Frost in the vegetable garden: the temperature each vegetable can stand
Tomato suffers from 0 °C. Winter lettuce holds down to -9 °C, leek down to -15 °C. A fleece or a mulch gains a few degrees.
Your leeks, cabbages and last salads are still outside. The forecast says -5 °C. In fact, almost all the vegetables still in place in November will hold. Summer vegetables suffer from 0 °C, like tomato, courgette, bean and pepper. Winter lettuce holds down to about -9 °C. Leek, carrot and parsnip go down to about -15 °C, kale and spinach to about -18 °C. A fleece or a mulch gains a few degrees, no more.
Frost and hard freeze
NC State, the extension service of North Carolina State University, distinguishes two things. You speak of a frost when the air falls to 0 °C and water freezes on low surfaces. A hard freeze starts at about -3 to -1 °C. The damage depends on the temperature reached, but also on how long it lasts.
Your thermometer does not tell the whole story. The RMI measures the minimum temperature at 1.5 m above the ground. On a clear night, the ground loses its heat very fast. At plant level, the air can then be colder than the announced figure. On a calm night, cold air also settles in hollows. A vegetable garden at the foot of a slope freezes more often than the neighbour's, higher up.
Each vegetable's threshold
NC State sorts vegetables into three groups. The PNW 548 sheet from the universities of Oregon, Washington and Idaho gives a threshold per vegetable for winter varieties. The figures below are converted to degrees Celsius and rounded.
| Vegetable | Group (NC State) | Holds down to about (PNW 548) |
|---|---|---|
| Tomato, courgette, bean, pepper, aubergine, cucumber | frost-tender: damaged from 0 °C | does not get through winter |
| Potato (foliage) | frost-tender: damaged from 0 °C | does not get through winter |
| Lettuce | tolerates -3 to -1 °C for a while | -9 °C (winter cut-and-come-again lettuce) |
| Head cabbage | tolerates -3 to -1 °C for a while | -7 to -4 °C (winter cabbage) |
| Cauliflower | tolerates -3 to -1 °C for a while | -12 to -9 °C (winter cauliflower) |
| Onion | tolerates -3 to -1 °C for a while | -15 to -12 °C (winter onion) |
| Brussels sprout | hardy: survives below -3 °C | see below |
| Parsley | hardy: survives below -3 °C | -9 °C |
| Leek | not classed | -15 °C |
| Carrot, parsnip | not classed | -15 °C, root in the ground |
| Broad bean | not classed | -9 to -7 °C |
| Spinach, kale | hardy: survive below -3 °C | -18 °C |
| Lamb's lettuce | not classed | -18 °C |
| The move | Harvest the frost-tender ones before the first frost | Leave the hardy ones in place |
These thresholds have a limit. The Oregon sheet specifies that they come from seed companies, farmers and their own observations, for winter varieties. Take them as landmarks, not guarantees.
For the Brussels sprout, the two sources do not match. NC State places it among the hardiest. The Oregon sheet gives it a threshold well above kale. When in doubt, keep a fleece within reach for the coldest nights.
Strawberries, in spring
The strawberry plant gets through winter without trouble. Its flower, on the other hand, fears frost in April and May. The University of Maine gives the temperature that damages it at each stage.
| Strawberry stage | Damaged at |
|---|---|
| Bud still tight | -4 °C |
| White bud, ready to open | -3 °C |
| Open flower | -1 °C |
| Small green fruit | -2 °C |
An open flower therefore freezes almost at zero. To look after them well, read the strawberry guide.
In your region
Frost-tender vegetables only go out once the frost has passed. The table gives the Vegiwise catalogue planting dates for two of them.
At Liège, the RMI counts 43.7 frost days a year on average. There are 11.3 in January, 10.3 in February, 6.2 in March, 1.6 in April and none in May. An average date is not a guarantee. Some years, it freezes after 30 March. The tomato therefore waits until mid-May. The guide The Ice Saints in Belgium looks at these late frosts in the data.
One rule holds everywhere. Look at the average date of the last frost where you live, then leave a margin for the frost-tender ones. If your garden is in a hollow, lengthen the margin. Cold air builds up there (RMI).
How to gain a few degrees
No move turns a tomato into a leek. But a few degrees are often enough to get through a night.
The fleece. The RHS estimates that a thick fleece protects by about 2 °C. The Oregon sheet gives up to 2 °C for a 17 g per m² fleece and 2 to 4 °C for a heavier one. A double layer protects more. The comparison is in Winter fleece 17 g or 30 g.
The timing. Rutgers advises putting the fleece on in the afternoon. It then keeps the heat built up during the day, before the temperature drops.
A moist soil. According to NC State, a moist soil holds heat and gives it back slowly at night. Also avoid hoeing just before a frosty night, so as not to dry out the surface.
Mulch over the roots. The Oregon sheet notes that roots in the ground are partly protected from frost. A mulch increases this protection. Carrots, parsnips and leeks can therefore stay in place and be harvested through the winter.
The hoops in step 3 are not a detail.
After a frosty night
For cool-season vegetables, NC State notes that the foliage may be damaged. The harvest is sometimes weaker afterwards, but the plant survives. Wait for the thaw and a mild day before judging.
The cold even has an advantage. The Oregon sheet writes that many vegetables become sweeter after low temperatures. It says so in its chapter on root vegetables left in the ground.
To know what to harvest and what to protect month by month, read The vegetable garden in December and Winter vegetable garden, 15 vegetables to harvest from November to March. Reading the weather for your vegetable garden teaches you to read a frost forecast and its useful figures.
The planting dates of your frost-tender vegetables depend on your climate. Tell the app which seeds you have, and it gives you the dates where you live. Start here.
FAQ
At what temperature should I cover my salads?
A well-established winter lettuce holds down to about -9 °C according to the Oregon sheet. NC State places lettuce among the vegetables that tolerate -3 to -1 °C for a limited time. Cover young transplants from -3 °C.
Can my leeks stay outside all winter?
Yes. The Oregon sheet gives -15 °C for leek. According to the same sheet, a mulch at the base adds protection.
Does frost make cabbages taste better?
The Oregon sheet writes that many vegetables become sweeter after low temperatures. It says so about root vegetables left in the ground. For cabbages, we found no measurement.
Should you water before a frosty night?
NC State advises keeping the soil moist before the cold. A moist soil holds heat and gives it back slowly at night.
My strawberries are in flower and it is going to freeze. What to do?
Cover them in the evening with a double layer of fleece, as the RHS advises. Open it in the morning, so insects can reach the flowers during the day.
Photo: Nati, Pexels
Sources
- NC State Extension, C. Gunter, Frost and Freeze Protection for Vegetable Crops, 7 mai 2020, mis à jour le 12 mai 2023 (définitions, trois groupes de légumes, sol humide, retrait des voiles)
- Oregon State University, Washington State University et University of Idaho, N. Andrews et al., Winter Vegetable Production on Small Farms and Gardens West of the Cascades, PNW 548, mai 2022 (tableau 2 de résistance au froid, voiles, racines en terre)
- University of Maine Cooperative Extension, Highmoor Farm, Frost Protection Tips For Strawberries, 16 mai 2023 (températures critiques par stade de la fleur)
- Rutgers Cooperative Extension, H. Cooper, Spring Row Cover Management for Plasticulture Strawberries, 30 mars 2026 (voile posé l'après-midi, double couche, pollinisateurs)
- RHS (Royal Horticultural Society), Fleece and crop covers (voile de 17 g et de 35 g, environ 2 °C de protection, double couche)
- RHS, Fruit: protecting from frost (fraisiers : cloche ou double couche de voile, ouverte le jour pour les pollinisateurs)
- IRM, Dico météo : température minimale (mesure à 1,5 m, refroidissement du sol par nuit claire, air froid dans les vallées)
- IRM, Statistiques climatiques des communes belges, Liège (1991-2020) : jours de gel par mois, dernier et premier gel en moyenne
- Vitisphere, Wrap your vines in a fleece to protect them against the frost, 4 juin 2021 (essai de voile P30 sous gel de printemps)
He gardens in Belgium and tries out in his own vegetable patch what the app recommends. His articles always start from a source you can check and a step you can take this weekend.
