What's a fairy ring of mushrooms? Why is it a circle?

Fairy Rings – Wisconsin Horticulture

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What is Fairy Ring and How to Get Rid of It

What's a fairy ring of mushrooms? Why is it a circle?

ncif EarthSky

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Fairy Ring Mushrooms Growing & Mowing in Bartow County

Fairy Ring Nat Geo Photo of the Day

Fairy Ring, we have these in the yard at the other house. Grows

Marasmius oreades, Fairy Ring Champignon mushroom

These mushrooms that grew in a circle that I came upon today : r