A plant freezes when the water in its cells turns to ice: the crystals pierce the cell walls, and the leaves turn black the next day. The lower the temperature falls and the longer it stays low, the deeper the damage. A few minutes of frost at -0.5 °C often spares what a night at -4 °C destroys.
Why this indicator matters
The official thermometer measures the air at two metres. At ground level, under a clear sky and with no wind, the temperature can be three to four degrees lower, because the earth cools by radiating towards the sky. That is ground frost: the air shows 2 °C and the grass is white.
Knowing the time of the minimum helps you act at the right moment: cover plants in late afternoon, bring pots in before dusk, open the fleece the next day so the plants can breathe. Frost is also a risk for outdoor pipes and for the road.
How to read it
The box looks at the next thirty-six hours. If there is air frost, it gives the low, the time it is reached, the range during which the air stays below zero and the number of hours involved, with advice suited to the severity.
If the air stays above zero but the low is 3 °C or less with a clear sky and little wind, the sentence announces possible ground frost. The indicator stays silent when the cold is not enough to threaten the garden.
The thresholds used on this site
Every sentence in the “Today” box comes from a numerical rule. Here are the rules for this indicator, exactly as they are applied in the code.
- Frost: low below 0 °C; hard frost: −4 °C or less; severe frost: −10 °C or less.
- Window examined: the next 36 hours.
- Ground frost possible: low of 3 °C at most, cloud below 30 %, wind below 10 km/h; or a negative ground temperature.
A worked example: Toronto
In Toronto, with the forecasts of 2026-11-02 08:13, this indicator stays silent: none of the conditions above is met at the moment. The low over the next 36 hours is 8.2 °C.
Its limits
The forecast is for a single point per town: a sheltered valley, the bottom of a garden or an open area can fall several degrees lower on a still night, because cold air drains downhill. How well plants cope also depends on their stage, a swollen bud being more fragile than a dormant plant.
Frequently asked questions
Is horticultural fleece enough to protect plants?
It gains two to four degrees compared with the open air, which is enough for a light frost. Below -4 °C, tender plants need to be brought indoors.
Why is ground frost possible when the thermometer shows 2 °C?
Because the ground cools by radiation towards a clear sky, and can end up several degrees below the air temperature measured at two metres.
Should I water before a frosty night?
A damp soil stores a little more heat than a dry one, but watering the leaves makes them freeze. It is better to water the soil late in the day and cover the plants.