Trouble Beneath The Snow

April, and the trees are a-budding, the bulbs are a-blooming, and the lawns are a-greening – except for a few lawns, which have big brown spots.  This I know, partly because I’ve received some photos from distressed homeowners, under the title “Why Did My Lawn Die?” and partly because my own lawn has brown spots, too.  It might not surprise anyone to learn that the problem has roots in 2020, but it is odd to learn that a common phenomenon – snow – is involved.  Let me tell you about the snow molds.

A lot of fascinating fungi live hidden in lawns, waiting for  just the right conditions to show themselves, and this is certainly true of the snow molds.  We’ll focus on the two most common types, called pink and gray.  Unlike most plant diseases, which require warm or at least moderate weather, the snow molds like it chilly.  Their idea of a good place to live is in the top layers of unfrozen soil with a nice, insulating cover of snow.  The longer the snow stays in place, the more damage these diseases can cause to the grass.  So, typically, the farther north you live, the more likely are to have snow mold appear on your lawn.   

Pink snow mold, the flashier of the two, gets its common name from its mycelium (body) color and the fact that the spots of dead grass it makes often turn pink on the edges.  Sometimes these spots are dramatic, with bleached, matted grass blades surrounded by a bright Crayola-like band, other times the patches are more diffuse.  The weather we love to hate in the Northeast - repeated frosts, cold fogs, slow and drizzling rains, with temperatures from just above freezing to almost 60 F – are joy to pink snow mold.  When the sun comes out and things dry, pink packs it in.  Kentucky bluegrass and fine-leaved fescues show more resistance than other grasses.  Those of you who like a good scientific name will appreciate knowing this pathogen is technically called Microdochium nivale.   

Gray snow mold makes bigger patches of trouble, sometimes measuring from one to three feet across.  The grass will often be flattened, matted and a pale straw or grayish color.  But there is a little shred of good news here.  While its damage may be more dramatic than pink, gray often doesn’t kill the grass, and a lawn may have a better chance of recovery.  Interestingly, it takes a longer period of snow cover for gray snow mold to develop and spread than for pink, but gray snow mold is more common on home lawns.  Also, if a lasting snow doesn’t appear until well into January, the chances of a banner year for the snow molds decreases, but you won’t catch me feeling sorry for them.

What you do influences these creatures.  Avoid fertilizing your lawn late in the season, but keep mowing until growth stops.  Avoid making giant snow piles.  In spring, patches can be raked, re-seeded and lightly fertilized.