October 17, 2026

What happens inside plants freezing on a cold night

When a plant freezes, the first ice forms in the thin film of water between its cells. That ice pulls water out of the cells until they shrink and their membranes tear. The leaf can also be colder than the air your forecast reports. On a clear, still night a leaf loses heat to the open sky faster than the air around it cools, so frost can reach your plants while the forecast low sits above freezing.

Where the ice forms first

Water inside a plant sits in two places. Some is inside the cells, carrying sugars, salts and proteins. The rest sits in the cell walls and the gaps between cells, and it carries far less dissolved material. Dissolved material lowers the freezing point, so the water between the cells freezes first.

Once ice forms there, it draws water toward itself. Ice holds water molecules more tightly than liquid water does, so water moves out through the cell membrane and freezes onto the growing crystals. The cell loses water the way it would in a drought, only faster. It shrinks, its contents thicken, and its membranes press against the ice.

Most frost damage comes from that dehydration. Ice that forms inside a cell is rarer. It happens when tissue cools fast, and it kills the cell outright.

Plant tissue can stay liquid below the freezing point for a while, a state called supercooling. Something has to start the first crystal. Dust, frost on the leaf surface and some common leaf bacteria all do it. Once one crystal forms, ice spreads through the wet tissue quickly.

Why the damage shows after the thaw

A frozen leaf often looks fine at dawn. The damage shows as it warms. Membranes that tore during the freeze can no longer hold the cell's contents. As the ice melts, water floods the gaps between cells and stays there. The leaf turns dark, glossy and limp, a look gardeners call water-soaked. Over the next few days that tissue browns or blackens.

If you are working out whether last night reached your beds, we cover how to read the signs in frost damage on plants.

Why hardy plants survive the same night

Hardy plants freeze too. They survive because they tolerate the dehydration. Over weeks of cool weather they harden. Their cells build up sugars and other solutes, which lower the freezing point inside the cell and hold water against the pull of the ice. Their membranes change makeup so they stay flexible when squeezed.

Tender crops from warm climates, such as tomatoes, peppers, cucumbers and squash, never evolved that response. A frost that spinach shrugs off can kill them. Hardening also takes time. A sudden cold night after a warm spell catches even hardy plants before they adjust, so an early fall frost often does more harm than a colder night in midwinter.

Why leaves freeze above the forecast low

A forecast low is an air temperature. It describes air in a ventilated shelter above the ground, modeled across a patch of land far larger than your yard. Your leaf sits somewhere else.

After sunset every surface radiates heat toward the sky. Clouds send some of that heat back. On a clear night nothing returns it, so leaves and the ground surface cool below the air above them. On a still night no wind mixes warmer air down to replace what they lost. The air at forecast height can stay above freezing while ice forms on the leaves below it.

Cold air also moves. Because it weighs more than warm air, it slides downhill on calm nights and collects in hollows, along fences, and against walls at the foot of a slope. A garden in a low spot sits in that pool. The model behind your forecast does not see the hollow.

These effects stack. A low bed on a clear, calm night can run several degrees under the forecast before the leaf effect adds anything. We cover how to find that number for your own beds in frost temperature for plants.

What our garden recorded

Our station runs 7°F colder than the forecast. Siltwell learned that offset by comparing our station's nightly lows with the model's lows for the same nights.

The offset moves our frost dates. For our area, the published date for the first fall frost is October 22. Applying the offset moves it to September 30. In 2025 our station logged its first fall frost on October 3, which beat the published date by 19 days. In spring the published last frost is April 28, and we recorded frost on April 30 this year. The offset puts our last frost at May 20.

We set peppers and cucumbers out on May 22 and the squash on June 1, after that adjusted date. A plan built on the published April 28 date would have put those transplants in the ground ahead of the frost we recorded.

Suppose the forecast for tonight says 38°F. With our offset, our station would read near 31°F, and leaves on a clear night would sit colder than the station. Our offset averages the air at one station over recent nights. It cannot tell you how cold a particular leaf got, and a bed in a low corner can run colder than the station that set the number.

How frost alerts use your offset

Every garden gets Siltwell's frost alert. At 5 p.m. local time each day, the alert reads the forecast lows for the next two days. If an adjusted low reaches your frost threshold, 0 °C by default, you get a banner in the app plus a push and an email, unless you turn those off.

The adjustment is your microclimate offset. You can enter it yourself in Settings, under Edit garden. A negative number marks a yard that runs colder than the forecast.

Connect a weather station and Siltwell works out the offset for you from the station's outdoor air readings. It compares your station's nightly low with the model's low and trusts the result after about 14 overlapping days. Soil sensors play no part in it.

Without a station of your own, you can use the microclimate checker, which sets the model's lows for all recent nights against readings from home weather stations near you. It returns a range and a median across those stations. Treat the result as an area indicator for your neighborhood, since your own low corner can still run colder.

To find your own offset, put a thermometer that records the overnight minimum at bed height in your lowest bed. Compare its reading with the forecast low on the next few clear, calm nights, and enter the gap in Settings. Your frost alerts use it from the next afternoon's check.