Snow Mould: How a Good Fall Cleanup Helps Your Lawn

Snow mould is a lawn fungus that grows under the snow and shows up after the melt as matted grey or pink patches. A good fall cleanup is the best way to cut the risk. Keep mowing well into fall, clear the leaves off the grass, and cut and rake the lawn before the snow arrives. Most lawns recover on their own in spring.

What is snow mould?

Snow mould is caused by a few types of fungi. The University of Saskatchewan says they handle cold well and like wet conditions. You don't see them in the fall. They show up in the lawn after the snow melts.

It can be mild one year and bad the next. It depends on the winter. It isn't toxic, but USask says it can bother people with allergies or breathing problems.

There are two main kinds. Iowa State University Extension names them:

• Grey snow mould. Caused by a fungus called Typhula. Patches can be a few inches to a few feet across.
• Pink snow mould. Caused by a fungus called Microdochium nivale. Patches tend to be smaller, under 15 cm (6 inches) across.

Both kinds start the same way. The fungus grows in the cold, wet, dark space between the snow and the grass.

What does snow mould look like in spring?

You'll spot it as the snow pulls back in late winter or early spring. Iowa State describes round, straw-coloured patches. The grass in them is often matted down and wet.

Look closely and you may see fuzzy threads on the grass. USask says these can be white, grey or even pink, depending on the fungus.

Grey snow mould has one more clue. Penn State Extension says it leaves white or tan crusted patches that look like papier mâché. Tiny reddish-brown dots form on the leaves and crowns. They're 0.5 to 2.5 mm across, about the size of a pinhead.

Patches often show up first where snow sat the longest. That's usually beside the driveway, along the sidewalk or where drifts pile up.

Why does snow mould grow under the snow?

It needs a few things at once. Penn State lists the best conditions for grey snow mould:

• Temperatures just above freezing where the snow meets the grass.
• Wet grass blades.
• Darkness under the snow.

Penn State also says grey snow mould seldom shows up unless there's snow cover on ground that isn't frozen. Iowa State says the same: long snow cover on soil that isn't fully frozen sets the stage.

That's why the deep snow matters. Penn State says the deeper the snow and the longer it stays, the worse the damage tends to be. Once the snow melts away, the disease slows down.

That sounds like most of the Prairies and the north every winter. You can't change the snow. But you can change what's sitting on the grass when it falls.

What makes a lawn more likely to get snow mould?

USask lists the conditions that favour it. Three of them come down to how the lawn goes into winter:

• Long, matted grass. Lawns left long and floppy over winter.
• Leaf litter. Too many leaves left on the lawn in the fall.
• Piled snow. Spots where snow gets heaped up all winter.

Leaves are the big one. Purdue University's turf team says a thick layer of leaves holds in humidity that leads to snow mould. At best, it thins the grass. At worst, it kills it right down to the ground.

Grass type plays a part too. Penn State says Kentucky bluegrass and fine fescues get hurt less by grey snow mould than perennial ryegrass and tall fescue.

How does a fall cleanup prevent snow mould?

USask puts it simply: cut and rake the lawn before snow arrives in the fall. Iowa State and Penn State add a few more steps. Here's what that looks like in your yard.

Keep mowing well into the fall

Don't park the mower early. Penn State says mowing well into the fall helps lessen grey snow mould. Iowa State says to mow at the right height all through the fall.

What's the right height? Iowa State suggests dropping the mower to 6 to 7.5 cm (2.5 to 3 inches) in mid to late September. Never cut off more than one third of the blade at once. Then keep cutting until the grass stops growing. Our post on when grass stops growing explains how to tell.

Get the leaves off the grass

Iowa State lists raking up fallen leaves as one of the main ways to cut snow mould. Don't wait for the last leaf to drop. A wet mat of leaves under the first snow is exactly what the fungus likes.

A light layer can be chopped up with the mower instead. Our guide on mulching leaves with a lawn mower shows how. Heavy piles need to come off.

Do the last cut and rake right before the snow

This is the step people skip. Leaves keep falling after you think you're done. The last pass should happen as close to the first lasting snow as you can manage.

Walk the yard and check these spots:

• Along fences and under hedges, where leaves blow in and pile up.
• Beside the driveway and walk, where shovelled snow will sit all winter.
• Low corners that stay damp.
• Under trees that drop their leaves late.

Our fall lawn cleanup checklist covers the rest of the jobs to finish before winter.

Where should you pile snow to protect the lawn?

This one is up to you once winter hits. Penn State says to avoid making long-lasting snowbanks on the grass when you clear the driveway and sidewalk. Iowa State says to spread out big snow piles so they melt faster.

So when you shovel, don't dump it all on one patch of lawn every time. Spread it around. In spring, break up any big leftover pile so the grass under it sees the sun sooner.

What should you do if you find snow mould in spring?

Don't panic. USask says snow mould goes away on its own once the weather turns warmer and drier. Lawns usually recover well. Iowa State says the damage usually isn't serious. The patches green up, just slower than the rest of the lawn.

Penn State says grey snow mould usually kills the leaf blades but not the crowns and roots. That's why the grass grows back.

You can help it along:

• Rake the patches gently. Iowa State says this helps them dry out and stops more fungus growth.
• Wait for a dry day. USask says to rake when the lawn is dry.
• Wear a mask if you have allergies. USask suggests it for anyone sensitive.
• Break up the crust. Penn State says raking breaks up the crusted, matted leaves.

A spring cleanup does this along with the rest of the yard. It clears the winter mess and lets the lawn dry out.

One note on sprays: our partner crews don't spray, and Golden Oak doesn't give advice on chemicals.

Is snow mould worse on the Prairies and in the north?

It shows up wherever snow sits on the lawn for a long time. That's a big part of Canada. Long winters with steady snow cover set the stage, as Penn State and Iowa State describe.

So if you live in Saskatoon, Red Deer or Sudbury, the fall cleanup matters even more. Our Saskatoon and Alberta pages cover lawn care in those places.

Can a local crew do the fall cleanup for you?

Yes. Timing is the hard part. The last cut and rake should land right before the snow, and that's often a cold, busy week.

Golden Oak Lawn Care matches you with a vetted, insured local crew in your city. They do the work. We stand behind it with the Golden Oak Guarantee.

Your crew can keep up your lawn mowing into the fall, then do the fall cleanup. That means leaves off the grass and out of the beds before the snow. You get the same crew every visit and a photo after every cut.

We price every lawn by its size from satellite images, so no one has to come by. The price depends on lawn size, how many leaves you have, slopes, gates and whether the leaves get hauled away. Book the season with the Worry-Free Lawn Season and your first cut is free.

Want it handled? Get your price in 60 minutes, or call 1-866-904-LAWN.

Snow Mould Questions

Sources: University of Saskatchewan snow mould, Iowa State snow mould, Penn State Extension grey snow mould, Iowa State fall lawn care, Purdue University leaf mulching.