Fairy rings are distinct circular patterns that appear in turf, often presenting as a band of stressed or dead grass. This visual phenomenon is a sign of hidden biological activity occurring beneath the soil surface. The name itself comes from ancient folklore that attributed the circles to the dancing of fairies. Understanding why the grass dies requires a look at the fungus responsible and the specific conditions it creates in the soil.
The Hidden Cause: Fungal Mycelium
Fairy rings are caused by certain species of fungi, primarily basidiomycetes, which include many common mushrooms and puffballs. The visible mushroom, or fruiting body, is only the temporary reproductive structure of the main organism. The true culprit is the extensive, underground network of thread-like filaments called the mycelium.
This fungal mass begins from a single spore and grows outward radially through the soil, consuming organic matter as it expands. The growth front of this mycelial network creates the characteristic circular or arc shape visible from above. The main body of the fungus, the mycelial mat, can persist in the soil for years, slowly increasing the ring’s diameter by one to two feet annually. The central portion of the ring may eventually die off, which leaves the active, growing edge as the circular band of stressed turf.
The Mechanism of Grass Stress and Death
Grass death is an indirect result of the fungus changing the physical and chemical properties of the soil, rather than the fungus directly attacking the grass roots. The most common cause of grass death, referred to as a Type I fairy ring, is the creation of a localized drought condition. The dense fungal mycelium and the waxy organic compounds it produces repel water, a condition known as hydrophobicity.
This hydrophobic layer in the soil and thatch prevents water from penetrating the root zone, effectively creating a barrier against irrigation or rainfall. Grass roots within the ring cannot access moisture and die. This lack of available water causes the turf to wilt and turn brown, mimicking severe drought stress. The water-repellent nature of the soil in the ring can last for a year or more, even after the active fungal growth has passed through the area.
As the mycelium aggressively breaks down organic matter, it temporarily ties up available nutrients, particularly nitrogen, for its own growth. This nutrient depletion can stress the grass, causing it to appear lighter green or yellow compared to the surrounding turf. Conversely, a Type II fairy ring symptom is a band of darker, lusher green grass, which occurs just outside the active fungal front.
This stimulated growth occurs because the fungus releases a surge of nitrogen and other nutrients as a byproduct of decomposing organic material. The grass just beyond the active mycelial mat benefits from this sudden release of fertilizer, creating a highly visible dark green ring. In some instances, the concentration of nitrogen, specifically ammonium, can accumulate to toxic levels behind the advancing front, which can also contribute to the death of the grass.
Managing Fairy Rings in Turf
Addressing fairy rings focuses on mitigating the environmental stress they cause, particularly the soil’s water repellency. Complete eradication of the deep-seated mycelial mat is often difficult and impractical. The primary cultural control involves breaking the hydrophobic layer and re-wetting the root zone to restore water access for the grass.
The affected area should be treated with deep aeration, such as spiking or using a core aerator, to physically penetrate the water-repellent soil layer. Following aeration, a soil surfactant, also known as a wetting agent, should be applied. These chemical agents help to reduce the surface tension of the water, allowing it to move past the waxy fungal residue and rehydrate the soil profile. This process may require repeated applications and deep hand-watering over several days to be effective.
For Type II rings showing the dark green, stimulated growth, the symptoms can be masked by applying nitrogen fertilizer to the entire lawn. This encourages the surrounding, unaffected turf to green up, making the circular pattern less noticeable. If the grass is already under stress, using iron supplements instead of high nitrogen can improve color without promoting excessive growth. Fungicide use is generally considered a last resort, as the chemicals must be applied at a high volume and drenched deep into the soil to reach the active mycelium. This treatment is both costly and not always consistently effective.