A fairy ring is a naturally occurring phenomenon in a lawn, typically appearing as a circular or semi-circular pattern of mushrooms or altered grass growth. This formation is caused by the underground growth of a fungus, which starts from a single spore and expands radially outward over time. While the visible effects on the grass can be unsightly to a homeowner, the fungus itself does not directly infect the turf.
The Distinct Visual Markers
A fairy ring presents in a few different ways, but the most complex and noticeable type displays three distinct zones in the grass. At the leading edge of the ring, the grass often appears noticeably greener, lusher, and grows faster than the surrounding turf. This highly stimulated outer band can be several inches wide and forms a near-perfect circle or arc across the lawn.
Just inside this dark green band, a second zone of grass may be present, which appears stressed, brown, or completely dead. This ring is often the most damaging visual marker and can give the overall formation a bullseye appearance. The third, and sometimes only, visible marker is the appearance of the fungal fruiting bodies themselves, which are the mushrooms or puffballs.
These fungi emerge in a ring or arc, typically during periods of high moisture, and can be various species that produce mushrooms or puffballs. Fairy rings can range in size, starting as small as a foot in diameter but expanding by up to a foot each year, sometimes reaching fifty feet or more in older lawns. In some cases, the only visible sign is the ring of mushrooms, with no noticeable change in the color or growth rate of the grass.
How the Ring Forms
The visible rings are merely the surface manifestation of a vast network of fungal threads, called mycelium, growing below the soil. The fungus begins its life when a spore lands and finds a food source, such as buried organic matter like old tree roots, stumps, or construction debris. From that central point, the mycelium grows outward in a uniform, circular manner, consuming the organic material as it goes.
The striking dark green color of the outer grass is a result of the fungus breaking down this organic matter. As the mycelium advances, it releases enzymes that decompose the material, which temporarily releases a surge of nitrogen and other nutrients into the soil. The grass roots in this zone absorb the nitrogen, resulting in the stimulated, deep-green growth seen at the ring’s edge.
The dead or stressed grass, which forms the inner band of the ring, is caused by a different effect of the fungal mass. As the mycelium becomes dense and established in the soil, it makes the soil water-repellent. This dense, waxy layer prevents water from penetrating the soil and reaching the grass roots, causing a localized drought that stresses or kills the turf. This effect is often more pronounced in hot, dry weather.
Safety Concerns and Management
Homeowners should treat any mushrooms emerging in a fairy ring with caution, as many of the fungi species responsible for this phenomenon are poisonous. Since many fairy ring fungi are difficult to identify, the general rule is to never consume any wild mushrooms found in the lawn. If the fruiting bodies are a concern due to pets or children, they can simply be mowed over or raked away, as this only removes the temporary reproductive structure and does not affect the underground mycelium.
Managing the aesthetic damage requires disrupting the dense, water-repellent mycelial mat and correcting the nutrient imbalance. The most common and effective non-chemical approach involves aerating the affected area deeply with a garden fork or core aerator to break up the hydrophobic layer. Following aeration, the area should be deeply watered, sometimes with the addition of a wetting agent or a few drops of dish soap, to help the water penetrate the soil.
To mask the dark green ring, homeowners can apply a nitrogen-based fertilizer to the rest of the lawn to encourage uniform color and growth. Over the long term, removing the initial food source, such as buried tree roots or lumber, will eventually eliminate the fairy ring, though this process can take many years. Chemical fungicides are generally not recommended for home lawns due to the difficulty of getting the product to penetrate the dense mycelium.