August 11, 2026 · Updated September 12, 2026

Should you cover plants during a freeze warning

A freeze warning is drawn for a whole county or forecast zone. It warns that the region could drop below freezing. It cannot tell you the low in your own yard. Whether you cover plants depends on your yard's real low, and that can sit several degrees off the regional number.

Why a warning covers more ground than your yard

The forecast low behind a warning is modeled for a broad area at a standard height above open ground. Your yard has its own terrain. Cold air is denser, so it sinks. On a still, clear night it drains to the lowest ground and pools there. A bed at the bottom of a slope can freeze while a raised bed uphill stays above freezing.

Wind and cloud change this too. A breezy or overcast night mixes the air and holds your yard closer to the regional number. A calm, cloudless night lets the ground radiate its heat to the sky, and your plants give up theirs faster than the air cools. That is why one warning ends some gardens and spares others. In our own garden the ground runs about 7°F colder than the forecast.

How cold your yard gets

To act on a warning you need your yard's offset from the forecast. There are two ways to get one.

• A manual offset. Compare a yard thermometer to the forecast low over several cold nights and enter the difference in Settings. This is the free path.
• A learned offset. A connected station subtracts the forecast from its own reading each day and averages that gap. Siltwell acts on the result once about 14 nights of station and forecast data line up.

In our own garden the learned offset is -7°F. That shifts our effective frost dates from the published last of April 28 and first of October 22 to a last of May 20 and a first of September 30. The published calendar and the wide warning both read colder than we act on, and the offset is how we bridge them. Our own last frost date runs weeks off the almanac for the same reason.

With no station yet, the microclimate checker sets tonight's forecast low against the lows nearby home stations logged over the past several nights. It tells you how far the area usually runs above or below the forecast, and shows the station count and a confidence level. It reads the area, so it points you toward a likely offset rather than a guaranteed low for one bed.

When to cover, and at what temperature

Frost forms at 32°F, where the water inside a leaf begins to freeze and rupture cells. Cover tender plants when your adjusted low reaches that mark. Tender summer crops like tomatoes, peppers, cucumbers, and squash suffer at or slightly below 32°F. Frost-hardy greens and roots like spinach, kale, and carrots hold well into the twenties, so a warning that ends your peppers may not touch them.

An offset and the area checker tell you how cold your site tends to run. They do not promise tonight's exact low in one bed. A still night with strong radiative cooling can drop a low pocket further than either number suggests, so treat the coldest, lowest bed as the one most at risk.

What to do when the warning lands

Take the forecast low, apply your offset, and read the adjusted number against 32°F for each bed. If a tender bed lands at or below freezing, cover it before sunset so the cloth traps the day's soil heat, and pull the cover once the air rises the next morning. Our guide on timing the cover walks through which nights earn the effort.