Pocket BotanistThe first frost of autumn is the date the garden year turns on. Tender plants die, dahlias blacken overnight, and the last tomatoes stop ripening. It is also a date nobody can give you exactly, because it moves with the weather and with the shape of your own garden. What you can know is the average for where you live, how far your plot is likely to differ, and what the forecast looks like on the evening it is going to happen.
Average first frost dates
These are averages for a first air frost, when the air at head height falls below 0 °C (32 °F). A ground frost, on the grass and low leaves, comes a week or two earlier. Roughly one year in three the first frost is earlier than the average, and one in three later.
United Kingdom: late October to mid November across lowland England and Wales; early to mid October in Scotland, northern England and anywhere above about 200 meters; late November or December in London, on the south coast and in the far south west, where the sea and the city keep the nights milder. Ireland runs similar to the English south west.
United States, by USDA hardiness zone: zone 3 in early to mid September, zone 4 in mid to late September, zone 5 in early to mid October, zone 6 in mid to late October, zone 7 in late October to early November, zone 8 in mid to late November, zone 9 in late November to December, zones 10 and 11 rarely or never. Elevation and distance from large water shift these by weeks.
Germany, Austria and Switzerland: mid October to early November in most lowland areas, late September in the Alpine foothills and the eastern uplands, and mid to late November along the Rhine and in the big cities. France follows a similar gradient, with the Mediterranean coast often frost-free until December.
Why your garden differs from the map
Cold air is heavy and flows downhill, so a garden at the bottom of a slope or in a valley frosts a week or more before one halfway up, and a walled town garden may escape a frost that whitens the fields a mile away. Frost pockets form wherever cold air is trapped: behind a hedge or fence across a slope, in a dip, at the base of a wall facing away from the sun. South-facing walls and paving store the day's heat and release it at night, keeping the bed beside them a degree or two warmer. Water does the same: gardens beside lakes, rivers and the sea frost latest of all.
Keep your own record. The date of the first frost in your garden, written down each year, is worth more than any map after three or four years, and the app does this for you if you let it.
The signs a frost is coming tonight
A frost needs three things: cold air, a clear sky, and calm. Clouds act as a blanket and hold the day's warmth in; wind mixes the air and stops the cold pooling at ground level. So the evening to watch for is one where the forecast says clear skies, light or no wind, and a low of 4 °C (39 °F) or under. A forecast minimum is measured at head height in a screen; the ground and the leaves on it can be three or four degrees colder. Dry air makes it worse: a low dew point means the temperature can fall a long way before condensation slows it.
A cold, clear day that feels crisp by mid afternoon, with the wind dropping at sunset and a red sky, is the classic run-up. If the sky clouds over in the evening, the frost usually does not come.
What to do the evening before
Bring tender plants in pots inside, or against a house wall under a porch. Cover anything tender in the ground with horticultural fleece, an old sheet or a cardboard box, put on in the late afternoon while the soil still holds some warmth and taken off in the morning. Water the soil during the day, because moist soil holds and releases more heat than dry. Pick any remaining tomatoes, chillies and squashes. Do not cut back dahlias or cannas until the frost has blackened them; the foliage protects the tubers while it stands.
After the first frost
A first frost is rarely the start of winter. It is usually followed by weeks of mild weather, so the plants that survived it, and the ones you protected, carry on. The first frost is the signal to lift dahlia tubers, dig up gladioli, plant garlic and broad beans, and to stop feeding anything in a pot. The hardy garden takes it in its stride.
A warning the evening before
Pocket Botanist Premium watches the forecast for your location and sends a frost warning the evening before, with the plants in your garden that are at risk and what to cover. The free app shows your hardiness zone and the average dates for it. Free on iPhone and Android.
Sources
- Royal Horticultural Society, "Frost damage." rhs.org.uk
- United States Department of Agriculture, "USDA Plant Hardiness Zone Map." planthardiness.ars.usda.gov