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Earthworms in the Lawn - Pests with Benefits

Love them or hate them, earthworms are an essential component of good soil.


Earthworms in the Lawn - Pests with Benefits
© flickr user kdnewton

The Benefits of Earthworms

The presence of earthworms in the lawn is an indicator of healthy soil, it is when they make their presence known that they become a problem. Most of the time worms are unseen, conditioning the soil and living their lives deep in the root zone. During the spring and fall, or other periods of extreme moisture, earthworms tend to migrate to the surface and leave their castings on the lawn. Worm castings are small mounds or bumps on the lawn which are basically worm excrement. The excretions are nutrient packed remains of digested plant matter and soil which is extremely beneficial to the lawn. Earthworms help in breaking down thatch, increasing decomposition and creating usable nitrogen in the soil. In fact, five or more earthworms per square foot of soil provides the lawn with 25% of its seasonal nitrogen requirements. The worms' movement through the soil profile acts as natural aeration, helping air and water move from the surface throughout the root zone.

So What's the Problem?

Excessive earthworm castings on lawns is a common problem in many parts of North America. The main grievance with worm castings is their unsightliness. The small mound of nutrient rich excrement is basically a dead spot about the size of a quarter on the lawn. Multiplied by the potential number of worms and it can seriously effect the aesthetics of a lawn. A weak, thin lawn, mowed low will appear even worse with the presence of numerous worm castings. Over time, an abundance of castings on a lawn creates a bumpy surface causing scalping when mowed and can potentially be a tripping hazard. An excessive population of earthworms may also attract secondary pests like moles, which feed primarily on worms.

Dealing with Castings

Currently, there are no pesticides labeled for earthworm control although some insecticides and fungicides are toxic to them. Using pesticides "off label" is illegal, dangerous and not recommended. Besides, the benefit worms provide the soil outweigh dealing with the castings.

    Cultural Control of Earthworm Castings on a Lawn
  • Rake or sweep castings into the lawn when they are dry. Power-rake the lawn in the fall to prevent uneven turf from developing.
  • Don't overwater. Earthworms will stay near the surface if it's continuously moist.
  • Mow high and keep the lawn healthy to hide the casts and minimize the unsightliness.
  • A roller can be used to press down the casts but over time, bumpiness will still occur and it can cause compaction of the soil.

Lawns made uneven and bumpy due to years of casting build-up may need restoration or renovation depending on the severity. Topdressing and over-seeding will take care of moderately bumpy turf. In more severe cases aerating and rolling may be necessary to achieve smoothness.

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