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Small holes in your lawn? Nightcrawlers may be to blame

Source: Karen Johnson, University of Minnesota Extension

Have you seen small holes with little piles of dirt next to them in your lawn? Wondering what it may be? In the last couple of days, I have received a number of calls about these small holes. The most common answer to this question is that the signs you are seeing in your yard are caused by nightcrawlers or more commonly known as earthworms.

In an article called “Managing lawns and turf insects: generalist feeders”, Vera Krischik and Mark Ascerno discuss nightcrawlers (Phylum Annelida: earthworms). The article states, Nightcrawlers are beneficial as they provide natural aeration of soil allowing water and oxygen to penetrate more easily into the ground. Their feeding and excrement helps recycle nutrients and fertilize the soil. Nightcrawlers also feed on thatch, a layer of live and dead plant material that can accumulate at the soil surface and reduce the penetration of water and fertilizer. However, nightcrawlers leave behind a waste product called castings. Nightcrawlers can deposit castings at their burrow entrances forming conical mounds at the soil surface.

These mounds do not harm turf but this lumpiness can be unsightly and a nuisance to home lawns, athletic fields and golf courses. These mounds are most common in early to mid-spring when nightcrawlers are first active. During late spring and summer, when weather becomes warmer, nightcrawlers move deeper into the soil and are not normally seen.
Vertical mowing can help reduce the lumpiness and also the amount of food available for night crawler development. Vertical mowing is best done in late summer, mid-August through September. You can also power rake in spring once the ground is firm underfoot and before hot weather sets in. If you power rake in spring, apply a pre-emergent herbicide to prevent annual weeds from sprouting. Do not do vertical mowing in hot weather as it causes stress to the lawn.
Despite the nuisance of these mounds, tolerate nightcrawlers whenever possible. Treating them with an insecticide is strongly discouraged. The value nightcrawlers have in keeping soil healthy far outweighs any problems their mounds create. If nightcrawler populations are low, turf will suffer in the long run.
Nightcrawlers can also be a nuisance when they occur in large numbers on sidewalks, driveways, patios, pools, and other places where they are not wanted. This usually occurs after a heavy or steady rainfall and soils are saturated. This can also occur when lawns are overwatered. When nightcrawlers are numerous on sidewalks, driveways, etc., just wash or sweep them off. If you ignore them, they will eventually move off on their own or dry up and die. Never use a pesticide on nightcrawlers under these circumstances.


Karen Johnson
Extension Educator, Ag Production Systems
(320) 484-4303
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