"Earthworms are good" is a generally held and worthy gardening belief. Earthworms are not usually associated with any negative attributes by horticulturalists and soil scientists. Golfers are the only group I know of that uniformly hates earthworms. They, in their never-ending quest for personal-best scores, obsessively clear all earthworm castings from meticulously manicured, daily-mown grass greens.
Gardeners thankfully have long been taught to make their lawns and garden soils worm havens to reap the benefits of improved root, and thus overall plant, growth. Worm castings are seen as proof positive that the gardener is achieving a gardening holy grail — worm-rich soil.
Fine Gardening, a trusted source of valuable gardening information, recently produced an excellent article (February 2005) extolling the benefits of having vibrant earthworm populations in garden soils. Gardeners were reminded that earthworms, through digestion of undecayed vegetable matter, hasten the release of nutrients needed by growing plants. Myriads of worm tunnels aerate soils, facilitating the exchange of oxygen and carbon dioxide with the above-ground atmosphere. In the world of horticultural commerce there are many new industrial-scale businesses bagging certifiably pure worm castings and worm-digested municipal compost suited to the home gardener.
However, amidst all the positive attributes of encouraging worm-enriched gardening practices, there is an earthworm concern emerging in the forestry world.
I was introduced to the negative impacts of earthworms on forest ecosystems one evening at a potluck dinner, part of a garden club meeting. A respected visiting lecturer, chatting about problems establishing alpine plants in scree (gravel-based) gardens, mentioned that earthworms are not a part of alpine ecosystems. He conjectured that they might be disturbing delicate alpine plant root systems. It was his next, almost incidental, afterthought that sparked my antennae. "Earthworms are now being blamed for destroying some North American forests too," he said.
I have to admit I raised my eyebrows when I heard this. I had an almost empty wine glass in hand, a satiated appetite thanks to a splendid table by accomplished cooks, and a noisy roomful of gardeners trading "Did you know ..." stories. I wondered if I had just been handed a can of worms — pun intended!
My interest in this intriguing worm story was heightened by an entrepreneurial experience in my youth when I had a role in sending earthworms into forested areas. In the 1950s, as an adjunct to many hours spent fishing in nearby southern Ontario lakes, my home-based student venture was as a dew worm picker and worm vendor to fishermen. My possible contributions to forest declines, from leftover worms being thrown into woods at the end of fishing trips, were inconsequential, with respect to timing and quantity, I am sure — at least based on the size of my business account!
Soon my professorial gardening friend emailed me his Internet links to the worm world. Geological and ecological worm facts soon opened up like Pandora's Box!
Ensuing Internet searches revealed that, geologically speaking, North America had glaciers scour soil from much of the northern states and provinces and deposit it further south. This I knew already. But I had not considered that pre-glaciation worm populations moved with that soil, leaving wormless environments for future forests to colonize. Soil-building and northward earthworm migrations proceed at geologically slow rates and, until the arrival of early settlers, northern post-glacial soils and colonizing trees developed in harmony without earthworms. Evolving forest ecosystems adapted to slow soil formation and slow nutrient release from vegetation decomposition in a cool soil environment.
With the coming of early settlers, earthworms quickly found their way into our northern ecosystems. These settlers and their descendants both purposefully (for improved agricultural practices) and unknowingly (in soil constituting ships' ballast) imported earthworms into North America. Fishermen and I have made our more recent contributions, too.
This is where the passage of time and the continuation of old habits enter the discussion. Historically, the worm populations in the soils of northern forests have not been much studied. Studies of changing tree composition in forests were not priority research projects. However, the Internet reveals that alarm bells are now beginning to be sounded by forestry and ecology researchers. Increasingly, they report, earthworm-filled northern forest soils are becoming more suited to growing non-native trees and shrubs. With more oxygen-rich air entering the duff layer through tunnels formed by earthworms, organic materials accumulated over the centuries are disappearing faster than they are building up. In some cases, a decade is all that is required to wipe out millenniums' worth of organic material. Forest soils are becoming more compacted, nutrient rich, and warmer due to the presence of earthworms. As in farming areas, forest soils are changing – and not for the better as far as historical tree species composition is concerned.
Gardeners in cottage areas are unsuspectingly contributing to this accelerated change in forest ecology by importing alien trees, shrubs, and garden plants to personalize their summer retreats. This plant importation, with unintended plant escapes and introduced pollen pools, further threatens the survival of native plants in forested areas. This is where selecting native plants for lakeside properties can help maintain traditional forest ecosystems.
Sadly, from a nursery-owner's perspective, I have yet to see a significant conversion of gardeners to planting native species anywhere — let alone in isolated, forested lake sides. We encourage all gardeners to accept our role in maintaining native plants for the benefit of forest-dependent native birds and animals. As fishers, we should not cavalierly toss unused fish bait worms or the media used to transport them (they may contain viable worm eggs) into the woods. Unused worms and transportation soils/media taken into remote areas should be brought back to our southern urban residential plots, where they can be incorporated into compost piles or gardens already populated with earthworms.
The following Web resources will give interested readers a wealth of literature entrées into the world of earthworm biology and research on their impact on forest soils.
Society for Conservation Biology, "Non-native Earthworms May Be Wiping Out Rare Plants," Conservation Biology, December 2002, http://conbio.net/SCB/Services/tips/2002-9-December.cfm#A3.
Jennifer A. Ramsay and Stuart Hill, "Earth Worms: The Agriculturalist's Friends," Macdonald Journal 39(10), 6-8 Oct. 1978, http://www.eap.mcgill.ca/publications/eap6.htm.
For 21 years Phil Reilly and family have operated a family-run specialty nursery, Reilly's Country Gardens, in west-end Ottawa. Their nursery features an acre of demonstration gardens showcasing over 1,800 varieties of perennials. They practise (and preach) organic gardening techniques in their show gardens. Their home garden has been certified as part of CWF's Backyard Habitat Certification Program.
Visit http://www.rcgardens.ca/ for gardening fact sheets (including using native plants in the garden) and photo galleries of their nursery specialties: peonies, hostas, ornamental grasses, and, new in 2005, ponds and aquatic plants.