Meadow Mushroom
Agaricus campestris
Also called field mushroom, pink bottom
The classic white grassland Agaricus, with free gills that shift from bright pink to chocolate-brown, a simple fragile ring and a dark brown spore print. It appears in troops and rings on old pasture after rain.
Overview
Agaricus campestris is the type species of Agaricus, a large genus of dark-spored agarics defined by free gills that darken from pale through pink to chocolate-brown, a partial veil that leaves a ring on the stem, and the absence of any volva or sac at the stem base.
It is the mushroom most people picture when they think of a mushroom: a plain white cap on short grass. Its most useful character is the gill colour sequence. Buttons have vivid, almost salmon-pink gills - much brighter than any other common grassland species - which mature through purplish-brown to a dark chocolate-brown, matching the spore print.
A. campestris is a grassland specialist, quite distinct from woodland Agaricus species. It is closely tied to old, undisturbed, grazed pasture and reacts strongly to the nitrogen enrichment left by livestock, so a good site may produce the same rings and arcs year after year for decades.
How to identify it
Cap
4-10 cm across, hemispherical to convex at first with an inrolled margin, then expanding flat. The surface is dry and white to cream, smooth in young specimens and becoming faintly silky-fibrillose or slightly scurfy with age, sometimes with pale brownish fibrils in exposed conditions. The margin often carries fragments of the veil.
Gills
Free from the stem and crowded, and the colour change is the key: bright pink to almost salmon in young caps, then dull purplish-brown, and finally dark chocolate-brown. Even in buttons the gills are already pink, not white, once the veil has begun to stretch.
Stipe
3-8 cm long, relatively short and stout in proportion to the cap, tapering slightly at the base, white and smooth. The ring is thin, simple, single-layered and fragile, sitting roughly mid-stem and often disappearing entirely in older specimens. There is no volva and no sac at the base - the stem simply ends in a blunt point.
Flesh & spore print
Flesh is thick and white, at most slowly and faintly pinkish where cut, especially in the stem; it does not turn bright chrome-yellow anywhere, including the very base. The spore print is dark chocolate-brown.
Where and when it grows
It grows on soil in short, nutrient-rich grassland: grazed pasture, meadows, downland, lawns, verges, playing fields and dunes. It has a strong association with land grazed by horses, cattle or sheep, and it dislikes ploughed or heavily improved ground, so long-established permanent pasture is the prime habitat.
Fruiting typically begins in late summer and continues through autumn, with big flushes a few days after heavy rain, often in troops, arcs or complete rings. The species is widespread and common across Europe, North America, temperate Asia and introduced grassland worldwide, but has declined in intensively farmed regions where old pasture has been lost.
Ecological role
Agaricus campestris is a saprophyte that decomposes organic matter, dead grass roots and dung-enriched humus in the topsoil of grassland. It is not mycorrhizal, which is why it thrives in treeless open pasture, and its nitrogen preference explains its close link with grazing animals.
The mycelium is perennial, spreading through the turf and frequently forming rings and arcs like other grassland fungi. By breaking down root litter and dung residues it recycles nitrogen and phosphorus back into the sward. Its fruitbodies are heavily used by fly larvae, beetles and small mammals, and the species is regarded as an indicator of long-established, unimproved grassland.
Similar species
The Horse Mushroom (Agaricus arvensis) is larger and taller, has a complex double 'cogwheel' ring with a star-patterned underside, greyish-white rather than pink young gills, an aniseed odour, and flesh that ages slowly yellowish. The Yellow Stainer (A. xanthodermus) has a squarish young cap and stains bright chrome-yellow immediately when the cap edge is rubbed and especially in the cut stem base, with an ink-like odour.
The Wood Mushroom (A. silvicola) is a slender woodland species with a bulbous stem base and a large pendulous ring. Most importantly, white Amanita species such as the Destroying Angel (A. virosa) differ structurally in every respect: their gills stay pure white and never turn pink or brown, their spore print is white, and their stem base sits in a distinct membranous sac (volva), usually buried and easily missed unless the whole base is lifted from the soil.