Snow Mould on Lawns
The matted patches that show up as the snow melts: what they are, whether the grass will come back, and what to change in fall.
Snow mould on lawns is a group of fungal diseases that grow in cold, wet conditions under snow. The University of Massachusetts describes the most common ones as grey snow mould and pink snow mould (also called Microdochium patch), and notes they appear as matted, straw-coloured or grey patches once the snow melts.
The good news, from the same source: in most cases only the leaves are killed, and the grass grows back from the crowns.
Snow mould at a glance: At snowmelt, rake matted patches to let them dry and reseed only where nothing regrows. To prevent it, keep mowing until the grass stops growing, avoid heavy nitrogen in late fall, keep thatch down, and don’t pile shovelled snow on the lawn.
Planning fall lawn work? See the Lawn Care Calendar for Canada for when the last mow and fall feeding fit.
Grey or Pink?
| Grey snow mould | Pink snow mould | |
|---|---|---|
| Looks like | Circular patches 30–90 cm across, yellow to grey-brown, matted; grey lint-like growth at melt | Smaller spots, usually under 20 cm; bleached tan, often with a pink edge |
| Favoured by | Long snow cover over unfrozen soil | Cool, wet weather; can occur without snow |
| Small dots on leaves | Yes, brown sclerotia | No |
From the University of Massachusetts. Washington State University adds that pink snow mould (which it calls Fusarium patch) is common in cool, wet spring and fall weather in the Pacific Northwest, which is why coastal BC lawns can show it with little or no snow.
What to Do at Snowmelt
Rake the matted patches lightly so they dry and warm. The University of Massachusetts recommends promoting fast drying and warming in spring, a light feeding to encourage new growth, and reseeding only where regrowth does not come. Washington State University notes the grass often recovers by itself when the weather changes.
Prevent It in Fall
The University of Massachusetts lists the habits that matter: avoid heavy nitrogen in late fall; keep mowing until growth stops so a tall canopy does not mat down; avoid excessive thatch; do not pile shovelled snow on the lawn; keep foot traffic off snow-covered turf; and improve drainage. It also recommends a soil pH of 6.5 to 6.9 and adequate potassium against pink snow mould.
Your average first fall frost is a useful marker for wrapping up fall feeding; see when to fertilize your lawn.
Reseeding Damaged Patches
Where snow mould has killed patches outright, the University of Saskatchewan lists snow mould among the kinds of damage repaired by overseeding. Grass seed germinates best once soil is above 10°C, so wait for daytime temperatures of about 15°C. It says spring and late summer to early fall are good times to seed on the Prairies, and for small repairs it suggests spring, so weeds do not take over the bare spots first.
It also recommends raking the lawn well in spring to clear leaves and debris from the year before.
Frequently Asked Questions
What is snow mould?
A group of fungal lawn diseases that grow in cold, wet conditions, often under snow. Grey and pink snow mould are the most common.
Will grass come back after snow mould?
Usually. The University of Massachusetts says in most cases only the leaves are killed and the grass regrows from the crowns.
What should I do about snow mould in spring?
Rake the matted patches so they dry out, give a light feeding, and reseed only areas that do not regrow.
How do I prevent snow mould?
Keep mowing until the grass stops growing, avoid heavy nitrogen in late fall, keep thatch down, and do not pile snow on the lawn.
Why does my lawn get snow mould without much snow?
Pink snow mould can develop in any cool, wet weather. Washington State University notes it is common in spring and fall in the Pacific Northwest.
Should I spray a fungicide after snowmelt?
The University of Massachusetts says fungicides applied after snowmelt are ineffective, and Washington State University does not recommend them for home lawns because grass often recovers.
Sources
- University of Massachusetts Amherst Extension — The Snow Molds
- Washington State University Hortsense — Lawn and Turf: Microdochium patch (Pink snow mold)
- University of Saskatchewan, Gardening at USask — Lawns