Worm Casts in Your Lawn: Why They're There, and What (Not) to Do About Them

You're about to get the mower out after a mild, wet night, and the lawn is covered in small heaps of dark, sticky mud, dozens of them. Nothing's eaten the grass and nothing's dug it up. Push a finger into one and it's just fine, worked soil. Those are worm casts: a sign of healthy soil rather than a lawn problem. The main nuisance is smearing them with the mower, and you avoid that by brushing them off once they've dried.

What worm casts are

A worm cast is soil that has been through an earthworm. The deep-burrowing species mainly responsible, Aporrectodea and Lumbricus, live in permanent vertical burrows reaching well below the surface and feed by drawing leaves and other plant debris down into them. What comes out at the burrow entrance is fine, muddy soil. It isn't pest damage, disease or anything eating the lawn from below.

It's easy to mistake for a molehill at a glance, especially a scatter of small ones, but the two look different close up. A worm cast is a loose, fine-textured heap a few centimetres across, sitting flush on the grass with no hole visible underneath. A molehill is bigger, coarser, and sits over a real excavated tunnel, often with a clear entrance hole at the centre once the heap is brushed aside. For moles, see moles in your lawn.

Why casts are worth tolerating

Casts aren't a hazard. They're just soil, so there's no reason to keep children or pets off a cast-covered lawn beyond muddy paws and shoes.

Earthworms are good for the soil under your lawn. Their burrows let water and air move through ground that would otherwise compact and waterlog, and they pull decomposing plant material down into the soil. Garden Organic reports that casts contain around 5 times more nitrogen, 7 times more phosphorus and 1,000 times more beneficial bacteria than the surrounding soil.

That's why the advice is tolerance rather than elimination. There's no worm killer to reach for: the products sold to home gardeners are sulphur-based deterrents that work by lowering soil pH. They don't kill worms, results are mixed because they depend on how far the soil's pH shifts, and they are likely to need repeating.

Earthworms feed on decomposing plant material, so a lawn with a healthy worm population is unlikely to be building thatch. Casts don't mean the lawn has thatch or disease either. Thatch is a springy, fibrous brown layer of dead grass and roots at soil level, and lawn diseases usually show as discoloured rings or patches in the grass itself. A small, loose heap of dark soil sitting on top of the grass is something different.

The real nuisance

None of that makes casts pleasant. The practical problem is what happens when they get walked on or run over before they've dried: fresh casts are wet and easily flattened, and mower wheels or feet smear them into a muddy film rather than leaving neat heaps. On a lawn cut fine and short, that smearing leaves patches that look and feel poor, and flattened, disturbed ground is the kind of gap moss and weeds are quick to move into. Casts may also bring weed seeds from the soil up to the surface where they can germinateA seed sprouting: breaking dormancy and putting out its first root and shoot, given the right warmth and moisture. Full definition →.

Managing worm casts

• Wait until the casts have dried, then brush or lightly rake them off the surface. Wet casts smear; dry ones break up and scatter cleanly with a stiff brush or the back of a rake. The RHS suggests a wire rake with the teeth facing upwards, moved from side to side over the lawn, to make casts less noticeable.
• Do this before mowing, not after. Mowing over undispersed casts is what causes the smearing and the poor cut in the first place, and it leaves a sour, earthy smell on the underside of the deck.
• Raise the mower height slightly during the main casting months if casts are a recurring irritation. A higher cut is less likely to scalp into flattened, smeared patches than a close cut.
• Time it around dry spells where you can. Casts that have had a day or two of drier weather to firm up come apart with a light brush; casts from an overnight downpour often need another day.
• A robotic mower makes this harder to manage, since it cuts more often and shorter than a weekly pass with a push mower, giving fresh casts less time to dry before they're driven over. Brushing the lawn by hand before its scheduled cut avoids the smearing.
• Leave the rest alone. Once brushed off, the job is done, with no follow-up chemical step.

When casts show up

Casting tends to peak in autumn and early spring, and in a mild winter it can continue right through. That's when the soil is most reliably damp and mild. Earthworms are usually dormant in cold or hot, dry weather, so casting drops off in a hard freeze or a dry summer spell and picks up again when conditions ease.

Why Scottish lawns see them

The west of Scotland has noticeably more of the damp-soil conditions that keep earthworms active than the east. Met Office 30-year averages (1991-2020) for two stations either side of the central belt show the gap in how many days of the year see rain, not just how much falls:

That's roughly six weeks' worth of extra wet days a year in the west, on two stations either side of the central belt, with similar numbers of frost days (45.57 and 48.01). Nobody has counted worm casts on Scottish lawns, so this is the likely explanation rather than a measured result, but it would fit a lawn in the wetter west casting for more of the year than one in the east. If that's yours, keep the mower height a little higher for longer.

Acidic soils turn up in pockets all over the UK but are more generally found in south-western England, Wales and Scotland, because of the underlying bedrock. Earthworms are scarce in extremely acidic soil, so a lawn full of casts suggests the ground isn't at the acid extreme, even where acid soil is common. A cheap soil pH testing kit used on a sample from your own lawn tells you where you stand.

Common questions

Dealing with a different lawn problem? See moss in your lawn, mushrooms and fairy rings, red thread, chafer grubs, drought or the dog, iron sulphate overuse, and moles.

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