Agaricus arvensis, commonly known as the horse mushroom, is a species of mushroom in the Agaricus genus, which also contains Agaricus campestris, the field mushroom.

Arvensis is a Latin term for “of the field,” chosen because of the habitat in which the horse mushroom can be found. Its common name is derived from the fact that the mushroom is frequently found near horse stables. It can also be found in grassy roadside areas, pastures, open forests, and residential yards. They can be found growing in circles that are several meters in diameter, also known as fairy rings.

The horse mushroom is a cosmopolitan species, meaning its distribution spans the globe, including parts of North America. It is edible and can be foraged, however it’s important to note that the horse mushroom has a tendency to accumulate heavy metals, and therefore should be ingested sparingly. If you are going to forage for this mushroom, it is critical to understand how to correctly identify the species, because it shares similar characteristics with two toxic species, Agaricus xanthodermus (yellow stainer) and Amanita virosa (destroying angel).

The cap of a mature horse mushroom measures between 10 and 20 centimeters. Initially it has a white coloring, but with age, it gradually yellows. Young mushrooms have caps that are spherical in shape, but as they mature, the cap flattens out. When bruised, the cap turns yellowish. The cap itself also has a mild, sweet smell. The gills of the horse mushroom are pale pink or brown, depending on maturity. White gills would signal that you’ve actually found a destroying angel. The stipe, or stem, of the horse mushroom is white, and usually has a double ring that forms the shape of a cog wheel on the underside of the cap.

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To make sure you’ve correctly identified the mushroom, observe its coloring when the stem base is cut. If it rapidly displays a strong yellow color, it is actually a yellow stainer. The horse mushroom can also be confused with the related field mushroom, Agaricus campestris. Both are edible, but if you’re trying to differentiate between the two, A. campestris is generally smaller, and doesn’t have the cog wheel ring on the underside of the cap.

Agaricus arvensis is a saprotrophic mushroom, meaning that it obtains its nutrients by breaking down organic material. Because of this, in addition to growing in pastures and near stables, they are often found growing near the base of spruce trees, feeding on fallen twigs.