As the (earth)worm turns

Karl Danneberger

Walking across my lawn in May, I feel bumpiness under my feet. Although not visually apparent, I know bumpy turf is a sign of earthworm casts. Helping verify my diagnosis are mole tunnels throughout my lawn from the moles hunting for earthworms as food. Moles crave earthworms so much that they eat the equivalent of their body weight of earthworms in a day.

Earthworms are beneficial to turfgrass systems, producing tunnels through the soil that help reduce soil compaction and let air and water move in and through the soil. Feeding on grass clippings and thatch, earthworms are important in the initial steps of microbial breakdown of plant litter. Dan Potter, Ph.D., at the University of Kentucky, has reported that an acre of turf can support more than a million earthworms that consume more than 4 tons of plant debris. Earthworms affect turfgrass soil fertility and land worldwide.

Commenting on the global adaptation of earthworms, Charles Darwin — in one of his last — papers speculated that almost all the fertile soil in the world passed through the gut of an earthworm. Earthworm activity is desirable and a sign of a healthy soil.

The downside to some, but not all, earthworm feeding is the deposition of their fecal material in small mounds, called casts, on the turf or soil surface. Casts are most evident during cool, wet weather, primarily in fall, but also at other times of the year.

I’ll live with the bumpiness in my lawn caused by the casts through the “shock absorbers” of my forearms on the lawn mower. But on golf courses, excessive earthworm casts can disrupt playability and playing surface aesthetics. Castings cause disruption to ball roll, and traffic can result in a muddy smear over the turf. In some cases that I observed in Europe, golf courses close temporarily because of the excessive mudding and smearing of earthworm casts.

Cultural practices to reduce earthworm casts include using soil-acidifying fertilizers, rolling, topdressing with angular sands or aggregates and removing clippings. Earthworm populations, at least of the casting type, decline with a drop in soil pH. At a pH below 5.0, earthworm populations are low. The use of acidifying fertilizers lowers the pH at the soil surface, reducing earthworm populations.

Rolling on courses to flatten castings was the most popular practice in the late 1800s and early 1900s. Disadvantages to rolling, depending on the type of roller, were the potential for soil compaction and the fact that the casts often smeared across the turf, making for a muddy surface.

Heavy topdressing with angular sands or aggregates is believed to agitate earthworms and discourage them from coming to the surface and creating casts. In the early 1900s, greens sometimes were constructed with a cinder layer in the subsurface to discourage earthworms from migrating upward in the green mix.

Recent research studies at the University of Arkansas and other places have looked at light sand topdressing for earthworm-cast suppression. Although preliminary results show variability, it will be interesting to see how these studies pan out.

The results from lowering pH, rolling, topdressing and removing clippings have been inconsistent at best in reducing earthworm casts.

There has been considerable attention paid recently to a naturally occurring plant compound known as saporins for controlling earthworms. Saporins are found in hundreds of plants. However, tea seed meal, a byproduct of tea oil production, is especially high in saporins. Field and laboratory studies have confirmed that saporins from tea seed meal are effective in controlling earthworms. Tea seed meal works as an expellant that causes the earthworms to come to the surface and die.

Earthworms present a dichotomy for golf course superintendents. On one hand, they provide many agronomic benefits to the turf. At the same time, they can cause significant disruption to the playing surface. Balancing earthworm activity on golf courses has been — and will continue to be — an ongoing challenge.