We hardly notice them most of the time… they’re out of sight, underground, aerating the soil, creating humus, increasing fertility. It’s only after a rain storm, when the ground is saturated, that they come up for air. Then we see their desiccated carcasses strewn across the pavement. Robins eat them, anglers use them for bait, and little kids bring them home in their pockets as pets. Most of us dissected one in biology, carefully counting the five aortic arches while debating the coolness of being squeamish. Yet, for all their inconspicuous habits, earthworms play a major role in both our gardens and in the wild.
To gardeners, earthworms are a boon. Burrowing into the soil with waves of muscular contractions, they create mucus-lined tunnels that allow air and water to penetrate. As they move along, they ingest tiny particles of plant matter. Inside the worm, these particles are mixed with ground up dirt particles and partially digested, creating humus. It has been found that worm castings, the excreted remains of the worm’s dinner, have five times the nitrogen, seven times the phosphates, and eleven times the potash (potassium) than the surrounding soil. One worm can produce more than ten pounds of casts a year, and an acre of farmland may contain well over a million worms. That’s a lot of fertilizer.
What can you do to encourage lots of worms in your garden? In a word: mulch. Several inches of straw, grass clippings, shredded leaves, or other organic mulch, acts to keep the soil at an even temperature and moisture level. Worms are attracted to the moderate conditions and high amounts of organic matter, and arrive seemingly out of nowhere. Incorporating a three to four inch layer of an organic amendment, such as compost, into the soil also leads to higher worm populations.
Avoiding harsh chemicals, including concentrated fertilizers and pesticides, helps too. In naturally acidic soils, fertilizers high in nitrogen decrease the pH to the point where it is actually toxic to the worms. (However, research at Colorado State University showed no problems in Colorado’s more alkaline conditions.) Many pesticides are harmful to earthworms—always read the label.
Many gardeners want to know if they can buy earthworms to add to their garden. However, those are usually “red worms” for sale, the kind of earthworms that need extremely high levels of plant matter, such as compost piles. They will not survive in your garden soil. (These are the worms you’d use in an indoor worm composter.) Save your money. You don’t need to buy worms. Create the right conditions and they will come.
With all the celebrated benefits of earthworms in the soil, it might come as a surprise to learn that they are not always beneficial. About a third of the earthworm species in the United States and Canada are immigrants. Some of these non-natives have spread to the forests around the Great Lakes—an area that hasn’t had earthworms since the last ice age—where they are actually harming the environment. Many native plants, including ferns, flower, and trees, require an intact layer of leaf mulch. The worms’ activity is decreasing that layer, plant communities are changing, and the entire forest ecosystem is being disrupted. Unfortunately, there is little that can be done at this point to stop the worm invasion.
In spite of the problems they cause elsewhere, earthworms are certainly welcome in our gardens. By providing damp, well-drained soil covered with a generous layer of mulch, you’ll have plenty of worms to work your soil, plus enough extras to keep those early birds well fed.
 Audubon Magazine, University of Minnesota, Department of Forest Resources
 Conversation with CSU Professor David Whiting, circa 2004.