Snow mould is one of the most consistently reported lawn problems across Canada. It appears in the weeks after snowmelt in spring — typically as circular or irregular patches of matted, discoloured grass ranging from a few centimetres to over a metre in diameter. The underlying fungal activity occurred during the winter, under the snowpack, and the visual damage only becomes apparent once the snow is gone.
Understanding the conditions that lead to snow mould, and the modest adjustments that reduce its severity, is practical knowledge for any Canadian homeowner who maintains a grass lawn.
What Snow Mould Is
Snow mould is caused by fungal pathogens that are active at temperatures just above freezing — roughly 0°C to 7°C. These are conditions that prevail under a snowpack in many Canadian regions throughout winter. The fungi colonise grass blades and crowns, breaking down plant tissue. The damage accumulates over weeks or months of continuous cold and moisture.
The two main types encountered in Canada are grey snow mould (Typhula spp.) and pink snow mould (Microdochium nivale, formerly classified as Fusarium nivale). They have different characteristics and respond somewhat differently to management.
Grey Snow Mould vs. Pink Snow Mould
| Characteristic | Grey Snow Mould | Pink Snow Mould |
|---|---|---|
| Pathogen | Typhula spp. | Microdochium nivale |
| Appearance | Grey or straw-coloured circular patches; fluffy grey mycelium visible shortly after snowmelt | Salmon-pink or copper-toned patches; less distinct mycelium |
| Snow required? | Yes — requires prolonged snowpack | No — can occur in cool wet conditions without snow |
| Crown damage | Usually affects blades only; crown often survives | Can damage or kill the crown; more severe outcome |
| Recovery | Often self-correcting with raking and time | May require reseeding in damaged areas |
The distinction matters for response. Grey snow mould patches where the crown is intact will typically recover with light raking and air circulation — the grass blades may be dead but the plant can regenerate from the crown. Pink snow mould that has killed the crown requires overseeding to restore the stand. If the centre of a patch remains bare after four to six weeks of spring growth, the crowns were likely destroyed and reseeding is necessary.
Conditions That Promote Snow Mould
Several site and management factors increase the likelihood of significant snow mould damage:
- Long, uninterrupted snowpack. The longer snow sits on grass, the more time the fungus has to colonise. Areas where snow accumulates deeply — the north side of buildings, near snow piles from plowing — show more severe damage.
- Tall grass going into winter. Grass left long before first snowfall mats under the weight of snow, creating a dense, moist environment with reduced airflow. This is the most commonly cited contributing factor in provincial agricultural extension resources.
- Late-season nitrogen application. High-nitrogen fertilizer applied too late in autumn encourages tender, soft growth that is more susceptible to fungal damage. Nitrogen applied after mid-October in most Canadian zones remains largely unused and increases disease vulnerability.
- Thatch accumulation. A thatch layer thicker than roughly 1.5 cm retains moisture and provides the organic material the fungus uses.
- Poor drainage. Areas that collect and hold water are more likely to support fungal development.
Autumn Prevention Steps
Most of the effective prevention work happens in autumn, before the snow arrives.
Final Mowing Height
Gradually reduce mowing height through September and October, finishing the last cut of the season at approximately 5 to 6 centimetres. This height balances two competing concerns: too short and the plant loses carbohydrate reserves needed for cold hardiness; too long and the grass mats under snow.
Leaf and Debris Removal
Leaves left on the lawn trap moisture and block light even before snow falls. They compound snow mould risk by creating a humid microclimate at the soil surface. Removing or thoroughly mulching leaves before the first lasting snowfall is one of the higher-impact autumn tasks.
Avoid Piling Snow on Grass
When plowing or shoveling driveways and walkways, snow piles deposited on lawn areas extend the snowpack duration significantly in those spots. Where possible, direct snow piles to paved areas or to parts of the yard with minimal grass.
Fertilizer Timing
The autumn fertilizer application — the "winterizer" — should be completed by mid-October in most Canadian zones. After that point, a fertilizer higher in nitrogen promotes growth that can't harden off before freezing. The Ontario Ministry of Agriculture guidance on lawn care identifies late nitrogen as a contributing factor in winter disease susceptibility.
Thatch Management
If the lawn has a measurable thatch layer above 1.5 cm, autumn is a good time for core aeration to improve drainage and reduce the substrate available to snow mould fungi. Power raking (dethatching) in early autumn, followed by overseeding, can address heavier accumulations.
Spring Recovery
Once the snow has gone and the lawn surface is no longer waterlogged — typically mid-April in southern Ontario and late April to early May in Prairie cities — the following steps support recovery.
Light Raking
Gently rake affected patches with a flexible tine rake to separate matted grass blades and improve air circulation. Avoid aggressive raking that disturbs soil or tears out roots. The goal is to break up the mat and expose the crowns to light and airflow.
Observation Period
Allow two to four weeks before concluding that a patch is not recovering. Crowns that survived will send up new growth once soil temperatures warm above 8–10°C. Checking too early and reseeding prematurely wastes seed and may disturb recovering grass.
Fungicide Use
Fungicide applications are generally used on commercial turf or golf courses rather than residential lawns. For home yards, mechanical and cultural approaches (raking, good mowing height, leaf removal) are the primary tools. Fungicide use introduces cost, timing complexity, and product selection considerations that make it impractical as a standard residential approach.
When to Reseed Affected Areas
Reseeding is warranted when patches show no new grass growth after four to six weeks of spring temperatures above 10°C. For best results:
- Use a seed mix suited to the existing lawn type — a grass reference table for Canadian zones is available from Natural Resources Canada's forestry and urban tree resources section.
- Lightly loosen the soil surface in bare patches before applying seed.
- Apply seed at the overseeding rate recommended on the product label, not the new lawn rate.
- Keep the seed area moist but not saturated for two to three weeks after sowing.
- Delay first mowing of the reseeded area until the new grass reaches approximately 8 cm.
For the full spring-to-autumn maintenance sequence, see the seasonal lawn care calendar. Equipment preparation for spring, including blade sharpening and fuel system checks, is covered in the equipment maintenance checklist.