Worm casts are small mounds of soil left on the lawn surface after earthworms burrow up from below. While earthworms play an important role in soil health, their casts can create a mess on lawns, especially during damp autumn and spring months. These muddy deposits smear underfoot and interfere with mowing, leaving an uneven finish and muddy patches across what should be a clean, tidy lawn. The challenge is to manage worm casts without harming the beneficial worm population.
What Causes Worm Casts?
Worm casts appear when earthworms surface to feed, breathe or move around—especially during periods of damp, mild weather. As they tunnel upwards, they bring fine soil with them, which is left behind on the lawn as a cast. These are more common in autumn and early spring, when the soil is moist, temperatures are mild, and worms are most active.
While worm activity improves soil aeration and drainage, too many casts on the surface can lead to unsightly lawns, poor mowing results, and even encourage moss and weed growth if smeared across the turf.
Avoid Smearing the Casts
One of the biggest problems with worm casts is not the cast itself, but what happens when it’s flattened. Smearing casts during mowing or foot traffic compacts the soil on the surface, blocking airflow and creating ideal conditions for moss and weeds.
If casts are dry, simply brush them away with a stiff broom before mowing. Never mow directly over wet casts, as this spreads soil across the grass and creates bare, muddy patches. Waiting until the lawn is dry allows the casts to crumble more easily and reduces long-term damage.
Improve Drainage and Reduce Surface Moisture
Earthworms favour damp soil. By improving drainage and reducing surface water, you can make your lawn less inviting for frequent worm surfacing. Aerating the lawn, particularly in compacted or waterlogged areas, helps move excess moisture away from the surface and encourages deeper worm activity rather than repeated surfacing.
Keeping the grass longer in autumn can also reduce surface wetness, while topdressing with sand or a sand-soil mix improves structure and helps the lawn dry out more quickly after rain.
Avoid Overfeeding with Organic Matter
Lawn feeds and organic fertilisers can boost worm activity if overused. Worms are attracted to high levels of organic material in the soil, including lawn clippings, compost and manure-based products. If worm casts are becoming a serious issue, consider switching to a low-organic or mineral-based fertiliser, especially in autumn when worms are most active.
You can also reduce organic build-up by collecting clippings instead of mulching during mowing.
Use Acidifying Products Where Appropriate
Worms prefer neutral or slightly alkaline soil. Applying a mild lawn acidifier, such as sulphate of iron or ammonium-based lawn treatments, can make the surface environment less appealing to worms without harming the lawn itself. These products should be used carefully and only in moderation, as they can alter soil pH over time. Always follow product instructions and avoid over-application.
It’s important to note that while these treatments may reduce surface casting, they should not be used to eliminate worms entirely.
Embrace a Balanced Approach
While worm casts are frustrating, it’s worth remembering that earthworms are vital for healthy soil. They improve drainage, increase nutrient availability, and support a stronger, more resilient lawn in the long run. The goal should be to manage casting activity, not eliminate the worms altogether.
Encourage deep tunnelling by maintaining a healthy lawn with good soil structure, and deal with casts promptly and gently to avoid smearing or long-term damage.
Final Word
Worm casts are a natural part of lawn life, especially during wet months, but with the right care they don’t have to ruin your lawn’s appearance. By improving drainage, brushing casts away when dry, and avoiding overuse of organic products, you can reduce surface activity without harming the worm population that keeps your soil healthy. It’s all about balance, protecting the lawn while respecting the life beneath it.
