August 5, 2026

Protecting Tomatoes From Frost: Cover on the Night It Actually Freezes in Your Yard

Most tomato-frost advice stops at "cover them when frost is forecast." Fair enough — a sheet or frost cloth over the plant on a cold night is the whole technique. But tomatoes are frost-tender, so one wrong call kills the plant outright, and the number you're betting on — the regional forecast low — is often two or three degrees off from what your actual yard records. The real question isn't how to cover. It's which night.

Tomatoes are all-or-nothing with frost

Some plants shrug off a light frost. Tomatoes don't. They're a warm-season crop with no cold tolerance to speak of — leaves blacken and collapse at around 0 to 2°C, and a genuine frost usually kills the plant to the ground overnight. There's no partial credit, no bouncing back next week. That's what makes the stakes so lopsided: covering costs you five minutes and an old bedsheet, while not covering on the wrong night costs you the plant and the rest of its fruit.

Because the downside is so severe, the sensible move is to cover whenever a frost is genuinely possible. But "possible" is doing a lot of work in that sentence — and this is exactly where a generic forecast lets you down.

The forecast low is a regional number, not your yard's

The overnight low on your weather app is a broad-area figure, usually anchored to an airport or a grid cell that can be kilometres from your beds. On a still, clear autumn night, cold air drains downhill and pools in low spots. A garden at the bottom of a slope, or boxed in by fences and buildings, can sit several degrees colder than the regional number — cold enough to frost when the forecast confidently said 4°C.

That gap is the whole problem. "Cover when frost is forecast" fails silently on the night the area forecast reads 3°C and your tomato patch quietly drops to freezing. You wake up to a blackened plant and a forecast that technically never called for frost. The fix isn't a better regional forecast — it's knowing how your yard behaves relative to it.

How Siltwell warns you on the night your yard actually freezes

Siltwell's frost alert works off a threshold you set for your garden, and it looks at tomorrow night and the night after. When the expected low drops to or below your threshold, it sends a critical alert telling you to cover tender plants. The important part is which low it uses:

• On a Pro connected station, Siltwell compares your station's readings against the Open-Meteo forecast and learns your site's bias — how much colder or warmer your yard runs. Once it has that, the alert is adjusted to your station, so a cold low spot gets warned even when the area forecast wouldn't.
• On the free tier, there's no connected station, but you can enter a manual temperature offset in Settings if you already know your yard runs, say, two degrees cold. The alert then adjusts for your site using that number.
• With neither, you get the plain forecast-based alert against your threshold — still useful, just without the yard-specific correction.

One honest number worth knowing: the learned offset isn't instant. Siltwell only trusts it after about 14 overlapping days of station-versus-forecast data. Before that, it leans on the plain forecast (or your manual offset). So if you connect a station in the last week before your first expected frost, you may be entering that first cold snap on the un-adjusted number — connect earlier in the season if you can.

So do you cover every night in fall?

No — and you shouldn't want to. Covering every night is a nuisance, and leaving cloth on plants pointlessly can trap dampness and block light. The goal is to cover only on the nights that actually threaten frost, which is precisely what a yard-tuned alert gives you: a heads-up the evening before a genuine cold night, and silence on the nights that are fine.

When you do cover, drape the cloth or sheet all the way to the ground rather than wrapping just the plant — you're trying to trap the day's warmth radiating up from the soil, not insulate the leaves. Take it off in the morning once the air warms. Late in the season, if a killing frost is coming and the plant has done its work, it's often smarter to harvest the mature green tomatoes and ripen them indoors than to fight every night to the bitter end.

Check your microclimate before you buy anything

Curious whether your yard even runs cold enough to worry about before spending on hardware? The public microclimate checker compares the forecast low to what nearby home stations recorded over recent nights and tells you how much colder or warmer your area tends to run, along with how many stations it found and a confidence level. It's an area indicator, not a guaranteed date for your exact tomato bed — but if it shows your area running a couple of degrees below the forecast, that's your answer about why the app's low keeps lying to you.