What are the little heaps of soil on my lawn?
Worm casts appear in numbers in autumn, smear under a mower and look like a problem. Why they are a sign of a healthy lawn, how to tell them from molehills and ant hills, and the only management that is worth doing.
- Difficulty
- beginner
- Time
- 12 min
- Read
- 4 min
Short answer
Small squiggly heaps of fine soil, a few centimetres across, appearing in mild damp weather are worm casts and they are a sign of good soil biology rather than a pest problem. Let them dry, then brush or switch them across the lawn with a besom or a stiff brush before mowing — smearing them wet is what creates the muddy patches where weeds then germinate.
Worms pull leaf material down into the soil, digest it, and deposit the mineral residue on the surface. That process is the main mechanism by which a lawn's soil is aerated, drained and fed, and a lawn with worms is in far better shape than one without. What makes casts a nuisance is purely cosmetic and mechanical: they are wet, fine soil, so a mower or a boot smears them into a patch of bare mud that then grows moss and weeds. Autumn and spring are their peak, because worms are most active in mild, moist conditions and dormant in cold or dry ones.
Step by step
- Confirm what you are actually looking at.Worm casts are small — usually two to five centimetres across — made of fine, dark, squiggly extruded soil, and scattered across the lawn. Molehills are much larger heaps of loose soil, often in a line, with a tunnel beneath. Ant hills are fine dry soil in a low dome, usually in hot dry weather and in the same spot each year.
- Wait for them to dry.This is the whole technique. A dry cast crumbles to dust and brushes away to nothing. A wet one smears into a patch that kills the grass beneath it. A dry morning after a dry night is the moment.
- Brush them across the lawn before mowing.A stiff broom, a besom, or a drag mat scatters the fine soil evenly across the surface where it does nothing but good. Doing this immediately before mowing is a habit worth forming in autumn.
- Raise the mower height a little in casting season.A higher cut passes over most casts rather than through them, and the grass shades the surface so any smearing recovers faster.
- Consider whether the conditions are making it worse.Worm activity is highest where there is plenty of organic matter at the surface and the soil is neutral to alkaline. A lawn that is heavily topdressed, or one where clippings and leaves are always left, will have more casts. That is a trade-off, not a fault.
- Accept that there is no lawful chemical control.There is no worm killer approved for use on lawns in the UK, and this is deliberate — earthworms are protected as beneficial soil organisms and the products that used to be used were withdrawn. Anything sold as a worm treatment works by discouraging activity, not by killing.
- Deal with the smeared patches rather than the worms.If casts have already been smeared, let them dry and rub them out, then overseed the bare patches. That is a lawn repair job, and the catalogue covers patchy lawns and overseeding.
Common mistakes
- Trying to get rid of the worms — They are doing the work that keeps a lawn draining and feeding itself. A lawn without worms compacts, drains badly and accumulates thatch — which are all much harder problems than a few heaps of soil.
- Brushing casts when they are wet — It converts a heap that would have crumbled into a smear that kills the grass. Timing is the entire skill here.
If it doesn't work
Mower leaves muddy streaks across the lawn
Cause: Mowing over wet casts — Fix: Brush first, mow later, and only when dry. This single change removes almost all of the visible problem.
Bare patches appearing where casts were
Cause: Smeared soil smothering the grass, then weeds and moss colonising — Fix: Rake out the dead material, and overseed in autumn or spring. Casting itself did not cause this; mowing over wet casts did.
Larger heaps in a line across the lawn
Cause: Moles, not worms — Fix: Entirely different problem with entirely different answers — the catalogue covers getting rid of moles. Ironically, a lawn with plenty of worms is attractive to moles for exactly that reason.
Fine dry soil domes in hot weather
Cause: Ants — Fix: Also different. Ant hills appear in dry summer conditions rather than mild damp autumn ones, and the catalogue covers ants.
Casts on a lawn that also feels spongy underfoot
Cause: Thatch as well as casts — Fix: Two separate things. The catalogue covers a spongy thatchy lawn and scarifying, and dealing with thatch in autumn also reduces the surface organic matter that drives casting.
Questions people ask
Do worm casts damage the lawn?
Not directly. The damage is done by smearing them, which smothers grass and creates a seedbed for weeds and moss. Managed dry, they are a free topdressing of fine soil.
Why so many in autumn?
Worms are most active in mild, moist soil, and there is abundant leaf material at the surface for them to pull down. Cold or dry conditions send them deeper and casting stops, which is why the problem disappears in a hard frost or a summer drought.
Can I reduce them without harming the worms?
Somewhat. Removing leaves and clippings promptly gives them less surface material to work with, and avoiding heavy organic topdressing reduces activity. Lowering soil pH is sometimes suggested and it is a blunt instrument that affects the grass too, so it is rarely worth it on an ordinary lawn.