Why frost is not the same as freezing
Water freezes at 0 °C (32 °F). Frost regularly appears when the reported air temperature is 2, 3 or 4 degrees above that, and the reason is where the measurement is taken.
Official air temperature comes from a shaded, ventilated sensor about 1.5 metres above open ground. That is a sensible standard, and it is not where your plants are.
On a clear night every surface radiates heat directly to space. With no cloud to return it, the ground, the grass, the leaves and your car roof all cool faster than the air, and end up two to four degrees below it. So a reported 3 °C can mean a leaf surface at −1 °C, with ice on it.
This is also why frost appears on a car roof and a lawn while the pavement beside them stays clear — thin surfaces with a clear view of the sky cool fastest, while paving is fed heat from the mass of ground beneath it.
The three conditions
Frost needs all three. Any one missing and it usually does not happen.
A low enough temperature. Below about 4 °C (39 °F) reported, frost becomes possible. Below 2 °C it becomes likely.
A clear sky. Cloud is the single biggest factor. It absorbs the heat radiating from the ground and returns much of it, and an overcast night at 1 °C frequently produces no frost at all.
Light wind. Wind mixes the cold surface layer with warmer air above and prevents the cold from concentrating at ground level. A breeze of even 10 mph often saves a crop that a still night would have taken.
Which gives the rule worth remembering: clear, calm and below 4 °C means expect frost. Cloudy or breezy at the same temperature usually means you are fine.
What the official warnings mean
| Warning | Roughly | What it threatens |
|---|---|---|
| Frost advisory | 0 to 2 °C (32–36 °F) | Tender plants, soft new growth, unharvested crops |
| Freeze warning | −2 °C (28 °F) or below | Most annuals, irrigation, exposed plumbing |
| Hard freeze | −4 °C (25 °F) or below, for hours | End of the growing season; risk to pipes |
Two things follow from the table. The advisory threshold is set above freezing precisely because of the radiation effect described above — the forecasters are accounting for the gap between air and surface. And a freeze warning is the point where the concern widens beyond plants: at −4 °C and below you are also into the territory covered by what temperature pipes freeze at.
White frost and black frost
White frost — hoar frost — is what people picture. There is enough moisture in the air that water vapour deposits directly as ice crystals on the cold surface. It is visible, and its visibility is useful: you can see exactly which parts of the garden froze.
Black frost happens when the air is very dry. The temperature falls below freezing but there is not enough moisture to form crystals, so nothing appears. Plant tissue freezes and dies with no visible ice at all, and the damage shows up the next day as blackened, water-soaked foliage.
Black frosts catch people out because the morning looks clear and unremarkable. If the forecast low was below freezing and you see no frost, do not conclude nothing happened.
Which parts of your garden go first
The frost line through a garden follows radiation and cold air drainage, and it is remarkably consistent year to year.
Frosts first: open lawn, raised foliage, the lowest ground, anything behind a solid fence across a slope, and the far end of a garden away from the house.
Frosts last: against a south or west-facing wall, under a tree canopy or pergola, beside a large body of water, close to the house, and partway up a slope rather than at the bottom.
The valley-bottom effect is worth understanding properly, because it is often the difference between losing a plant and not — temperature inversions explains why cold air behaves like a liquid on a still night.
Not the low itself, but the low together with the sky. A forecast of 2 °C and clear is a frost night. A forecast of 0 °C with cloud and a breeze often is not. If the app shows you the low, the sky condition and the wind, you have everything the decision needs.
What to do when the warning comes
Briefly, because it has its own page:
- Water the soil during the afternoon. Moist soil holds and releases more heat overnight than dry soil.
- Cover before dusk, not at bedtime — you are trapping heat that is already there, so the cover has to go on while the ground is still warm.
- Use fabric, not plastic, in contact with foliage. Plastic touching leaves conducts cold straight into them.
- Move containers against a wall or under cover. They are the easiest win and the most vulnerable, because roots in a pot have no soil mass to buffer them.
- Harvest what is ready. Tomatoes, peppers, squash and beans will not improve after a frost.
The full version, including what each material is worth in degrees, is in when to cover your plants.