Ever notice the circles of mushrooms that frequently appear on our neutral grounds and in our yards the day after a hard rain during our humid summer months? People often refer to them as “fairy rings” because fairies supposedly dance in them at night.
The “fairy ring” mushroom (Chlorophylum molybdites) is easily identified since it is the only mushroom in the world that has green spores. You can check for spore color by making a spore print in the same way as scientists who study mushrooms (they are called mycologists). When the mushroom has opened, remove the umbrella-shaped top (the cap) and place it, gilled surface down, on a piece of white paper. Cover the cap with a bowl to prevent air flow. Within a few hours, enough spores will have fallen onto the paper to leave a spore print and you can clearly see the color.
To understand why this mushroom grows in a circle, you must understand something about its life history. When you see a mushroom you are looking at the reproductive portion of the organism - the equivalent of a rose on a rosebush. The vegetative portion of the mushroom (the “rosebush”) is called a mycelium, a gray stringy material that grows like a root system beneath the surface of the soil.
The mycelium begins life from the spore and, as it grows outward, consumes nutrients within the soil. Since reproduction - i.e. the production of the mushroom - is a high-energy activity, it makes sense that mushrooms will be produced at the periphery of the mycelium, where nutrients are most abundant. Hence the ring. It is not uncommon to see half-circles or rings within rings. And because the mushrooms require nutrients in different combinations than grasses, they have little or no affect on the quality of the lawn.
Mention should be made that the fairy ring mushroom is not edible; it causes nausea, dizziness and headaches for most people. Unfortunately, it closely resembles Lepiota procera and L. rachodes, highly edible mushrooms from the northern United States, which explains why Northerners moving to the South often mistake these species. In fact, the most common form of mushroom poisoning in the New Orleans area is attributed to the fairy ring mushroom.