For Release April 13, 1999

Earthworms, Friend or Foe?

by Chuck Otte, Geary County Extension Agent

It’s been a real common question the past couple of weeks. "What are these little piles of dirt in my yard, why are they killing my grass and what can I do about it?" The answer to "what" is easy, earthworms and night crawlers. From there, unfortunately, the situation gets real muddled.

Earthworms are a natural and vital component of most healthy soils. They burrow through the ground literally eating dirt and digesting the organic matter contained there in. As they burrow they excrete this processed soil in what we call castings. These castings are often carried to the soil surface, especially in cooler weather, wetter weather and early in the season. These castings can build up to be quite a mound, several inches tall and wide. If you examine these piles closely and break them apart you’ll find that they are composed of many small pellets of soil. If you carefully scrape them away you’ll usually find a hole leading into the soil. Interestingly enough, most of these larger mounds are from the large warms we’d call night crawlers and night crawlers are not native to most North American soils.

This worm action is a very important process for soils. The worms break down organic matter, thereby releasing many nutrients into the soil that plants can use. The worms create a rather extensive network of macro-pores that improve water infiltration during rainfall events. The digging and burrowing action also helps to break up compaction of soil and literally moves soil around. The worms are very beneficial.

The mound of castings is most visible in areas of bare soil such as in flower beds and gardens or areas of the lawn where the grass is thin and clumpy. Since the castings are seen in the thinner areas of lawns, homeowners often mistakenly assume that the worms are killing the grass. This is not the case. Thin lawn turf is caused by many other management factors. In many cases the thin lawn is underneath a spreading tree. Because of the shade from the tree there simply is not enough light for the grass to grow thick and luxurious. Because of the shade the ground is often a little moister and a little cooler than surrounding lawn areas, therefore it is also more attractive to the worms. There is a cause and effect at work here, just not the one most homeowners assume.

I have seldom seen a spring when we do not have a lot of earthworm activity, but this may be one of the best worm springs in quite some time. In lawns with very good earthworm populations the castings can be so numerous and so large that it actually becomes difficult, if not dangerous, to try to walk across the yard. It can also make for a very bumpy ride with a riding lawnmower. I can certainly understand why a homeowner would want to take steps to reduce or eliminate the problem.

Before a pesticide can be legally recommended for use against a "pest" it must be registered as such on the label and by the Environmental Protection Agency (EPA). Since earthworms are inherently good creatures, the EPA has been reluctant to label any product for worm control. Therefore, while there are pesticides that will kill worms, I can not legally recommend any pesticide treatment. You can rake the piles down or use a power rake to do this but they will return over time. Excessive lawn watering will make the problem worse, and an extended drought will send the worms deeper. As the spring wears on the problem should diminish, but unfortunately, in the meantime, there’s not much that can be done.


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