What is Snow Mold?

What snow mold looks like

You find it at snowmelt. The affected areas are circular or irregular patches of matted, flattened grass, straw or tan colored, often with the blades stuck together as if they were glued. The patches can keep enlarging while the grass stays cold and wet, and as they age the matted area may take on a white, crusty appearance.

Two forms show up in Wisconsin and Minnesota lawns. Gray snow mold needs extended snow cover to develop and generally stays in the leaf tissue. Pink snow mold, which shows a pinkish cast at the edge of the patch in damp conditions, can develop without deep snow during long stretches of cold, wet weather, and it is the one more likely to injure the crown of the plant rather than just the leaf.

Why some winters bring it and others do not

Snow mold does not appear every year. It is most common in winters when an early, deep snow falls before the ground has frozen. Unfrozen soil under an insulating snowpack holds the turf at a temperature just above freezing, wet and without air movement, for weeks or months. That is the environment the fungus needs.

A winter that freezes the ground hard before the first significant snow usually produces far less of it. The same is true of a winter with little snow cover. This is also why the worst patches in any given spring are predictable: they are wherever snow lasted longest, which usually means the shaded north and east sides of the house, along fences and tree lines, and anywhere plowed or shoveled snow was piled.

Cleaning it up in spring

If the infection is not severe, control is straightforward. In most cases the affected areas are simply slower to green up than the rest of the lawn, and they recover on their own as the soil warms.

• Rake the matted areas gently. Loosening the matted grass lets air circulate down near the crowns and soil, which dries the area and lets the grass resume growing. Rake lightly so the turf is not uprooted.
• Do not use a power rake. A power rake on turf that is still weak and wet pulls out live grass along with the matted material and turns a patch that would have recovered into bare soil.
• Break down remaining snow piles. The longer snow sits on grass, the more likely snow mold is to develop and keep spreading. Spreading a pile out so it melts faster is the easiest fix, and it matters most in shaded areas where piles linger for weeks.
• Wait before deciding to reseed. Give the area two to three weeks of growing weather. Most patches fill in. Only the spots where the crown was killed need seed, and those are usually smaller than they looked at snowmelt.

Preventing snow mold before winter

With most lawn disease, preventive maintenance beats repair. Snow mold is a good example, because everything that reduces it is done in the fall.

Follow a balanced fertilization program

The goal is a lawn going into winter with adequate nutrition but not a flush of soft new growth. Heavy nitrogen applied too late in the fall pushes tender top growth that mats under snow and feeds the fungus. Timed fall applications are part of a lawn care program for this reason.

Manage thatch

Thatch holds moisture and harbors the pathogen. Core aerate at least once a year and keep thatch below half an inch. Fall aeration also improves drainage, which helps the surface dry faster in spring.

Mow the lawn short for its last cut

Cut the lawn to one and a half to two inches at the final mowing of the season. Tall grass lays over on itself under snow, creating the matted, airless layer snow mold thrives in. Do this gradually rather than all at once: lower the mower a notch a week until it is set at one and a half to two inches. Dropping from three and a half inches to two inches in a single pass scalps the lawn and stresses it going into winter, which is its own problem.

Clean up before the snow flies

Remove leaves and debris from the lawn. A wet mat of leaves under snow does the same thing matted grass does. While you are at it, plan where plowed and shoveled snow will be piled so it is not stacked on the most shaded turf, and make sure your irrigation system has had its fall blowout before the ground freezes.

When to call for help

Call if patches are still dead after several weeks of spring growth, if the same areas come back every year, or if the matted areas cover a large share of the lawn. Repeat damage in the same spots usually points to a drainage problem, a shade problem, or a snow storage problem that can be corrected rather than treated every spring.