Fairy rings
Circles or arcs of dark green or dead grass in a lawn, often with mushrooms, caused by fungi feeding on buried organic matter.
Also known as: Fairy ring fungus
Symptoms and identification
A ring or arc of darker green, fast-growing grass, sometimes with a zone of dead or dry grass inside or beside it. Mushrooms or puffballs may appear in the ring after rain, and soil beneath can be dry and repel water.
Causes
Soil fungi spread outward from a central point, breaking down buried wood, old roots or thick thatch. The mat of fungal threads in the soil blocks water, so the grass in the dead zone goes dry.
Organic treatment
Rake off mushrooms before they spread spores, spike the ring with a fork to let water through, and water deeply. Feed the lawn to mask the dark green ring. Digging out the soil to 30 cm beyond the ring and replacing it is a last resort.
Prevention
Remove old tree stumps, roots and builders debris before laying a lawn, keep thatch thin and water deeply so moisture soaks in.
Chemical treatment (use with care)
Fungicides rarely work well on fairy rings, because the fungus lives deep in the soil.