Snow mold: how to control it in your lawn.

Here it is:  the glass half-full.  As someone who suffers from intense hayfever, the cold, snowy weather and the subsequent setback of plants breaking dormancy has meant that it will be a good month or so yet before I start feeling poorly from the production of effusive amounts of pollen.

Now for the glass half-empty:  there seems to be an abundance of snow mold this year, lolling on lawns in the neighbourhood.  I’ve been sniffling and sneezing for nearly two weeks.  (Okay, I think I’m done whining now.  All expressions of sympathy can be directed to the “comments” section.  I’m big on sympathy).

So, what is snow mold and what can we do about it?  It does indeed look gross:  circular grey-white fuzzy stuff called mycelium matting all over the dormant grass.  It has a sinister quality, just lurking there on top of your lawn like a silent killing machine (not to mention, making some of us sick.  Did I mention the sneezing?).  Snow molds are fungi, and there are several different types:  the kind we probably have here in Calgary is a grey type called Typhula ishikariensis.  Fortunately, despite its appearance,  it’s not likely to cause any sort of permanent damage to turf, as it only affects leaf tissue.  (There is a pink snow mold called Fusarium nivale that can be deadly, as it destroys the crowns of plants affected.  Thankfully it’s not as common as grey snow mold).  Snow mold occurs whenever there is an extended period of snow cover on ground that is not solidly frozen.  The fungus actually lies dormant during the long, hot days of the summer, taking a siesta on the leaves of grasses as tiny black specks called sclerotia.   Any time you mow or walk across the lawn, these miniscule sclerotia go for a ride and gleefully spread, carried on your shoes or your dog’s paws or the mower blades.  Once the temperatures drop to the freezing mark in late fall, the sclerotia spring into action, germinating and producing thousands of spores.  Beneath the cozy blanket of winter snow, the fungus munches on dead organic matter and thrives until exposure in the spring.  (Snow mold cannot freeze because it manufactures special enzymes that keep it from doing so).

While there’s not much you can do about the fungus, as once it’s on the lawn, it’s pretty much taking up permanent residence, you can keep it from making a mess in the spring (and giving me sniffles).  Make sure you clean up all of your leaf litter in the fall, as the fungus holds dead leaves in the same high esteem as I would a gigantic hot fudge sundae.  And keep your lawn short – don’t forgo that last mowing of the season, and don’t leave clippings on the lawn after that final cut.  Don’t fertilize too late in the season, as a huge nitrogen spike at the wrong time can actually contribute to serious winter kill of your turf, and in less extreme cases, aid in the growth of snow mold.  And don’t walk on the lawns in the winter, if you can help it – even what seems to be slight compaction on top of the snow can motivate fungal production.  In late spring, it may be a good idea to spread the snow banks around, dispersing snow over the entire lawn to aid in a more “even” melt.

Even meticulous grooming might not prevent snow mold from rearing its ugly mycelium in the spring, but the fix is easy:  let it be.  No, seriously.  The fungus hates heat and drought and it will go dormant as the weather improves.  You can also rake the grass with a fine-tined tool and that will help immensely (but wear a mask if you don’t want to inhale the spores and sneeze repeatedly.  Like me).  I have also been told that you can use the high pressure setting on the nozzle on your garden hose and give the grass a good rinse – but do it when the day promises heat and drying winds.  If, for some reason, the lawn can’t recover from the effects of the mold, you will need to consider a prescription of topdressing and overseeding, but that’s a rare occurrence.

Glass half full:  snow mold means spring has arrived.  How fantastic is that?!


Dr. Tom Hsiang, a professor of the Department of Environmental Biology at the University of Guelph, has written some interesting stuff about snow mold at

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