Agaricus arvensis, horse mushroom., (stem with ring).
A substantial harvest from these large fungi, it's common name 'horse' mushroom, is an indication to where it likes to grow, feasting on manure in fields, farms and meadows, but some people like to think it's name it due to its great size.
Mushroom ID key
Cap: Starts of spherical, flattening when older. Matures to 8-20cm, occasionally growing to around 30cm. White, yellowing with age or when bruised. Smooth to finely scaled. Flesh of cap is white and firm.
Gills: Pale pink-dark brown, free and crowded.
Stem: Usually up to 10cm tall, and 2-3.5cm thick. Parallel with a bulbous base. Double ringed, one ring when cap is closed, second ring drops when cap opens, the second ring forms a cogwheel pattern, when viewed from underneath. The stem is smooth above the rings, and finely scaled below. When the base of the stem is cut, it stays white and gradually turns a faint yellow, rather than instantly staining bright chrome yellow, like it's poisonous lookalike the yellow stainer.
Spores: Dark purple - brown.
Odour: Strong aniseed, especially in older fungi.
Taste: no distinctive flavour, but a delicious Portobello like mushroom, that can be used any way you would use standard shop bought mushrooms.
Habitat: manured meadows, anywhere where there is plenty of decaying organic matter, as that's what this mycelium feeds on. Often seen in large rings, but sometimes appear more informally.
Season: August-November, on average appearing 2weeks later than field mushrooms.
Similar species: The poisonous Yellow Stainer, Agaricus xanthodermus. As mentioned it rapidly turns bright yellow when cut or bruised, it also smells of iodine or ink rather than aniseed. And Agaricus urinascens/Agaricus macrosporus, the macro mushroom. This is very similar in appearance, and does not turn bright chrome yellow when cut or bruised. It grows in grassland and woodland clearings, so is often easily confused. The name 'urinascens' comes from the urine like odour the mushroom has, which gets stronger once picked, a key feature for differentiating the horse mushroom from the macro.