Fairy Ring Champignon

The Fairy Ring Champignon (Marasmius oreades) is the classic ring-forming lawn fungus of temperate grassland. Marasmius takes its name from the Greek for withering: its species shrivel in dry weather and then re-expand almost unchanged when rain returns, thanks to tough, resilient tissues that do not collapse on drying. Oreades refers to mountain nymphs, matching the folklore of the rings themselves.

The rings are the species' most famous property. A single mycelium radiates outwards from a starting point, exhausting nutrients behind it, so fruitbodies appear on the advancing front as an arc or complete circle that widens by several centimetres a year. The grass immediately inside the front is often darker green from released nitrogen, and a band of thinner grass may follow.

Structurally the fungus is small but distinctive: a buff to tan hygrophanous cap with a persistent broad umbo, unusually broad and widely spaced pale gills, and a slender but remarkably tough stipe that twists rather than snaps. Those three features together set it apart from the many small brown lawn agarics.

Cap

2 to 5 cm across, convex when young and expanding to flat with a persistent broad, low central umbo. The surface is smooth and dry, buff to tan or pale reddish-brown when moist, drying much paler cream-buff from the margin inwards. The margin is often faintly grooved and becomes wavy in older specimens.

Gills

Pale cream to whitish, notably broad, thick and widely spaced, free or only narrowly attached to the stipe. The wide spacing is one of the best field characters, visible at a glance from below.

Stipe

3 to 7 cm tall and 2 to 5 mm thick, cylindrical and equal, whitish to tan, finely velvety towards the base. It is solid and extremely tough and pliant — it can be twisted around a finger without breaking, unlike the brittle stipes of most lawn fungi.

Flesh & spore print

The flesh is thin, whitish and tough. Fruitbodies shrivel in drought and revive when rewetted. Odour is faintly sweet. The spore print is white, and the spores are smooth, ellipsoid and rather large for the size of the fungus.

This species occupies short, well-drained grassland of almost every kind: lawns, playing fields, golf courses, churchyards, park turf, roadside verges, pasture, dunes and grassy woodland clearings. It is largely absent from long rank grass and from waterlogged ground, and does not grow on wood or in deep leaf litter.

It has an exceptionally long season, appearing from late spring through to the first hard frosts, with flushes triggered by rain after warm weather. It is abundant and cosmopolitan across temperate Europe, North America, Asia and introduced grasslands worldwide.

Marasmius oreades is saprotrophic, decomposing the dense mat of dead grass roots, stem bases and thatch beneath living turf. As the mycelium advances it releases nitrogen from decomposed organic matter, which fertilises the grass just behind the front and produces the characteristic dark green ring; it can also form a dense hydrophobic mycelial layer that temporarily starves the grass of water, giving the paler band sometimes seen.

Rings expand steadily year on year and very old examples can be tens of metres across, representing a single individual many decades old. Where two rings meet, the mycelia usually inhibit one another and both fronts stop, leaving flattened arcs. Spore dispersal is by air from the widely spaced gills, and the tough, revivable tissues allow fruitbodies to shed spores over several wetting cycles.

Clitocybe rivulosa and C. dealbata grow in the same turf and often in rings, but their caps are whitish to greyish with a frosted or zoned appearance, their gills are crowded and clearly decurrent, running down the stipe, and their stipes are brittle rather than pliant. The contrast between crowded decurrent gills and broad, well-spaced free gills is the decisive character.

Panaeolina foenisecii, the Haymaker's Mushroom, shares lawns but has mottled brown gills and a dark brown spore print. Gymnopus dryophilus has a similar tough stipe but grows in woodland leaf litter with much more crowded gills. Conocybe species on lawns are far more fragile with conical caps and a rusty-brown spore print, and Agrocybe species have brown prints and a larger, cracked cap.