August 11, 2026

How Many Degrees a Frost Blanket Really Buys You — and the Night You Actually Need It

Search "frost blanket for plants" and every result sells you the fabric and repeats the same vague promise: it adds "a few degrees." True enough — a frost blanket really does buy a modest, genuine buffer. What none of those pages tell you is the part that decides whether the blanket works: what your yard's actual overnight low is going to be, and which night crosses the line. The fabric buys degrees; your real low decides if that's enough.

What a frost blanket actually does

A frost blanket doesn't heat anything. During the day the soil soaks up warmth; at night that heat radiates back out and escapes to the sky. A cover traps some of that rising warmth in a pocket of air around the plant, so the leaves stay a little warmer than the open air. That's the whole mechanism, and it's why the protection is real but modest — usually a few degrees, more with heavier fabric, less on a windy night that strips the warm air away.

The practical upshot: a blanket shifts the odds, it doesn't set a guarantee. Lightweight spun row cover leaves more light through and can stay on for days; a heavier blanket traps more warmth for one hard night but shades the plant, so you'll pull it off after. Either way the number that matters isn't printed on the packaging — it's the low your yard will actually hit tonight.

Why "a few degrees" is meaningless without your real low

Here's the gap every listicle skips. Say a blanket buys you three degrees. Whether that saves your tomatoes depends entirely on where your overnight low lands. If the forecast says 1°C and your yard runs true, the fabric is overkill. If your low-lying, cold-pooling backyard actually drops to −3°C while the regional forecast still reads 1°C, that same blanket lands you at freezing anyway — and you'll wake up to damage while wondering why the cover "didn't work."

The forecast is for a wide area, usually anchored to an airport or a grid cell miles away. Your yard is its own microclimate: a hollow that collects cold air, a wall that radiates stored heat, a patch shaded from the afternoon sun. The blanket's few degrees only mean something once you know the number they're being subtracted from.

Naming the night, not just the fabric

This is where a connected weather station changes the job. When you connect a station on Siltwell Pro, it starts comparing its own readings to the forecast for your location. After about 14 overlapping days of data, Siltwell trusts the learned bias between your site and the forecast — roughly how much colder or warmer your specific yard tends to run.

Once that offset is learned, a frost alert doesn't just parrot the regional forecast. It fires when the adjusted overnight low for tomorrow or the day after drops to your garden's threshold, and it says so — "Overnight low X°C expected Thursday (adjusted to your station). Cover tender plants." That's the sentence that tells you which night to actually throw the blanket on, rather than covering every borderline night out of anxiety or, worse, skipping the one that mattered.

• Free tier: you enter a manual site offset in Settings, and frost alerts read "(adjusted for your site)." Honest, but it's your estimate.
• Pro with a connected station: the offset is learned from your station versus the forecast over ~14 days, so the alert is anchored to real measurements at your yard.
• No offset yet: Siltwell still alerts on the plain forecast low — it just won't claim to know your site until it does.

How to use both together

The blanket and the alert do different jobs, and they're strongest paired. The alert tells you which night your yard dips below the line; the blanket buys you the margin on that night. Knowing your yard runs, say, three degrees colder than the forecast tells you whether one layer is enough or whether you need to double up, prop the cover on hoops so it isn't crushing the foliage, and get it on before dusk while there's still ground heat to trap.

If you're still deciding whether your yard even needs the fuss, the public microclimate checker compares tonight's forecast low to what nearby home stations recorded over all recent nights, and tells you how much colder or warmer your area tends to run. It's an area indicator with a station count and confidence — not a guaranteed reading for your exact bed — but it's a far better starting point than the raw forecast. For the timing side of things, the frost cloth timing guide gets into when to cover and uncover.