Nightcrawlers, a type of earthworm, often grow as long as 8 inches, making them hard to miss when you see them above ground at night or after it rains. They provide a valuable service to your lawn and flowerbeds, keeping the soil aerated while providing fertilizer in the form of castings -- or manure -- for your plants. They tend to prefer certain soil types over others.
Nightcrawlers, like other earthworm species, need moist soil to function properly. They breathe through their skin by diffusion, and this only works if their skin is damp. Burrowing through damp soil helps keep their skin at the correct moisture level to help with respiration. They secrete a mucus around their bodies to help retain the moisture, but it's not going to help much if the soil is bone dry.
Earthworms tend to stick to soil where food is easy to come by. This includes most areas of your yard. They eat mostly decomposing plant material, but they also feast on fungi and other micro-organisms that live in the soil. Nightcrawlers are one of the few creatures that help to naturally break down the thatch layer in your lawn, keeping it from getting so thick that the lawn develops bare spots. Too much thatch also creates an ideal environment for weeds, but nightcrawlers help keep it manageable for a healthy lawn. Nightcrawlers even venture to the surface of your lawn after a rain or while dew keeps the soil moist to munch on your thatch.
Neutral is Better
Nightcrawlers don't function well in acidic soils, or soils with a pH of less than 6.0. They prefer neutral or slightly alkaline soils, such as a 7.0 or 7.5. They're less likely to hang out around acidic-loving plants such as azalea (Rhododendron) that grow in U.S. Department of Agriculture plant hardiness zones 4 through 9, than they are around those that thrive in neutral soil, such as your grass.
Problems With Nightcrawlers
Without question, nightcrawlers provide a valuable service to the plants in your yard. However, your soil may be ideal for nightcrawlers, bringing them in by the tens or hundreds. Their tunnels sometimes cause ridges in your lawn, and many dump castings in piles on top of the soil. These castings make excellent fertilizer, but they also mar the look of your lawn. Raking down the casting piles can help, but you can encourage the nightcrawlers to stay deeper in the soil by watering your plants and grass infrequently, letting the top couple of inches of soil dry out. Use long waterings once a week rather than short daily ones, for example. These deeper waterings encourage your grass and plant roots to develop deeper in the soil, which leads to a healthier lawn and garden.