On your lawn, you may see tiny mounds of soil appearing on the surface. They crop up in random places all over the lawn. They most often appear in the autumn or winter, but can crop up at any time of the year. These are worm casts.
Worm casts are created by earthworms. These worms consume and digest organic matter in the soil. They then expel it from their gut onto the soil surface. This creates the coil of smooth soil which we call worm casts.
In the UK, worm casts are created by three different species of worms. The lob or common worm (lumbricus terrestris), The grey worm (aporrectodea calignosa) and the black-headed worm (aporrectodea longa). The lob worm is the most common worm cast creator. It only comes out at night or during periods of heavy rain.
On your lawn at home a worm cast can be squashed underfoot making a muddy patch on the grass. The mud can be smeared on the patio or stepped on and trodden indoors, causing a mess. A squashed worm casts is also a perfect germination patch for a weed seed.
Worm casts can be a nuisance in the lawn care industry. Especially on fine turf lawns such as golf courses and bowling greens. These rely on a smooth surface to roll a ball on. A golf green dotted with worm casts can become unplayable. Worms soften the surface and their casts blemish the green.
It is frustrating that the high nutrient content and good soil required for fine turf is a haven for earthworms. The better your lawn, the more difficult the problem of worm casts can become. Green keepers have battled with earthworms for decades.
Should you try to control worm casts on your lawn?
On a lawn at home, worm casts are less of an issue. The longer grass and less reliance on a flat surface make worm casts less problematic. In most domestic lawn care situations, I discourage trying to control worms. Worms bring too many benefits to soil and the general ecology of your garden. Trying to control them may only bring further problems.
If it does bother you though, what can you do about it?
Natural ways to control worms casts
- Leave your lawn longer. A longer grass brings a healthier lawn. Blemishes in the soil surface tend to not be a problem if the grass leaves are longer. Especially in the winter. Keep your mower blades high (at least 30mm) and the worm casts will be less of a problem.
- Reduce the worm food source. Keep the leaves off your lawn in the winter. Also make sure you remove the grass clippings when mowing. Reducing the amount of organic matter on your lawn will reduce the amount of worm activity in the soil.
- Improve the drainage. Worms tend to put up more casts and seem to be more active in wetter soils. Improve the drainage in your lawn and you will reduce your worm problems.
- Brush casts away when they appear. If you see worm casts appearing, use a stiff broom to brush them away. This will distribute the nutritious worm casts soil and remove the problem. This is best done before mowing and when the casts are dry.
Chemical control of worm casts
- Carbendazim is a worm control used by professionals. It suppresses worms by keeping them lower down in the soil. In my experience it can also kill worms, which is why I no longer offer the treatment. Carbendazim is not available to the general public. Some lawn care companies will apply worm controls so get in touch with your local operator. There are rumours that it is in danger of being added to the banned chemicals list. So it may be withdrawn at any time. Edit: It was confirmed as withdrawn in 2016.
- There are some worm control products which are available for home use. These are usually based on sulphur. Sulphur lowers the pH of the soil, making it more acidic. This tends to discourage worms. Attempting to adjusting your soil pH can bring mixed results. At worst it can cause other turf problems. It is best to conduct a soil test and try in small amounts if you wish to try this route.
Worm cast control in summary
Unless you are aiming for a smooth surface, trying to control worm casts in your lawn is unnecessary. Try to reduce the number of casting worms naturally and maintain your lawn correctly. You will find that you will learn to love the humble earthworm. They are your friends in your garden.