April showers not only bring May flowers but also bumps in the lawn. As you are out in the landscape inspecting and preparing for spring, you might find numerous small mounds of soil popping up all over the place. The randomly spaced tablespoon-size deposits make it hard to mow and walk without twisting an ankle. Believe it or not, the culprit is nightcrawlers.

Nightcrawlers are large worms  usually 4 to 8 inches or more long. They belong to a group of earthworms known as deep burrowers. The deep burrowers build large, vertical, permanent holes that can reach as deep as 5 or 6 feet.

Nightcrawlers pull plant material down into their burrows for later feeding. The bumps you see on the ground are called “middens” and are a mixture of plant residues and castings (worm feces). These middens may be used for protection and food reserves.

The burrows can have a significant positive effect on soil by opening channels for water and air to penetrate. Roots also like these channels because of the ease of penetration and the nutrients found in the casting material lining the burrow. So nightcrawlers actually help the soil even though they make it difficult to mow.

Getting rid of the middens or mounds is not easy. Rolling the lawn while the middens are soft may help temporarily, but mounds will be rebuilt when nightcrawlers become active again.

Most avid gardeners try to protect the nightcrawlers because of their positive effect on soils. Therefore, they choose pesticides that are not toxic to the worms. Those that have no effect include diazinon, Dylox (Proxol) and Oftanol.

Malathion may be slightly toxic while Sevin, Benomyl, copper sulfate and the arsenicals (MSMA, DSMA) are extremely toxic. Avoid using the latter group while nightcrawlers are active.

 Bruce Chladny is horticulture agent at K-State Research and Extension-Douglas County. For more information, call him at 843-7058 from 8 a.m. to 5 p.m. weekdays.