Worm casts are small, muddy mounds of soil that appear on the surface of your lawn, representing the digestive waste of earthworms. While these creatures are beneficial for the health of the soil, the resulting casts can be a frustrating nuisance for homeowners trying to maintain a uniform turf. When stepped on or run over by a mower, the casts smear across the grass, creating an uneven surface, aesthetic damage, and ideal seedbeds for weeds and moss to germinate. Managing this issue effectively requires a multi-pronged approach that focuses on altering the lawn environment rather than trying to eliminate the earthworms entirely.
Identifying the Cause of Casts
The surface mounds are produced primarily by a few specific types of earthworms, often referred to as anecic species, such as Lumbricus terrestris and Aporrectodea longa. These worms live in permanent, vertical burrows and surface to feed on organic matter, pulling things like fallen leaves and grass clippings down into their tunnels. The soil they ingest during this process is then excreted as the familiar, nutrient-rich casts on the surface.
Casting activity peaks during the cool, damp conditions of autumn and early spring. Earthworms require moisture to breathe through their skin and are drawn closer to the surface when the soil is wet and poorly drained. They are also attracted to areas with a high concentration of organic matter, which serves as their primary food source.
Cultural Practices for Prevention and Removal
Reducing Food Sources
Since earthworm activity is tied to moisture and food supply, managing these two factors is the most effective long-term strategy for prevention. Removing surface litter, such as fallen leaves and grass clippings, reduces the worms’ available food source. When mowing, use a grass collector or “box off” the clippings instead of letting them return to the turf, which starves the surface-feeding worms.
Improving Drainage and Soil Structure
Improving soil drainage is a major preventative measure because worms surface less frequently in drier conditions. Practices like core aeration relieve compacted soil, allowing water to penetrate and drain more effectively, which encourages the worms to stay deeper. Top dressing the lawn with coarse, angular sand can also help, as the abrasive nature of the sand makes the surface less hospitable to the worms’ sensitive skin.
Adjusting Soil pH
Earthworms prefer neutral to slightly alkaline soil, and casting is often worse when the pH is higher. Strategically lowering the soil pH can help to deter activity, though this must be done carefully to avoid harming the grass. Using acidic-reacting materials, such as ammonium sulfate-based fertilizers, can shift the pH balance slightly away from the worms’ preferred range. Test the soil’s pH before making any adjustments, and avoid applying lime products, which would increase alkalinity and encourage worm activity.
Physical Removal of Casts
Once casts have appeared, physical removal is the only way to restore the smooth surface, but timing is crucial. Only disperse the casts when they are completely dry and crumbly to prevent smearing and muddy patches. A stiff brush, a wire rake with the teeth facing upward, or a drag mat can be used to gently break up and scatter the dried mounds across the turf. Attempting to brush or rake wet casts will smear the muddy material, smothering the grass blades and creating an ideal environment for moss and weed seeds.
Using Applied Treatments to Discourage Casting
Direct chemical control is no longer a viable option for homeowners, as most potent chemical vermicides, such as carbendazim, have been banned for general lawn use due to environmental concerns. The focus has shifted to using authorized products that act as environmental deterrents without killing the beneficial earthworms. These treatments work by making the topsoil environment less appealing, causing the worms to retreat deeper into the soil profile.
Iron Sulfate
Iron sulfate, also known as ferrous sulfate, is a common and authorized treatment that indirectly helps by acidifying the turf surface. Earthworms are sensitive to this acidic change, and regular applications can temporarily discourage casting. This product is also widely used to control moss and provide a deep green color to the grass, offering multiple benefits.
Non-Pesticidal Suppressants
Proprietary non-pesticidal liquid treatments are also available that contain plant nutrients or natural surfactants, like saponins, which irritate the worms and drive them down. These products are considered cultural soil amendments, not worm killers, and are designed to alter the local soil surface environment. These suppressants often require repeated applications to remain effective throughout the peak casting seasons of autumn and spring.
The Role of Earthworms in Lawn Health
Earthworms are a sign of a healthy, functioning soil ecosystem. Their burrowing activity is a natural form of aeration, creating tunnels that improve the movement of water, air, and nutrients to the grass roots. Earthworms also consume and decompose thatch and organic matter, recycling these materials into nutrient-rich castings that fertilize the lawn.
The goal of management should be cast reduction, not eradication. A lawn without earthworms would likely have issues with compaction, thatch buildup, and poor nutrient cycling. Therefore, the most sustainable strategy involves making the surface less inviting so the worms stay deeper, allowing the lawn to benefit from their activity without the visible casts.