Most of us grew up with the belief that earthworms are the superheroes of soil health, and that their presence in our gardens and wild environment is nothing but auspicious. After all, earthworms loosen compacted soil, break down decaying organic matter, and leave behind nutrient-rich poop many gardeners call "black gold." We can even buy worms by the pound — most commonly the red wriggler (Eisenia fetida, previously E. foetida) to start "colonies" in special worm bins for vermicomposting.
Then we learned that up to 60% of earthworms are non-native, having crossed the Atlantic with European explorers and settlers, but we shrugged it off. Hey, if it weren't for cultural diversity, we'd be eating nothing but maize, tomatoes, and Wonder Bread here in North America and without earthworms, wouldn't our gardens be barren wastelands? Hold onto your butts, Seed Needs readers, because you might be as shocked as we were when we learned that releasing non-native earthworms — including those used for composting — may be illegal in some areas, and they might be mucking up our soil biomes as much as they're helping. This may cause us to take a more thoughtful approach to using unprocessed worm castings and maintaining vermicomposting systems.
We're going to dig beneath the surface to unearth the pros and cons of earthworms in North American gardens so you can make your own informed decisions. Follow the embedded links for more information, or reach out to us for our advice!
What are worm castings, and why are they so awesome
Worms expel tiny pellets containing worm feces, soil aggregates, and beneficial microbes. Depending on the quality, moisture content, and worm species, you can pay up to $5.00/lb for packaged worm poop. Nutrient levels vary, but the University of California Cooperative Extension assigns worm castings with a 5.5.3 NPK (nitrogen, phosphorus, and potassium) rating. They're also rich in iron, sulfur, and calcium. Worm castings are water-soluble, but their mucus covering helps them act as time-release capsules for plant nutrition. Castings also help retain soil moisture and structure.
Worm castings have a pH of 7, which is neutral to slightly acidic. Bacteria and enzymes in the worm castings, and the poop itself, support soil microbiomes by encouraging beneficial bacterial and fungi.
Studies show that castings have a positive effect on plant biomass, grain, and fruit yield. Container-grown seedlings treated with worm castings may experience less salinity shock as compared to concentrated chemical fertilizers, in part because its slow dissolution reduces the risk of salt buildups. Reputable sources, which include academic studies and agriculture extension services, recommend a soil mix of 15% to 40% castings by weight for seedlings and container plants; castings make an excellent substrate for seed germination, as they're not likely to burn plants when they're the most vulnerable. Worm poop also makes a good side- or top-dressing for bed-grown plants.
How else do live earthworms help in the garden?
The University of Minnesota Extension groups earthworms into three soil habitats:
- Litter dwellers live among fallen plant debris and rarely burrow below the soil's surface. You'll easily find them under leaf piles and rotting logs.
- Soil dwellers live in the topsoil, up to 20 inches deep.
- Deep burrowers will feed on surface-strewn litter, but they spend most of their time up to six feet below ground.
In addition to releasing nutrients in their castings, earthworms "churn" soil, helping distribute organic matter into sandy or clay substrate. Tunnels created by deep-dwelling worms help distribute water to root systems. Worms thrive off the microbes living on soil particles and decaying plant matter, breaking down leaf litter and minerals so they're more accessible to our plants, but to the other beneficial critters in our soil's biome.
But in areas where earthworms aren't native, they conflict with natural soil renewal processes and compete with other animal species.
How can non-native worms damage the ecosystem?
The University of Minnesota explains that most earthworms found in the northern U.S. states and Canada were introduced from Europe and Asia, and in these regions, non-native earthworms pose an ecological threat. Some worms, like many nightcrawlers and the jumping worm (Amynthas agretis) deplete so much plant debris from hardwood forest floors that duff is unable to accumulate — robbing birds, amphibians, reptiles, and other insects of food and habitat. "If you were to think about the soil food web as the African savanna, it's like taking out all the animals and just putting in elephants — a ton of elephants," said Yale forest ecologist Annise Dobson in a recent interview with The Atlantic. The jumping worm (Amynthas agretis), also known as the crazy worm, is an Asian species at the top of her heap of worries.
The efficiency by which these worms convert decaying leaf matter to soil nutrients is also shocking, but mostly to the native plants. It's as if they get one big megadose of vitamins, followed by... nothing.
Many invasive worms don't overwinter well in cold winter areas, but enough survive to pose a long-term risk to forest health. The red wriggler (E. fetida) appears to be somewhat safe, since the species doesn't burrow deep enough to avoid hard frosts, and they're most active when the soil is about 70°. Red wrigglers reproduce quickly, but they're also more vulnerable to predation. About 90% of the worms commercially sold for composting kitchen and garden waste are red wrigglers, also called branding worms, tiger worms, or red worms. Larger, deeper-dwelling European nightcrawlers (Eisenia hortensis) and African nightcrawlers (Eudrilus eugeniae), though, are becoming popular.
Self-contained vermicomposting systems
As mentioned above, litter-dwelling earthworms like red wrigglers are efficient at converting organic matter to castings. You can shell out a pretty penny for manufactured worm bins; these are usually stacking trays that force worms to migrate to their food supplies, leaving behind "finished" castings and egg cocoons. Simple, home-built rectangular boxes work on the same principle: you bury food to the side of the last location until you get to one end, and harvest compost from the other. Even the most efficient systems require a certain amount of hands-on labor but they're popular projects for classrooms and garden-savvy weirdos like us.
How to harvest and apply castings without introducing non-native worms
Worms migrate far beyond your garden's borders but you can still benefit from worm castings without the risk of releasing worms and their egg cocoons into the "wild."
Screen out worms and their cocoons
If you keep your own "worm farm," you can separate worms and cocoons from the castings using a two-phase screening process: 1/4" hardware cloth to remove the worms, and 1/8" to remove the eggs. Because the average red wriggler cocoon diameter is about 1/8" in diameter, some will inevitably get through. Experiment with finer screens as a third phase, but have patience as they'll clog up quickly, especially if the castings have a high moisture content. Remaining castings and cocoons can go back in with the adult worms. We've also heard of gardeners dumping mostly-harvested castings into kiddie pools and letting their ducks or chickens clean up the stragglers.
Make and use worm tea
Worm tea is an alternative to solids, which — even when aged, packaged, and shipped — might still contain viable egg cocoons.
Remember, you can't overdose your plants with worm castings. A little goes a long way, and excess only equals waste. Worm tea recipes range from 1:4 casting to water ratio to a cup of castings per gallon of water. All recommend letting the mixture sit for about 24 hours, and using the final product as quickly as possible while the microbes and nutrients are most viable. Use tea to water at the ground level, or strain it to apply as a foliar spray.
For best results, neutralize chlorine in treated city water by leaving it out for 24 hours before making your worm tea, stirring it once in a while. We can't always irrigate our gardens with untreated water, but when we're making our own seedling water supply or special mixes, the extra step is worthwhile.
Can composting worms survive in my garden?
If you live in a northern climate, it's likely enough of the duff-dwelling non-native species will survive but you can contain them using raised beds with thick, pro-grade, woven polyester landscaping cloth attached to the bottom. Unless there's significant organic matter rotting on the surface, they won't multiply as well as they would in natural areas or in "fed" composting bins.
Outside of containers, worms will migrate to happier spots: Compost and debris piles, underneath shredded mulch, or below the frost line in the case of larger worms. There, they'll slow down their metabolisms until warm weather kicks in again.
What about nematodes?
What, you haven't had your fill of worm talk for the day? We explored the world of teeny, tiny nematodes in our gardening blog and we encourage you to read up if you haven't yet done so!
Worm poop is good for your garden. Crappy seeds? Not so much.
We think you should do everything you can to take advantage of worm casting nutrients, but planting high-quality, fresh seeds from disease-resistant plant genetics is the first step to your season's success. Contact us to order your vegetable, herb, and ornamental seeds today; we only stock what we expect to sell in a single season, and if you don't see what you're looking for in our online catalog, we'd love to hear your requests. We're usually able to help with special orders and quantities, too!