September 11, 2026 · Updated September 12, 2026

Frost cloth for plants and how long to leave it on

Frost cloth traps part of the heat your soil radiates back to the sky at night, so it buys tender plants a few degrees and no more. It helps most on still, clear nights and barely at all when the wind blows. Which material you pick and how long you leave it on both change how much it does.

How frost cloth holds heat

After sunset the ground gives back the heat it soaked up all day, and it gives it straight to the open sky. On a still, clear night your plants lose heat faster than the air around them cools, so leaf surfaces can freeze while a thermometer at head height still reads above freezing. This is radiative frost, and it is the kind a cover fights.

Frost cloth works as a lid over that escaping heat. It slows the loss from the ground and the plant, and it holds a pocket of slightly warmer air underneath. Wind breaks that pocket, which is why a cover does little on a blustery night. For how many degrees different covers hold, see our guide on frost blanket degrees and the right night.

Which material to use

The material decides both how well the cover traps heat and whether it can stay on.

• Spunbonded row cover. Light, breathable polypropylene made for this job. It holds heat while letting light, air, and water through, so it can sit over a bed for days.
• Old sheets and blankets. They trap radiated heat well for one night. Dew soaks into them and they block light, so lift them off at sunrise.
• Plastic sheeting. The weakest choice. Plastic holds little heat, and where it touches a leaf it conducts cold straight through, so it damages the tissue it rests on.

How long to leave it on

Breathable cloth can stay through a cold spell because the plant still gets light and water underneath it. Weight it at the edges so the warm pocket doesn't leak, and lift it once the nights turn mild so growth doesn't stall in the shade.

Plastic and any sealed cover must come off each morning without exception. A closed cover on a sunny day builds heat fast, and the same trap that saved the plant overnight now bakes it. When the freeze risk passes for the season, store the cloth rather than leave it slumped on the bed.

The nights your yard freezes

A cover only earns its keep on the nights your own yard drops below the plant's threshold, and the regional forecast rarely matches your yard. Cold air weighs more than warm air. On still nights it slides downhill and settles in low spots, so a sheltered dip can hold several degrees below what your phone shows. Cover on the wrong nights and you shade the plant for nothing; skip the right one and you lose it.

In our own garden the station reads 7°F colder than the forecast, so the published last-frost date of April 28 sits closer to May 20 for us. We recorded our last spring frost on April 30 and our first fall frost on October 3. Those gaps are why we deploy cloth against our own measured low.

Our public microclimate checker shows how much colder your area runs. It sets the forecast low against what nearby home stations logged over recent nights, then reports the gap along with how many stations it counted. It is an area indicator, so treat it as a lean rather than a guaranteed date for your exact spot. To learn a per-yard offset, a connected station watches your own site over about 14 overlapping days before Siltwell trusts the number; a free account applies a manual offset you enter yourself.

Tonight the step is simple. Check tomorrow's low, adjust it toward your yard's real reading, and if that adjusted low sits below what your tender crops can take, cover them with breathable cloth before sunset. For the timing side of this, read when to cover plants with frost cloth.