Some overly enthusiastic worm fanciers believe it is useful to import large numbers of earthworms. I do not agree. These same self-interested individuals tend to breed and sell worms. If the variety being offered is Eisenia foetida, the brandling, red wiggler, or manure worm used in vermicomposting, adding them to soil is a complete waste of money. This species does not survive well in ordinary soil and can breed in large numbers only in decomposing manure or other proteinaceous organic waste with a low C/N. All worm species breed prolifically. If there are any desirable worms present in soil, their population will soon match the available food supply and soil conditions. The way to increase worm populations is to increase organic matter, up mineral fertility, and eliminate acidity.
Earthworms and their beneficial activities are easily overlooked and left out of our contemplations on proper gardening technique. But understanding their breeding cycle allows gardeners to easily assist the worms efforts to multiply. In temperate climates, young earthworms hatch out in the fall when soil is cooling and moisture levels are high. As long as the soil is not too cold they feed actively and grow. By early spring these young worms are busily laying eggs. With summer's heat the soil warms and dries out. Even if the gardener irrigates, earthworms naturally become less active. They still lay a few eggs but many mature worms die. During high summer the few earthworms found will be small and young. Unhatched eggs are plentiful but not readily noticed by casual inspection so gardeners may mistakenly think they have few worms and may worry about how to increase their populations. With autumn the population cycle begins anew.
Soil management can greatly alter worm populations. But, how the field is handled during summer has only a slight effect. Spring and summer tillage does kill a few worms but does not damage eggs. By mulching, the soil can be kept cooler and more favorable to worm activities during summer while surface layers are kept moister. Irrigation helps similarly. Doing these things will allow a gardener the dubious satisfaction of seeing a few more worms during the main gardening season. However, soil is supposed to become inhospitably hot and dry during summer (worm's eye view) and there's not much point in struggling to maintain large earthworm populations during that part of the year. Unfortunately, summer is when gardeners pay the closest attention to the soil.
Worms maintain their year-round population by overwintering and then laying eggs that hatch late in the growing season. The most harm to worm multiplication happens by exposing bare soil during winter. Worm activity should be at a peak during cool weather. Though worms inadvertently pass a lot of soil through their bodies as they tunnel, soil is not their food. Garden worms and nightcrawlers intentionally rise to the surface to feed. They consume decaying vegetation lying on the surface. Without this food supply they die off. And in northern winters worms must be protected from suddenly experiencing freezing temperatures while they "harden off" and adapt themselves to surviving in almost frozen soil. Under sod or where protected by insulating mulch or a layer of organic debris, soil temperature drops gradually as winter comes on. But the first day or two of cold winter weather may freeze bare soil solid and kill off an entire field full of worms before they've had a chance to adapt.
