Worm casts in lawns

What are worm casts?

Worm casts appear on the surface of lawns as wet strings of sludgy mud. These are the waste product (castings) of earthworms and are a sign of a good, healthy soil. Worm casts are most often seen on lawns in autumn and early spring. They can be left in place or gently brushed off when dry, and should be tolerated as part a healthy garden ecosystem.

How are worm castings made?

Earthworms live in permanent vertical tunnels in the soil and draw plant detritus, such as leaf litter, into their tunnels and feed on them. In eating plant detritus they not only help break down spent plant material but 'recycle' their nutrients, making them available for living plants to feed from. Many species of earthworm deposit their castings underground but some, including those in the Aporrectodea and Lumbricus genera, deposit their castings on the surface of the soil.

Earthworms are therefore an essential part of the garden ecosystem, ensuring your soil is well structured, aerated, oxygenated and nutrient-rich, so that ornamental and vegetable plants can grow healthily. What's more, earthworms are an essential part of the food chain, being a vital source of food for birds and their chicks, hedgehogs and amphibians.

Can you make your own worm castings?

Because the activities of earthworms are recognised as beneficial to gardeners, you can buy or make a bespoke wormery, which mimics the natural process of making worm castings. Most wormeries consist of a tiered system of three or four boxes into which you add newspaper and food waste, along with a colony of brandling worms, which are better suited to the closed environment of a wormery than most earthworms. Brandling, or compost worms, are often found in compost bins anyway, so creating a wormery to house them in is an effective way of making the most of their castings. In most tiered wormery systems you add waste to the bottom box until it's full and then start filling the next box up. The compost worms eat the food waste in the bottom bin and then travel up into the box above it, leaving you to remove the bottom box and harvest their castings.

How to remove worm castings from a lawn

Earthworms are part of a healthy garden ecosystem and should be tolerated. You can rake or brush worm casts off the lawn when dry, but we don't advise taking further action to remove worms from your soil as this may damage the ecology of your garden. In autumn and early spring, when worm casts are most prevalent, the casts are usually too wet to brush off, and so should be left in place. We recommend avoiding walking on the lawn during this time as it can lead to the smearing of the worm casts on the surface, leaving space for weeds to germinate in spring.

Earthworms are much less frequent in acidic soils, and there are products available which may reduce the pH of the soil. These products don't kill the worms but act as a deterrent, and need to be applied regularly. However we don't recommend them as earthworms are so beneficial to soil, gardens and the wider ecosystem as a whole. What's more, repeated applications of these products may adversely affect grass growth.