Have you ever noticed several small holes and mounds in your lawn? At first glance, this may seem like insect damage, but chances are they were earthworm holes. The mounds surrounding the hole openings are earthworm castings.
Earthworms are beneficial to all soils and to your lawn. They do not feed on your turf. Earthworms burrow through the soil and feed on decomposing organic matter and microorganisms. Earthworm activity can greatly improve the condition of your soil. Their borrowing helps to improve aeration and drainage.
After earthworms feed, they move to the soil surface and deposit their excrement, or castings. These castings are rich in organic matter and nutrients and are very beneficial to your turf. However, sometimes too much earthworm activity can leave large amounts of unsightly castings all over your lawn.
If you have large piles of earthworm castings, there are a few steps you can take to remediate the issue. First, smooth out the soil surface by raking down the castings. Next, if no natural rain is present, irrigate your lawn with 1-inch of water per week. This will encourage deep root development. Deeper roots allow your lawn to survive longer without frequent artificial irrigation. This will help dry out the upper layers of soil and lessen the impact of earthworm activity, as they are less likely to surface when the soil is dry.
Keeping your turf mowed to the recommended height will also help hide large piles of castings. For tall fescue, the recommended mowing height is 2-3 inches. For Bermuda and zoysia, the recommended height is 1.5 inches. Proper cutting height will also allow for deeper root systems.
Moles are predators of earthworms, but they are not friends of your lawn. There are a few other natural enemies of earthworms that can help control their populations, including birds, snakes, toads, centipedes, ants, beetles, and nematodes. Thankfully, most of these predators don't cause problems in the lawn like moles do.
I have received questions as to whether or not any pesticides can be used to control earthworms. Some pesticides and fertilizers are known to have an impact on earthworms, none can be officially recommended as controls. Because of the strict pesticide regulation process, today's pesticides have relatively little effect on earthworm populations. Any potential pesticides that have high mortality rates on beneficial and non-target organisms could not obtain a registration label.