The biological mechanism behind the mysterious circles of the fairy ring

Ideas & Philosophy · Published 2 October 2026 Swipe more like this →

In short: Long dismissed as the footprints of witches or pixies, these expanding rings of mushrooms and dead grass are actually the result of a sophisticated underground nutrient struggle. Discover how a single fungus creates a living, moving frontier that can persist for centuries.

While folklore in Western Europe often links these rings to the Devil or good fortune, the reality is a process of outward expansion. The most significant culprit is the fungus Marasmius olarides. This fungus lives within the turf's root zone, where its mycelium—a network of thread-like filaments—spreads gradually outward. As the colony advances, the center eventually dies off because the nutrients have been exhausted, leaving behind a circular, advancing 'front'.

The visible symptoms of a fairy ring are often a combination of dead and hyper-vibrant grass. A 'necrotic zone' of dead grass can appear when the mycelium coats roots and makes them water-repellent, particularly during dry weather. Conversely, you might see a ring of much greener grass. This happens because as the dead grass decays, it releases nitrogen, or because certain fungi produce gibberellins—chemicals that act like hormones to stimulate rapid, luxuriant growth.

The scale of these formations can be immense. While they often appear as simple circles, they can manifest as arcs, sickles, or even doubled arcs. Some colonies expand by approximately 30cm per year and can exist in large areas of turf for hundreds of years. In certain environments, like the chalky soils of Dorset and Wiltshire, the cycle may even be influenced by local wildlife. For instance, rabbits can help maintain these rings by cropping the grass short and providing nitrogen-rich droppings, which can eventually trigger secondary rings to grow inside the original ones.

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