Frost and your lawn: what actually causes damage

“Stay off frosted grass” is one of the few pieces of lawn advice where the mechanism is simple and the damage is genuinely visible afterwards.

What happens inside the leaf

A grass leaf is mostly water held inside cells. When the leaf freezes, ice forms first in the spaces between those cells, then, if it gets cold enough, inside them.

Ice crystals are rigid and sharp-edged. An unfrozen leaf bends under your foot and springs back up. A frozen one cannot bend, because the cell walls are braced by ice, so the pressure ruptures them instead. The contents leak out and the tissue dies.

None of this shows on the day. You see it three or four days later, when the crushed tissue browns off and your footprints appear across the lawn in dead grass. On a lawn walked over regularly during a cold spell, the path stays visible for weeks, because the grass has to grow out entirely and it is not growing. It is winter.

Mowing frosted grass does the same thing across the whole lawn at once, with the tearing that comes of cutting brittle leaves on top.

Grass frosts before the air does

Here is the part that catches people out. The thermometer says 3°C, so there cannot be a frost. There is one on the lawn all the same.

On a clear, still night the grass canopy radiates heat to the sky and gets almost nothing back. It cools below the air around it, typically by three or four degrees when it is clear and calm. Forecasters treat this as a separate quantity, the grass minimum temperature, as distinct from the air minimum.

Two things suppress it. Cloud radiates heat back down, so an overcast night produces little or no ground frost even at low air temperatures. Wind mixes the air near the surface and keeps the canopy closer to air temperature.

Which means the frost risk is highest on exactly the nights that look most pleasant from a window: clear and still.

When it lifts

Frost lifts once the canopy warms back above freezing, which in practice means once the sun is properly on it. On a clear winter morning that is a couple of hours after sunrise, earlier where the sun reaches first and later in the shade.

That last point deserves stating plainly, because it is where the damage happens: parts of your lawn thaw at different times. The strip along a north-facing fence or under a hedge can stay frozen through most of a short winter day while the middle of the lawn is fine.

What to do

Keep off it while it is frosted, and that includes the dog and the route you take with the wheelie bin.

Do not mow it. There is no mower setting that makes this acceptable.

Wait for it to thaw properly rather than merely to look green again, because the shaded parts lag behind the rest.

Do not try to melt it with water. You are adding water to a lawn that will freeze again tonight.

And do not panic about the frost itself. Cool-season grass is well adapted to freezing. The damage comes from traffic on frozen leaves, not from the cold.

The one that surprises people

A hard frost that sits all day is, oddly, safer than a light one, because nobody is tempted to walk across an obviously white lawn.

The dangerous morning is the marginal one. The frost has half-lifted, the lawn looks merely damp, and the shaded third of it is still frozen solid.

Check whether your lawn is frosted right now. We estimate canopy temperature rather than just reading air temperature, so the model catches the clear, still mornings a thermometer would miss.