September 7, 2026

Frost Covers for Plants: What Each Type Buys You in Degrees — and the Nights You Actually Need One

Search "frost covers for plants" and you get buying guides that rank fabric by price and GSM, as if the thickest blanket is always the right answer. But a cover doesn't heat anything — it traps a few degrees of ground warmth and slows your plants from radiating their heat to a cold sky. That means the real question isn't which cover is thickest; it's whether this night, in your yard, is cold enough to bother covering at all.

What a frost cover actually does

All day, the soil soaks up warmth. After sunset, that heat leaks back out — and on a still night, plants shed their own warmth upward to the open sky faster than the air around them cools. That's why frost forms on leaves and car roofs before the air thermometer reads freezing. A cover interrupts that: it puts a lid over the plant so the ground's warmth stays trapped underneath and the leaves stop radiating to the sky.

The practical upshot is that a cover buys you a modest, borrowed few degrees — not a furnace. It works best when it reaches the ground on all sides to seal in that rising warmth, and worst when it's draped tight over the foliage like a flag. Heavier fabric holds more heat than thin cloth, which is the real difference between a bedsheet and a proper frost blanket.

What each type buys you

• Lightweight frost fleece: breathable, cheap, and easy to leave on for a run of cold days. It buys a couple of degrees — enough for a light, brief frost, and you can water through it.
• Heavy blankets or doubled fabric: more trapped warmth, more degrees, but heavy enough to crush soft growth if it rests directly on the plant. Use hoops or stakes to hold it off the leaves.
• Cardboard boxes and buckets: surprisingly good for a single hard night over a small plant — a rigid air pocket that traps ground heat well. Uncover in the morning.
• Plastic sheeting: cheap and windproof, but it conducts cold wherever it touches a leaf and can cook plants once the sun hits. If you use it, keep it off the foliage and pull it at dawn.

Notice that every one of these ties back to the same thing: they add a fixed handful of degrees on top of whatever your low actually is. Two degrees of protection saves a plant on a 30°F night and does nothing on a 24°F night. So the cover you need is decided by the number you don't usually know — your real overnight low.

The night matters more than the cover

This is the gap in every buying-guide SERP: they compare fabric and never tie the choice to how cold your specific yard actually gets. A regional "freeze warning" is issued for a wide area at the nearest official station — often an airport miles away, at a different elevation, over different ground. Your yard might sit in a cold pocket that runs several degrees colder, or a sheltered spot near the house that barely frosts. The forecast low and your yard's low are two different numbers.

Get that wrong in either direction and you lose. Trust an optimistic forecast and skip the cover on a night your low spot dips below the danger line — dead plants. Cover reflexively every night the app flashes a warning and you're out there wrestling fabric on nights that never threatened anything. The honest move is to match the cover to the low your yard will really hit, and to only deploy it on the handful of nights that actually reach the danger zone.

How Siltwell narrows the guess

Siltwell's frost alerts work off the overnight low adjusted to your site, not the regional forecast. If you enter a manual temperature offset in Settings on the free tier, its frost alert reads "adjusted for your site." If you're on Pro with a connected weather station, Siltwell compares your station's readings to the forecast and learns your yard's bias automatically — but only after about 14 overlapping station-and-forecast days, so it isn't an instant number. Once that calibration is trusted, the alert tells you the adjusted low for tomorrow or the night after and says "adjusted to your station."

That's the piece that decides whether tonight is a cover night. If your station shows the yard runs 3°F colder than the forecast, a broadcast "low of 34°F" is really a 31°F night for you — cover the tender stuff. If your spot runs warm, you can skip the scramble the app-less neighbor is doing. For the deeper mechanics of matching gear to your real low, the frost blanket guide and protecting plants against your real low both go further.

One more practical rule the buying guides skip: with breathable fabric you can cover at dusk and leave it through a run of cold days, but pull sealed plastic off in the morning before the sun turns the pocket into an oven. And if you're deciding what else to throw over a bed in a hurry, the what-to-cover-plants-with guide runs through the household options.