Michelle Grabowski, UMN Extension Educator
Photo 1: Snow mold damage on a lawn in spring 2010Photo by T.Burnes.
In the spring of 2010 snow melted away from yards and landscapes to reveal round dinner plate sized patches of tan or gray matted turf. Two fungi, known as snow molds, were responsible for the damage. Many gardeners were dismayed to learn that little could be done in the spring to cure snow mold. Rather they had to wait for the weather to change and the grass to recover. This is because the time to prevent snow mold is not in the spring. The time is now.
Snow mold is caused by two different fungi, Microdochium nivale and Typhula sp. Both of these fungi thrive at temperatures just above or below freezing with high levels of moisture. Although snow cover is not a requirement for the growth for snow mold, snow cover provides ideal conditions for the fungi. Of course, gardeners cannot control how much snow Minnesota will receive this winter or how long that snow will stay. Gardeners can prepare their lawns for winter in a way that provides the best chance of a healthy spring.
Photo 2: Fluffy white mycelia of snow mold can often be seen in moist conditions Photo by T.Burnes.
Several factors can increase the chances of snow mold the following spring. Long turf that is bent over by snow creates a canopy that traps humidity at the base of the turf plant. Piles of leaves or other debris on the lawn have a similar effect. Snow mold thrives in these humid microclimates. Fertilizing lawns in late September or early October can cause the grass to produce a flush of young succulent growth. This new succulent turf often does not have time to harden off before winter comes and is an easy meal for the snow mold fungi.
Several simple steps can be followed to prepare turf for the winter and reduce the risk of snow mold the following spring.
1. Rake up leaves and any other debris on the lawn.
2. Continue mowing until the grass goes dormant. A height of 2 inches will help the turf remain upright and facilitate air movement and drying of the turf.
3. Do not fertilize lawns until next year. A late season application of fertilizer can be done around Labor Day, but there is little benefit of fertilizing beyond that date.