Snow mold is a fungal disease visible on turfgrass immediately after the snow melts in late winter or early spring. This condition thrives when a heavy, insulating layer of snow falls onto grass that has not fully frozen, creating a consistently cold, wet, and matted environment. The fungi are cold-tolerant and actively grow in temperatures near or slightly above freezing, typically between 32 and 45 degrees Fahrenheit. Extended periods of snow cover, especially on long or debris-laden grass, trap moisture and prevent air circulation, allowing the fungus to flourish.
Identifying Snow Mold Types
Two types of snow mold affect lawns, each presenting distinct characteristics and causing varying levels of damage. Pink Snow Mold, caused by the fungus Microdochium nivale, appears as circular patches that are orange-brown to bleached light gray with whitish-pink fungal growth (mycelium) at the margins. This type is more destructive because the fungus can kill the grass crown and roots, requiring extensive repair.
Gray Snow Mold, or Typhula blight, is caused by Typhula species and presents as circular, straw-colored patches that can expand up to several feet in diameter. The fungal growth is white to gray, and the matted grass blades may contain tiny, hard, dark structures called sclerotia, which are resistant survival bodies. Gray Snow Mold is less severe, as it typically only damages the grass blades, leaving the crowns and roots intact for spring regrowth.
Immediate Steps for Removal
Once the snow has melted and the mold patches are visible, the first step is to gently break up the matted grass to allow the area to dry and increase air circulation. Waiting until the soil is no longer saturated is important, as raking wet soil can lead to compaction and damage to the grass crowns. Use a leaf rake or a soft garden rake to lightly fluff the affected areas, lifting the blades without aggressively tearing the turf.
This process disrupts the fungal growth and exposes the area to sunlight and air, which are detrimental to the cold-loving fungus. For Gray Snow Mold, gentle raking is often the only necessary treatment, as the grass crowns are usually alive and will recover as temperatures rise. Fungicide applications are generally unnecessary for most residential outbreaks. If a severe case of Pink Snow Mold is confirmed, a specific fungicide may be applied, but it is usually only effective if used immediately upon snow melt or as a preventative measure in the fall.
Repairing Lawn Damage
After the mold has been addressed and the area has dried, the focus shifts to repairing the damage and encouraging new growth. Lightly raking the area again to remove dead debris is helpful, as this material can hinder seed germination. For patches with severe damage, especially those caused by Pink Snow Mold where the grass crowns are dead, overseeding is necessary to restore turf density.
The best time for spring overseeding is when soil temperatures are reliably between 50 and 65 degrees Fahrenheit for cool-season grasses. Before seeding, add a thin layer of topsoil or compost and lightly loosen the soil surface to provide a good seedbed. Applying a starter fertilizer, which contains phosphorus to promote root development, will help seedlings establish themselves. Consistent, light watering is required to keep the seedbed moist until the grass is established, while avoiding overwatering which could promote other fungal issues.
Strategies for Future Prevention
Preventing snow mold begins with cultural practices implemented in the fall before the first heavy snowfall. Ensure the grass is mowed shorter than usual, ideally down to two to three inches, before the turf goes dormant for the winter. This reduced height prevents the grass from matting down under the snow, which contributes to fungal growth.
It is important to manage organic debris; all fallen leaves and excessive thatch should be removed from the lawn. A thatch layer thicker than half an inch traps moisture and creates a favorable environment for the fungus. Avoid applying high-nitrogen fertilizer in late fall, as this encourages tender, new growth susceptible to snow mold damage. Preventative fungicide applications in late fall, before the first permanent snow cover, can be considered for lawns with a history of severe infections or prolonged snow coverage.