Earthworms are common organisms found thriving beneath the surface of turf and garden soils. These invertebrates are recognized for their positive contributions to a healthy environment, yet they can present a challenge for lawn owners. A robust earthworm population signals a healthy soil ecosystem, but their activities often result in visible surface disruption. Managing a lawn requires balancing subterranean health with surface aesthetics.
Earthworms as Essential Soil Engineers
Earthworms are natural cultivators of the soil. As they move through the lawn, they create an intricate network of permanent burrows that function as natural aeration channels. This tunneling action loosens compacted soil, allowing air and water to penetrate more effectively, which significantly improves drainage and helps grass roots grow deep and strong. Soils with healthy earthworm populations can drain up to ten times faster than those without.
Earthworms consume organic matter, such as dead roots and the thatch layer. They digest this material and excrete it as nutrient-rich castings, which are a form of natural, slow-release fertilizer. This activity supports microbial activity and improves soil fertility by incorporating organic material back into the soil.
Understanding Surface Casts
The conflict between earthworms and lawn aesthetics is the production of surface casts. These casts are small, muddy mounds of fine soil and excrement deposited on the turf surface. They are the result of deep-burrowing (anecic) earthworm species that pull organic matter into their vertical burrows for food, then deposit their waste above ground.
While casts are rich in nutrients, their presence creates a rough, uneven surface. These soil piles interfere with mowing, causing blades to dull quickly as they scrape the abrasive material. When wet, casts smear across the turf under foot or mower traffic, which can thin the grass stand and create seedbeds for unwanted weeds. Casting activity is most pronounced during periods of mild temperature and high soil moisture.
Practical Management Strategies for Lawns
Since earthworms are beneficial, the goal is management rather than elimination, especially as no chemical products are currently labeled for control.
Physical Dispersal
A primary strategy for addressing existing casts is physical dispersal, which should only be performed when the casts are completely dry. Using a stiff brush, a rake, or a drag mat to gently break up the dried mounds allows the nutrient-rich material to filter back down into the turf canopy without smearing.
Cultural Practices
To reduce the frequency of surface casting, homeowners can limit the earthworms’ food supply at the surface. Collecting grass clippings instead of leaving them on the lawn removes a substantial food source for the casting species. Adjusting irrigation practices to water less frequently but more deeply encourages earthworms to burrow further down, keeping them away from the surface where they deposit their waste.
Soil Amendments
Amending the soil environment can make the surface less appealing to casting worms. Earthworms prefer a near-neutral soil pH, so using acidifying fertilizers like ammonium sulfate or elemental sulfur can discourage their activity in the upper soil profile. Applying a topdressing of medium-coarse sand can also help, as the abrasive nature of the sand particles encourages worms to seek deeper soil. This sand topdressing ensures that any casts that appear are more granular and easier to disperse.