Worm Casts in Lawns

Worm casts are earthworm droppings. Worms digest soil and organic matter as they tunnel, depositing processed material on the surface. These casts are nutrient-rich, containing nitrogen, phosphorus, and potassium in plant-available forms.

The characteristic coiled or lumpy appearance results from the cast being extruded through the worm’s body. Fresh casts are soft and muddy; older ones dry out and become crumbly.

Casting activity peaks in spring and autumn when soil is moist but not waterlogged. Activity decreases during summer dry spells and winter freezes when worms retreat deeper underground.

What Are Worm Casts?

Worm casts are essentially earthworm droppings created when worms digest soil and organic matter while tunnelling, depositing processed material on the surface. The characteristic coiled or lumpy appearance comes from the cast being extruded through the worm’s body. Fresh casts appear soft and muddy; older ones dry and crumble.

Peak activity occurs in spring and autumn when soil is moist but not waterlogged. During summer dry spells and winter freezes, worms retreat deeper underground and casting activity decreases significantly.

Why Worms Are Good for Your Lawn

A healthy lawn can have over a million worms per acre. Worm tunnels naturally aerate soil, creating channels for air and water to reach grass roots. Worms break down thatch and organic matter, recycling nutrients back into the soil by pulling dead grass and leaves into their tunnels.

Worm casts contain significantly higher nutrient concentrations than surrounding soil — five times more nitrogen, seven times more phosphorus, and eleven times more potassium in plant-available forms. The casts themselves function as a premium natural fertiliser.

The Problems Casts Cause

Fresh casts are soft and sticky. Mowing over them smears mud across grass and potentially clogs mowers. Flattened casts smother grass beneath them, creating small dead patches.

Heavy casting creates an uneven surface. Accumulated soil raises lawn areas, making it bumpy to walk on and difficult to mow evenly.

Worm casts provide ideal seedbeds for weeds. The nutrient-rich, finely textured soil promotes weed seed germination.

The Simple Solution: Brush Them

The most effective management involves brushing casts across the lawn when dry. This disperses nutrient-rich material into the grass where it benefits your lawn rather than sitting in lumps.

Use a stiff brush, besom broom, or springbok rake. Work across the lawn when casts have dried out, typically on a dry morning after overnight casting activity. Dry casts crumble and scatter easily, disappearing into the grass.

Brush before mowing to prevent smearing. A quick brush takes minutes and significantly improves lawn appearance. During peak casting periods in spring and autumn, you may need to brush every few days.

Timing matters: wet casts smear rather than scatter. Wait for dry conditions or brush in the afternoon when morning moisture has evaporated.

Reducing Worm Activity

If casting is severe, take steps to make conditions less favourable for surface-casting worms without harming them.

Reduce surface organic matter by removing fallen leaves promptly and scarifying to reduce thatch. Worms are attracted to areas with abundant food.

Improve drainage. Worms prefer moist soil, so addressing waterlogging can reduce their activity in problem areas.

Slightly acidifying soil discourages worms, as they prefer neutral to alkaline conditions. However, this can affect grass health and is not recommended for most lawns.

What Doesn’t Work

Chemical worm killers are no longer available to home gardeners in the UK. Killing earthworms devastates soil health and creates far bigger problems than surface casts.

Watering with household chemicals or strong solutions harms worms and damages soil biology and potentially grass. Avoid any advice suggesting you pour substances on your lawn to kill worms.

Rolling the lawn to flatten casts does not help. You are simply compacting soil, which damages grass roots without addressing underlying casting. Casts reappear as soon as worm activity continues.

Living With Worm Casts

For most gardeners, accepting worm casts as part of a healthy lawn and managing them with regular brushing works best.

Consider that heavy worm activity indicates excellent soil biology. Worms aerate, fertilise, and improve soil structure for free. Surface casts are a minor inconvenience compared to these benefits.

Some lawn enthusiasts celebrate worm activity as a soil health indicator. A lawn with lots of worms indicates you are doing something right. A lawn with no worm casts might look tidier but likely grows in depleted, compacted soil.

Seasonal Patterns

Spring brings heavy activity as soil warms and worms become active after winter dormancy, coinciding with optimal grass growing conditions. Regular brushing followed by mowing keeps things under control.

Summer casting drops significantly during dry spells. Worms retreat deeper where soil stays moist, and surface activity declines.

Autumn brings another peak as soil moisture increases and temperatures remain warm enough for activity. Fallen leaves provide extra food, attracting worms to the surface.

Winter activity is minimal. Frozen or very cold soil sends worms deep underground where they are largely dormant.

Improving Overall Lawn Health

A healthy, vigorous lawn copes better with worm casting and recovers faster from disruption.

Regular feeding promotes strong grass growth that bounces back quickly after brushing and mowing. Thick grass also hides minor surface imperfections better than thin, patchy turf.

Proper mowing height helps too. Keeping grass slightly longer provides more leaf area to photosynthesise and recover from stress.

Good general lawn care creates conditions where worm casts become a minor management task rather than a major problem.