Earthworms and their impact on your lawn

In this article:

• Pests
• Are earthworms a problem?
• Top tips for controlling earthworm activity

Pests

There are a wide range of pests that can affect natural grass areas such as lawns to varying degrees, including rabbits, moles, leatherjackets (crane fly larvae), fly larvae and chafer grubs (chafer beetle larvae). One key pest for quality lawns in cooler, wetter regions is the earthworm although the extent to which they are considered a nuisance will depend on the sward composition, cutting height and aesthetics of an area.

Are earthworms a problem?

Earthworms live within the soil environment; there are a range of species and each exhibits unique behavioral and physiological characteristics. Their distribution is influenced by climate, soil pH, soil texture, organic matter, mode of feeding, lifestyle, and their location within the soil profile.

The most important role of the earthworm is in the decomposition process within the soil. Earthworms feed on dead and decaying plant and animal material, which in turn serves to control organic matter while also increasing nutrient availability and soil fertility. Other benefits include soil aeration leading to a modification (improvement) of soil structure, increased infiltration rates, and improved rooting for plants.

A small number of earthworms produce casts made up of indigestible waste material and it is these casts that are considered undesirable by turf managers. For general amenity areas comprised of vigorous growing perennial ryegrass species mown above 25 mm, such casts should not be problematic unless the area is being used as a sports pitch. Within a close-mown, high-quality ornamental lawn, however, these casts are considered unsightly. They will also serve to smother grass plants and encourage weeds, thereby further affecting the aesthetics of an area. In such a situation the disadvantages of earthworm activity may be considered to outweigh the advantages.

Top tips for controlling earthworm activity

Good management practices can help to discourage earthworm activity.

Organic matter is their main source of food. Controlling the level of organic material within the soil environment will reduce their food source and make an area less attractive. Therefore, box-off clippings where possible, limit the use of organic fertilizers, encourage the breakdown of organic material with regular aeration, and remove organic matter with a combined program of scarification and hollow coring.

Worms do not like acid conditions and much prefer neutral and alkaline soils. This is most useful in ornamental lawns dominated by fine bent and fescue species, which favor acid conditions. In such circumstances try to use acidic-reacting materials as opposed to alkaline ones. For example, ammonium sulfate-based fertilizers and iron sulfate both provide an acidifying effect when applied and should be included within a fertilizer program where worms are a problem.

Take care if using lime products to raise pH as too much will increase worm activity. As well as being a soil conditioner, lime can be present in various disguises. Care should be taken when selecting sands, especially for top-dressing—ensure that the material used contains no more than 0.5 % lime. Similarly, do not use limestone to backfill drain trenches. Limestone is easily broken down in acid soils, significantly reducing the effectiveness of the drain but also attracting earthworms which will tend to cast into the stone backfill, contaminating it with soil.

Applications of a medium-coarse sand top dressing can also serve to reduce worm activity over time as it tends to irritate the worms’ skin.

Worm casts can be removed from lawn areas using a rake or brush, but this should only be done in dry conditions. Trying to remove casts when they remain wet will result in smearing the surface, which may reduce surface infiltration rates and smother underlying grass.

In some territories carbendazim-based chemical control products are available to reduce worm casting. However, they are only available to professional users with appropriate spraying qualifications.