Worms and wormcasts on lawns & turf

Worm casts on a lawn
Worm casts are a common problem on may lawns throughout the autumn and winter months when the soil is moist.

Worms and worm casts are one of the most common problems during autumn and winter period when the soil is wet.

Worms are actually beneficial to the soil as they help aerate the root zone, they break down the organic thatch layer and they improve the drainage. In fact if you have a large worm population in your lawn it is the sign of a healthy soil.

Despite of these benefits that worms have, the disadvantages of worms out weight the advantages of worms in lawns. It is the casts that cause the most problems as they create muddy conditions and they also provide an ideal seed bed for weeds. They also encourage moles as worms are the main food source of moles.

Conditions in the lawn that favour worms

Earthworms favour the following conditions in lawns.

  • Wet and warm conditions - As we are experiencing more rainfall in the UK, earthworms are becoming more of a problem on lawns. In fact the whole climate has become wetter and warmer, causing problems with casting worms. We have also been experiencing fewer hard frosts which drive the worms deeper underground thus reducing any worm cast problems.
  • Favourable soil types - Worms favour heavier clay and loam soils which have a high thatch and organic matter content for the worms to feed on. However they can still be present in lighter free draining sand based soils. However they are less of a problem in lighter soils as the worm-casts are easily dispersed without smearing the surface and creating a muddy surface.
  • Ideal pH - The majority of earthworms prefer a neutral to alkaline (high pH) soil however some types of worms can survive in acidic soils. The pH is a scale used to measure how acidic or alkaline a soil is.
  • High organic matter content - Worms will always favour a soil with a high organic matter content as this is their food source.

Management of worms in lawns

Many years ago chemicals such as chlordane were used very successfully to control worms but were recently banned. The only chemical for use on worms at present is a product called Carbendazim, this product is actually a fungicide with worm control properties.

Carbendazim is widely used in the fine turf industry on sports grounds and golf courses but the product can't be purchased for home use. However there are various cultural methods you can employee to try and discourage worm activity in your lawn.

  • Lower the pH level - By gradually lowering the soil pH you will help discourage worms as they prefer a higher pH. You can do this by applying acidifying products and fertilisers such as sulphate of iron and lawn sand. A light monthly application of sulphate of iron at a rate of 8g/m2 during the autumn and winter months may reduce worm numbers. This product can be purchased from garden centers and it is quite cheap. It is mixed in hot water and applied with a sprayer or watering can. Ensure it is mixed thoroughly as it can block spray nozzles or you can put it through a filter before filling your sprayer. Be careful when using this product as it can stain clothes and concrete pathways. Please visit our Sulphate of Iron information page for advice on this product.
  • Reduce the thatch or organic matter content - If thatch build up (thatch = organic matter) is a problem in you lawn reducing it would help lower the worm population. Scarifying (ideally during the autumn) will remove excess thatch from the lawn. The removal of leaves from the lawn during late summer and by boxing of grass clipping when the lawn is mown will all help prevent thatch accumulation.
  • Manual removal of worm-casts - Tools and implements such as brushes, switches and drag mats are available for the removal of worm-casts. On heavy clay soil make sure that the worm-casts have dried sufficiently before removing or they will smear the grass.
  • Raising the height of cut on the mower - During periods of worm activity it would be beneficial to raise the height of cut on the mower. This leaves the grass longer and helps reduce the smearing effect.

Unfortunately there are no real effective methods left to totally remove earthworms and worm-casts on lawns. The most effective chemicals have now been banned for sometime now. We are now left with the cultural methods to try and reduce earthworm numbers, and these may not prove very effective if you have a severe problem.