Earthworms Aren't Always the Good Guys

By Therese Ciesinski, GardenSMART

Earthworms. We know they're important in our gardens, but often only notice them in their absence. Lots of earthworms = good soil. No earthworms = bad soil. We've been taught that they turn organic matter into castings that enrich the soil and feed our plants. They help move water and air through the soil and reduce compaction.

And all of this is true: in gardens. In certain areas of the country, however, earthworms are not helping the environment, but harming it.

There are concerns about what earthworms are doing to northern forests in the U.S. Research shows earthworms changing the pH and structure of the soil, eating and de-nutrifying organic matter instead of adding it, and possibly releasing carbon into the atmosphere as they do so.

Draw a line from central New Jersey, through Pennsylvania and Ohio out to the west. The forests north of that line have not had earthworms for 15,000 years, since the glaciers retreated. The forests evolved to survive without them. (In the southeastern U.S., there are more than a dozen species of native earthworms.)

The terrestrial earthworms that are in northern forests now are not native species. They were introduced from Asia and Europe over hundreds of years, via ballast in ships and in soil around the plants newcomers brought from home. Earthworms' popularity as fish bait also helped disperse them throughout the country.

Earthworm feeding destroys duff, the spongy layer of organic material on the forest floor that supports native plants, animals, and insects, and keeps soils from eroding. They bury the organic matter deeper into the soil where some plants can't access the nutrients – spring wildflowers in particular – and alter the habitat and food supply of other forest organisms, including native salamanders.

Forest floor in Eastern Canada before earthworms. Photo courtesy of Robert Lee, Bugwood.org

Forest floor after earthworms, with tree roots exposed. Photo courtesy of Robert Lee, Bugwood.org

Where some northern forests once had a rich carpet of fallen leaves and organic matter supporting layers of native vegetation that included flowers, shrubs and young trees, earthworms have left the forest floor almost bare.

And paradoxically, earthworms can even compact the soil. The Minnesota Department of Natural Resources explains:

"Earthworm-free hardwood forests in Minnesota have a naturally loose soil with a thick duff layer. Most of our native hardwood forest tree seedlings, wildflowers, and ferns grow best in these conditions. However, when earthworms invade they actually increase the compaction of hardwood forest soils. Compaction decreases water infiltration. Less infiltration combined with the removal of the duff and fallen tree leaves results in increased surface runoff and erosion."

It's hard to believe a creature gardeners have been taught to nurture can do such damage. So what can we do?

Eliminating non-native earthworms from these northern forests isn't possible, ecologists say, but gardeners and fishermen can keep the problem from becoming worse by not helping introduce new worm species in an area. Gardeners can avoid adding purchased worms to compost piles and freezing any purchased worm castings for a week to kill worm eggs before adding to the pile. Fishermen can help by dumping unused fishing bait in the water, rather than on land.

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