Earthworms in the Lawn: Pests With Benefits

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

Hand and earthworm
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The term earthworm is a common name that refers to any of the legless, tube-shaped, segmented worms that live in soil. They include many species, all falling into the animal phylum Annelida. Depending on species, they can range from tiny 1/2-inch worms to rather large nightcrawlers (Lumbricus terrestris) that can grow as long as 14 inches. The most common species found in North American landscapes are reddish or brown creatures that you are likely to see whenever you turn over a shovelful of lawn or garden soil.

 

Earthworms in Residential Lawns and Gardens

The presence of earthworms in the lawn is an indicator of healthy soil containing a high level of organic material, and in most cases, there is no reason to worry about worms in your soil. It is of much more concern if your soil has no worms in it, as this indicates a fairly sterile soil lacking in organic material. 

Most of the time worms, are entirely unseen, conditioning the soil and living their lives deep in the root zone. But 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--but highly nutritious excrement.  These excretions are nutrient-packed remains of digested plant matter and soil that have passed through the worms--a material that is extremely beneficial to the lawn.

Earthworms also 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 provide the lawn with 25% of its seasonal nitrogen requirements. Finally, the worms' movement through the soil profile acts as natural aeration, helping air and water move from the surface throughout the root zone.

On almost every level, earthworms and their castings are very good for a garden and lawn. However, homeowners intent on billiard-table smooth turf lawns sometimes agonize over the signs that earthworms leave on their lawns. 

So What's the Problem?

Excessive earthworm castings on lawns can be a problem in many parts of North America for people from whom a less-than-perfect turf lawn is offensive. The main grievance with worm castings is their unsightliness. The small mound of nutrient-rich excrement forms a dark spot about the size of a quarter on the lawn. Multiplied by the potential number of worms and it can seriously affect the aesthetics of a lawn. A weak, thin lawn that is mowed low will appear even worse when there are numerous worm castings present.  Over time, an abundance of castings on a lawn creates a bumpy surface, causing scalping when mowed, They can even potentially cause a tripping hazard. Finally,  an excessive population of earthworms may also attract secondary pests such as moles, which feed primarily on worms, grubs, and other below-ground creatures. 

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" and contrary to label instructions is illegal, dangerous and not recommended. Besides, the benefit worms provide the soil outweigh the relatively small inconvenience of dealing with their castings.

There are, however, some practical and earth-friendly ways to deal with worm castings on a lawn: 

  • Rake or sweep castings across the lawn when they are dry. This has the effect of spreading a natural fertilizer over the turf and breaking apart the unsightly lumps.  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, but they will go down deeper in search of moisture if the top few inches of soil are dryer. 
  • Mow high and keep the lawn healthy to hide the castings and minimize the unsightliness.
  • A roller can be used to press down the castings;  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.

Did You Know?

Of the roughly 180 common species of earthworms in North America, about 60 of them are non-native species that are regarded as invasive to some ecosystems. In natural forest areas, for example, earthworms consume the organic-rich leaf material blanketing forest floors that provide essential nutrients to many trees and plants. So while the presence of earthworms is a good sign in an urban or suburban lawn and garden, they are not necessarily a good sign in wilderness forests. Never release worms used for fishing bait in forested regions, and examine soil carefully if you are transplanting garden plants into the landscape around a cabin or vacation home in the woods. This is one area where you want to avoid earthworms.