Horse Mushroom

The Horse Mushroom is one of the largest and most handsome field mushrooms, frequently dwarfing the common Field Mushroom (Agaricus campestris) that grows in the same pastures.

Cap

The cap begins as a tight, white, almost spherical 'snowball' (hence one of its names) before expanding to a broad convex or flattened dome 8-20 cm across, occasionally far larger. Its surface is white to cream, smooth or only faintly silky-scaly, and bruises a slow, soft yellow where rubbed, ageing overall to a dull ochre or buff.

Gills

Turn a specimen over and check the gills: in the very youngest, tightly closed buttons they can look pale and almost whitish, but they very quickly take on a greyish-pink to dirty-cream tone (never the vivid clear pink of the Field Mushroom) and then darken steadily through grey-brown and chocolate to almost black as the spores ripen.

Because young white buttons are the stage most easily confused with a deadly Amanita, never rely on gill colour alone at the button stage - always dig up and inspect the base.

Stem

The stout white stem carries a large, drooping double ring; lift it and you will see the hallmark of the species - the lower face of the ring is split into a soft, ragged ring of segments resembling a cogwheel or many-pointed star, often with creamy-yellow flecks. The base is club-shaped but sits in bare soil with no sac-like cup.

Flesh

The flesh is thick, firm and white.

Smell & taste

The whole mushroom smells distinctly and pleasantly of aniseed or marzipan.

A creature of open grassland rather than woodland. Look for it in unimproved and grazed pastures, hay meadows, paddocks, roadside verges, dunes, lawns, parkland, golf courses and woodland glades - anywhere the turf is short and the soil is rich.

It has a strong affinity for nutrient-enriched ground, classically the manured pastures grazed by horses and cattle, which is the origin of its English name.

It commonly fruits gregariously, scattered in troops or arranged in conspicuous rings and arcs that may persist in the same spot for many years.

Chiefly a late-summer to autumn mushroom.

The first flushes often appear from July once warm soils are dampened by rain, the species builds to a peak from late August through September and into October, and fruiting tails off with the arrival of hard autumn frosts in November.

In any given pasture it tends to follow a few days behind significant rainfall, so the best hunting is during mild, moist spells.

Saprotrophic, feeding on the humus and decaying organic matter of grassland soils rather than partnering with tree roots. The underground mycelium spreads outward year on year, which is why fruit bodies so often appear in expanding rings and arcs.

Individual mushrooms develop quickly from buried 'eggs' to full size over a few days under favourable warm, damp conditions, and a productive site may yield repeated flushes through the season.

Grassland saprotroph: fruits after sustained autumn rain on warm soils, typically several days to a week and a half after significant rainfall; flushing slows once soils cool below roughly 8 C and stops with hard frost.

Four groups matter.

1. 1Most important, the deadly Destroying Angel (Amanita virosa / A. bisporigera) and the Death Cap (Amanita phalloides) can grow in or beside grassland and, as pure-white buttons, superficially resemble a young Horse Mushroom; they are separated by their PERMANENTLY WHITE gills, a white spore print, a thin fragile ring, and a membranous sac (volva) cupping the swollen stem base - features a Horse Mushroom never shows. The single most reliable field check is to dig up the whole base and look for a volva.
2. 2The Yellow Stainer (Agaricus xanthodermus) is the common poisonous Agaricus mimic: it shares a white cap and pasture habitat but bruises an INSTANT, bright chrome-yellow (especially at the very base of the cut stem) and smells unpleasantly of phenol, ink or chemicals rather than aniseed.
3. 3In North America and other warm regions the Green-spored Parasol (Chlorophyllum molybdites) shares the same lawns and pastures and is the most frequent cause of serious mushroom poisoning there; it is parasol-like with a scaly cap, gills that turn dull GREEN, and a GREEN spore print, so a spore print settles any doubt.
4. 4Several harmless relatives - the Field Mushroom (A. campestris, smaller, single thin ring, bright pink young gills, no anise smell), the Wood Mushroom (A. silvicola, slimmer, woodland, also anise-scented and edible) and the Macro Mushroom (A. urinascens / A. macrosporus) - are easily and safely confused with one another but require the same volva and bruising checks.

Choose firm, young to middle-aged specimens; the very young 'snowball' buttons are excellent and the fully expanded caps still good as long as the flesh remains firm and the gills have not gone soggy black.

Cut or gently twist the stem at the base, brush off soil, and check each cap individually.

Because the deadly Destroying Angel can share the same grassy habitat, NEVER gather Agaricus 'buttons' in a hurry - dig up the WHOLE base of every specimen and reject anything sitting in a sac-like volva or cup.

Do not rely on gill colour alone at the youngest button stage, where both an Amanita and an immature Agaricus can show pale gills.

Also slice the flesh: discard anything that stains bright chrome-yellow within seconds or smells of ink, phenol or chemicals (Yellow Stainer).

Avoid roadside and heavily sprayed verges, as this species readily accumulates heavy metals and other contaminants from rich or polluted ground.

A prized edible with a fuller, slightly sweeter, more aromatic flavour than the cultivated button mushroom, carrying a gentle hint of its aniseed scent.

The firm young caps are superb fried in butter, grilled whole, stuffed, or added to stews, soups, sauces and omelettes; larger flat caps make excellent grilling or baking 'steaks'.

Clean by brushing rather than soaking, and cook thoroughly - Agaricus species should always be eaten cooked, never raw.

The mushroom dries well for storage and rehydrates into a richly flavoured ingredient for winter cooking.

Edibility is excellent, but this is a genus that demands discipline because its grassland habitat overlaps with two of the most lethal fungi known. Adopt three non-negotiable rules:

1. 1ALWAYS dig up and expose the full stem base and reject anything sitting in a sac-like volva or cup - this is the most reliable defence against a deadly Amanita, because at the youngest button stage gill colour alone is NOT a safe guide (both a Horse Mushroom and an Amanita can show pale gills then).
2. 2Confirm that the gills become pink and then chocolate-brown as the mushroom matures, and reject any specimen whose gills stay pure white into maturity.
3. 3Cut the flesh and reject any mushroom that stains bright yellow within seconds or smells of ink, phenol or chemicals (Yellow Stainer), keeping only those with a clean aniseed or almond aroma and at most a slow, faint yellowing.

The Horse Mushroom can also concentrate cadmium and other heavy metals from rich soil, so eat it in moderation and avoid specimens from roadsides, industrial ground and heavily fertilised land.

A minority of people experience digestive upset from Agaricus species; try a small portion first.