Brown Mushrooms in Yard: How to Tell Them Apart

Brown is the least useful colour in mushroom identification, because it covers the most abundant harmless lawn species and several of the most dangerous fungi in North America at the same time. Mycologists call the whole cluster LBMs — little brown mushrooms — and treat them as a category where field identification often fails.

That is not a reason to panic about a brown mushroom in the grass. It is a reason to sort by the features that actually separate them:

• Where the stem comes from. Soil, thatch, or wood. Wood-growing brown mushrooms include the deadly ones.
• Gill colour, especially in maturity. Dark mottled grey-brown, rusty brown, chocolate brown and pale tan all point to different groups.
• Spore print colour. White, rusty brown, dark brown or black.

This is almost always Panaeolus foenisecii, the lawnmower's mushroom or haymaker, which is described as one of the most common and widely distributed lawn mushrooms in North America and often fruits in large numbers.

What to look for: a small cap, brown when moist and fading in a distinct band as it dries, mottled dark grey-brown gills, a thin brittle stem, and a habit of appearing across an open lawn rather than in rings or clusters. It is a thatch feeder, which is why it follows mowing and rain.

On risk: guidance treats it as toxic on a precautionary basis and not a mushroom to gather. A multicentre study of accidental ingestions in Germany and Switzerland, indexed on PubMed, reported that children who ate five or fewer specimens that were later carefully identified did not develop clinically significant symptoms. That study describes one identified species and small amounts. It is not permission to wait and see when a toddler has eaten something unidentified — call Poison Control at 1-800-222-1222 and let a specialist weigh it.

Brown mushrooms arranged in a curve are usually the fairy ring mushroom, Marasmius oreades. The Missouri Department of Conservation describes it as a cap ½ to 2 inches wide, pale tan to buff to reddish brown, bell-shaped becoming convex with a central knob; broadly spaced off-white gills, attached or free; a stalk 1 to 3 inches long and ⅟₁₆ to ¼ inch wide, cream to pale buff, dry, tough and feltlike; white spore print; fruiting May to September in lawns and meadows, often in arcs or circles.

The tough, wiry stem is the field character people remember: it is hard to snap and the mushroom revives after drying.

The Missouri guide also names the reason this is a bad beginner mushroom. It shares its habitat with poisonous Clitocybe and Inocybe species. Clitocybe dealbata is grey-pink with gills running down the stalk, and Inocybe species usually have grey-brown gills. Both contain muscarine and produce sweating, salivation and tears within a couple of hours.

This is where a brown lawn mushroom becomes a serious matter. If the stems rise from a stump, a buried root, a rotting fence post or a wood-chip bed rather than from soil, the candidates change.

• Galerina marginata. Small, tawny to rusty brown, with a ring on the stem and a rusty brown spore print, growing on wood including wood buried under turf. It contains amatoxins, the same class of liver-destroying compounds as the death cap, and the North American Mycological Association lists Galerina among the amatoxin-containing genera dangerous to dogs and cats. See Galerina marginata.
• Armillaria, the honey mushrooms. UC IPM describes Armillaria root rot as producing clusters of several to dozens of light brown mushrooms at the base of infected trees and shrubs, sometimes from a buried root out in the turf. They are tan to honey coloured with a ring on the stem. Supporting signs are white mycelial fans under the bark and black shoestring-like rhizomorphs.

A big brown cap on a stout stem with a ring, growing from lawn soil, is usually one of the Agaricus species or a brown-capped parasol type. Agaricus campestris, which UC IPM lists among lawn fungi, has gills that start pink and darken through brown to near black, and a dark brown spore print. That colour sequence is the opposite of Amanita, where gills stay white.

Watch for the trap at this size:

• Young Chlorophyllum molybdites. Before the gills green up, a green-spored parasol reads as a pale brown-scaled mushroom with white gills. The spore print settles it: green. It is the most frequently eaten poisonous mushroom in North America precisely because the young stage looks so ordinary.

If the cap is more than a few inches across and there is a ring, get a spore print before you draw any conclusion.

Do not wait for symptoms and do not rely on a photograph. Poison Control states that only experts in mushroom identification can tell poisonous species from harmless ones, and that poison specialists work directly with mycologists to identify what was eaten.

• People, United States: 1-800-222-1222, free and available 24 hours a day.
• Dogs and cats: your veterinarian, or ASPCA Animal Poison Control at (888) 426-4435. The North American Mycological Association advises collecting samples in paper bags or waxed paper and refrigerating them until they can be examined.
• Keep the specimen whole, including the base, and note what it was growing on. Wood versus soil narrows the possibilities immediately.

Amatoxin species are the reason the timing rule exists: the North American Mycological Association describes a 6 to 12 hour or longer delay before severe gastrointestinal illness in amatoxin poisoning. Feeling fine two hours later proves nothing.