Worm Casts in Lawns

Those small muddy piles appearing across your lawn are worm casts, and while they’re annoying, they’re actually a sign of healthy soil. Earthworms are doing important work underground, but their surface deposits can make lawns look messy and create problems when mowing. Here’s how to manage them without harming these beneficial creatures.

What Are Worm Casts?

Worm casts are essentially earthworm droppings. Worms digest soil and organic matter as they tunnel, and deposit the processed material on the surface. These casts are incredibly nutrient-rich, containing nitrogen, phosphorus, and potassium in forms that plants can easily absorb.

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

Casting activity peaks in spring and autumn when soil conditions suit worm movement, specifically when soil is moist but not waterlogged. You’ll see fewer casts during summer dry spells and winter freezes when worms retreat deeper underground.

Why Worms Are Good for Your Lawn

Before discussing how to manage casts, it’s worth understanding why earthworms are valuable. A healthy lawn can have over a million worms per acre, and they provide enormous benefits.

Worm tunnels aerate the soil naturally, creating channels for air and water to reach grass roots. This is essentially free aeration happening constantly beneath your feet.

Worms break down thatch and organic matter, recycling nutrients back into the soil. They pull dead grass and fallen leaves down into their tunnels, processing them into plant-available nutrients.

The casts themselves are a premium natural fertiliser. Studies show worm casts contain five times more nitrogen, seven times more phosphorus, and eleven times more potassium than surrounding soil. You’re essentially getting free topdressing.

The Problems Casts Cause

Despite their benefits, worm casts do create practical problems that many gardeners want to address.

Fresh casts are soft and sticky. Mowing over them smears mud across the grass, creating an uneven appearance and potentially clogging your mower. The flattened casts also smother grass beneath them, creating small dead patches.

Heavy casting creates an uneven surface over time. The accumulated soil raises areas of the lawn, making it bumpy to walk on and difficult to mow evenly.

Worm casts provide perfect seedbeds for weeds. The nutrient-rich, finely textured soil is ideal for weed seed germination. Casts left in place often sprout weeds before they break down.

The Simple Solution: Brush Them

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

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

Do this before mowing to prevent smearing. A quick brush takes just a few minutes and makes a significant difference to 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 a dry spell or brush in the afternoon when morning moisture has evaporated.

Reducing Worm Activity

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

Reduce surface organic matter. Worms are attracted to areas with plenty of food, so removing fallen leaves promptly and scarifying to reduce thatch gives them less to eat at the surface.

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

Acidifying the soil slightly discourages worms, as they prefer neutral to alkaline conditions. However, this can also affect grass health and isn’t recommended for most lawns.

What Doesn’t Work

Various supposed remedies circulate online, but most are ineffective or harmful.

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

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

Rolling the lawn to flatten casts doesn’t help. You’re just compacting soil, which damages grass roots and doesn’t address the underlying casting. The casts will reappear as soon as worm activity continues.

Living With Worm Casts

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

Consider that a lawn with heavy worm activity is a lawn with excellent soil biology. The worms are aerating, fertilising, and improving your soil structure for free. The surface casts are a minor inconvenience compared to these benefits.

Some lawn enthusiasts even celebrate worm activity as an indicator of soil health. If you have lots of worms, you’re doing something right. A lawn with no worm casts might look tidier but is likely growing in depleted, compacted soil.

Seasonal Patterns

Understanding when casting peaks helps you plan management.

Spring sees heavy activity as soil warms and worms become active after winter dormancy. This coincides with optimal grass growing conditions, so 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’re largely dormant.

Improving Overall Lawn Health

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

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