ripen, contrasting quite sharply with the paler cap.

Stipe

Slender, 2-6 cm long and only 2-4 mm thick, cylindrical, straight or slightly flexuous, pale yellow-buff to whitish and often browning from the base upwards. The surface is finely pruinose or minutely mealy, especially near the apex. There is no ring, and usually no more than the faintest trace of a veil, which is a key separation from other fieldcaps.

Flesh & spore print

The flesh is thin and pale buff, unchanging when cut, with a faint mealy odour. The spore print is dark brown, tending towards a cigar or tobacco tone; the spores are smooth and ellipsoid with a clear germ pore.

This is a fungus of open, short, well-lit grassland. It fruits in lawns, playing fields, roadside verges, grazed pasture, calcareous downland, fixed sand dunes and mossy turf, generally singly or in scattered troops rather than clusters. It shows a marked preference for ground enriched by animal droppings, and can be abundant in sheep- and cattle-grazed pasture, occasionally growing directly from old, well-weathered dung.

The fruiting season is long, stretching from late spring through summer and into autumn, with flushes following warm rain. Unlike most grassland agarics it is at its best in the summer months rather than late autumn. The species is effectively cosmopolitan, recorded across Europe, Asia, North and South America, Africa and Australasia wherever suitable short turf occurs.

Agrocybe pediades is a saprotroph of grassland litter. It decomposes the thatch of dead grass stems, roots and leaf bases in the upper few centimetres of turf, together with incorporated dung and other organic material, and returns the nutrients to the soil. It is not mycorrhizal and forms no partnership with the grasses among which it grows.

Its association with nutrient-enriched ground reflects a preference for readily available nitrogen, which is why grazed pasture and fertilised lawn are such productive habitats. The long, warm-season fruiting pattern, small fruit bodies and rapid turnover suit a fungus that exploits short-lived flushes of moisture in exposed grassland. Ecologically it is part of the same guild as the small brown Panaeolus and Conocybe species that share these habitats.

Bolbitius titubans, the Yellow Fieldcap, is much more fragile with a slimy, bright egg-yellow cap that is deeply grooved at the margin, and it collapses within a day; its spore print is rusty ochre rather than dark brown. Conocybe species have distinctly conical, thin-fleshed caps on very fragile stipes and drop rusty cinnamon spores.

Panaeolus species share the grassland and dung habitats but have mottled, unevenly darkening gills and a black or blackish spore print, and their caps often have a paler marginal band. Agrocybe praecox is much larger, carries a distinct membranous ring, has white rhizomorphs at the stipe base and grows chiefly on woodchip and hedge banks in spring rather than in short turf.