Why is there a dark green ring in my lawn?

The ring is the visible edge of a roughly circular colony of fungus in the soil, expanding a few centimetres to a third of a metre a year from wherever it started, often a buried root or a long-gone tree stump. It has been there for years before you see it. Understanding which of the three types you have decides whether it is a cosmetic annoyance or something that will actually kill grass.

What you'll need

• A spade
• Lawn feed (optional)
• A garden fork (optional)

Step by step

1. Look at what the ring is made of.Type 1 has a dark green outer ring, a band of dead or dying grass inside it, and often toadstools. Type 2 has the dark green ring with no dead zone. Type 3 has toadstools or puffballs in a ring with no colour change at all.
2. Understand the dark green.The fungus digests organic matter in the soil and releases nitrogen as it does. The grass over the active edge gets a free feed, which is why that band is darker and grows faster than the rest of the lawn.
3. Understand the dead band, if you have one.In type 1 the fungal mycelium forms a dense mat in the soil that water cannot penetrate. The grass above it dies of drought while the rest of the lawn is perfectly moist. That is a physical problem, not a toxicity one.
4. Check for a buried cause.Old tree stumps, buried timber, builders' rubble containing wood, and construction debris under a new-build lawn are the usual origins. Knowing this does not fix the ring but it explains why yours is where it is.
5. Decide honestly whether it needs anything.Type 2 and type 3 rings are cosmetic. Most people who ask this question are happier once they know that, and the sensible response is to do nothing except keep the rest of the lawn fed so the contrast fades.
6. For type 2, feed the whole lawn to even the colour.The ring is dark because it has more nitrogen than the rest. Bringing the rest up to the same level makes the ring nearly invisible. This is the standard practical management and it works.
7. For a dry dead band, break the water repellency.Fork or hollow-tine the affected band deeply and repeatedly, and water heavily and slowly so it soaks rather than runs off. A wetting agent helps water penetrate a hydrophobic mat. This treats the symptom for a season; the fungus continues outwards.
8. Knock the toadstools off if you have children or pets.Brush or mow them off as they appear. Some lawn toadstools are toxic if eaten, and none of them should be sampled to identify the ring. Removing them does not affect the fungus, which lives in the soil.
9. Accept what full eradication involves.The only reliable removal is excavating the soil to below the mycelium — typically 300mm or more, extending well beyond the visible ring — disposing of it, replacing with clean topsoil and re-turfing. On a domestic lawn that is a disproportionate job for a cosmetic problem, and it fails if any infected soil is left.

Tips

• Fairy rings are commonest on sandy, low-fertility, dry soils. A well-fed lawn on decent loam shows them least.
• Rings expand outwards each year and can merge, which sometimes makes people think the pattern is spreading rather than one colony growing.
• If a ring appeared where a tree came down, that is your answer, and it will continue expanding until it exhausts the buried wood.

Common mistakes

• Applying moss killer or lawn fungicide to a fairy ring — Neither touches the soil fungus. There is no product available to a householder in the UK that eradicates a fairy ring.
• Digging out only the visible ring — The mycelium extends well beyond the coloured band. Removing the ring you can see leaves the growing edge in place.
• Assuming the dead band means the soil is poisoned — It is dry, not poisoned. The mat repels water. Aeration and slow soaking is the response, not soil replacement.

If it doesn't work

Questions people ask