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Lawn Agaricus (by Bob Sellers)

From the Duff Archives circa January 1997.
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This month’s featured species turns up in the plastic bags of people attending the Fungus Fair perhaps more than any other fleshy fungus. Invariably, hordes of people will be lined up at the mushroom ID table with bags containing fresh to soggy specimens of Agaricus mushrooms they found “growing in my lawn. Are these edible? Will they poison my dog, cat, kids?”

Indeed, when abundant, lawn Agaricus probably elicit more questions for identification than any other species. They are obtrusively obvious, boldly bursting up through a lush carpet of green grass. People don’t have to venture very far to collect them. They are also exceedingly common, appearing shortly after the first fall rains before the weather gets too cold, even in the summer in well-watered lawns. Cemeteries contain them, city parks can produce them, and along with fairy ring mushrooms, they are the most abundant large mushrooms of lawns and mowed grassy areas.

Agaricus species are an easy group to recognize because of the store-bought variety with which everyone is familiar. When mature, all Agaricus species have dark brown gills which give a chocolate brown spore print. The stem will separate cleanly from the cap and gills, and the stem (stipe) will also have a ring or annulus, which may be thick and persistent or may collapse as the mushroom matures. The cap color is generally white, but there can be gray to brown patches or fibrils on the cap depending on the species and age. These features will help you determine if your mushroom is an Agaricus, but knowing which Agaricus you have will take a closer look.

Pull your mushroom out of the ground so that your get the entire stem. Clean off some dirt and scratch the very bottom of the stem with a fingernail to remove some mushroom tissue. Did the scratched area immediately stain bright yellow? If it did you have A. xanthodermus, which means yellow skin. (Other parts of the stem and cap may also stain yellow from handling. The base of the stem, however, is the critical part. This mushroom often grows in large clusters or fairy rings, and also under cypress and oak trees. The color of the entire mushroom is nearly white, but it often develops grayish to brownish or even metallic tones on the cap with age. The ring is thick and membranous with flat patches underneath. Crush the mushroom and notice the strong odor of phenol that exists along with the “normal” mushroomy scent. All of the phenol-smelling Agaricus species actually contain the compound phenol and are considered poisonous, although I’ve heard of individuals who don’t seem to be affected (i.e., vomit) after eating these species.

If the base of your lawn Agaricus didn’t stain yellow but there are yellow stains from handling on the cap or stalk, take another sniff and see if you can detect the odor of anise  or almond extract. If this odor is present, your mushroom is most likely A. arvensis, the horse mushroom, so named because of the musty or urine-like odor the mushroom develops when old. Also found in pastures along with A. osecanus (the giant horse mushroom), A arvensis has a nearly pure white cap and stalk and is a look-alike for A. xanthodermus, but doesn’t grow in clumps nor have a phenol odor. The annulus is persistent, thick and cottony with tooth-like patches on the underside. A arvensis is a choice edible (as are all the anise or almond smelling Agaricus species). Sliced and dried, it maintains its anise odor and lends a marvelous flavor to soups. You can eat it fresh or stuff the caps with chopped stems, onions, basil, parmesan cheese and broil.

The next two lawn Agaricus are difficult to tell apart unless you have a good nose and some young buttons of each species. If your mushroom doesn’t stain visibly yellow from handling, lacks and anise odor, but has a faint phenol odor, you probably have A. californicus.

The odor is not as apparent as in A. xanthodermus, but it is there — especially i young specimens that haven’t become waterlogged or rotten. This species seems to be especially abundant in Santa Cruz and most often in lawns, rarely in pastures. It will also appear in disturbed ground, roadsides, vacant lots or in grassy areas along the edges of the woods. When mature, this species has a flattened top which has brown, gray or even silvery areas in the center of the cap. It is often gregarious, growing in patches, but not in rings. The membranous ring in A. californicus has felt-like patches on the underside and will be evident even as the mushroom ages. As with A. xanthodermus, the phenol odor in A. californicus, often faint or difficult to detect, should dissuade you from eating it. However, some peole do just fine and actually enjoy this very common Agaricus.

The final lawn Agaricus we’ll mention is the meadow mushroom, A. campestris. As its name suggest, this species is usually found in meadows or pastures, but it occasionally strays into some fortunate person’s lawn. The meadow mushroom is easy to identify  in the button stage, for it is the only local Agaricus with pink gills when it is young (before the cap opens). As it matures, the pink color fades until the gills are brown as in all Agaricus. There is no phenol odor, only a lovely sweet mushroom scent. No part of the mushroom stains yellow, but red stains may appear when the mushroom is wet. The color is nearly all white, bur in age darker areas may appear near the center of the cap. The veil is thin, cottony and collapses or disappears entirely as the gills open from the button stage. The stalk is rather short and often tapers towards the base. This mushroom is far superior in flavor to the store-bought variety, and can be used in any recipe calling fresh mushrooms.

One final note: please be absolutely sure of your identifications before making a meal of your lawn Agaricus. Use all of your senses and consult a more thorough description in a guidebook if you are not entirely convinced which species you might have. I have tried to describe the most common Agaricus species you will encounter. There are others than can occur occasionally in lawns. However, Agaricus is a rather “safe” genus in that no species are deadly, and all of the poisonous species have a phenol odor, which is most notable in fresh younger specimens.