Winter fleece 17 g or 30 g: which to choose and when to take it off
Thin fleece keeps 85 % of the light and gains up to 2 °C. Thick fleece gains 2 to 4 °C. The heaviest only let through 30 to 50 % of the light.
In the shop, two rolls of fleece await you, 17 g and 30 g per m². The thin one lets through about 85 % of the light and gains up to 2 °C. The thicker one gains 2 to 4 °C but stops a good share of the light. Take the 17 g for vegetables that have to grow underneath for weeks. The 30 g, or two layers of 17 g, is for getting through a frosty night or a winter on vegetables that hardly grow. In both cases, take it off as soon as the cold has passed. And always when strawberries flower.
How many degrees each thickness gains
The PNW 548 sheet from the universities of Oregon, Washington and Idaho compares the two families of fleece. The RHS, the British horticultural society, and the University of Maine give similar landmarks.
| Thin fleece (17 g) | Thick fleece (30 g and more) | Two layers | |
|---|---|---|---|
| Weight | 17 to 19 g per m² | 31 to 68 g per m² (PNW 548), 35 g (RHS) | two fleeces one over the other |
| Degrees gained | up to 2 °C (PNW 548); 1 to 2 °C for thin fleeces (Maine) | 2 to 4 °C (PNW 548); about 2 °C (RHS) | 4 °C measured under a double P30 fleece (Vitisphere) |
| Light transmitted | 85 % | 30 to 50 % | less still |
| Lifespan | sometimes a single season (RHS) | not given | depends on the fleece |
| Use | spring sowings and plantings, winter salads that are growing | frosty nights, winter vegetables waiting for harvest | the coldest nights, strawberries in flower |
| The move | Leave it in place for weeks | Put it on at night, open it on mild days | Double it when below -3 °C is forecast |
The figures vary from one source to another. Each measures in its own climate, with its own fleeces. The order of magnitude stays stable. A fleece gains a few degrees, not ten.
Why a fleece only gains a few degrees
On a clear night, the ground loses its heat very fast by radiation to the sky, the RMI explains. The fleece does not produce heat. It slows this loss and keeps a little of the day's heat against the soil. Rutgers therefore advises putting it on in the afternoon, before the temperature drops.
The price the plant pays
Under a fleece, three things change for the plant.
Light. A thick fleece only lets through 30 to 50 %, according to the Oregon sheet. Rutgers notes that it can slow growth. For a leek or a cabbage waiting for harvest in winter, it does not matter. For a lettuce you want to see grow in spring, take the 17 g.
Heat, in the sun. On a day of strong sun, the temperature rises fast under the fleece. NC State advises removing it before heat stresses the plants. The Oregon sheet gives the same warning for fleeces and cloches.
Insects. The fleece stops pests. The RHS cites leek moth and cabbage caterpillars. It also stops the insects that pollinate. The RHS therefore advises against covering, during flowering, the crops that depend on them, such as strawberries and courgettes.
When to take it off, crop by crop
Strawberries. Fleece at night when it freezes, open by day from flowering. An open flower freezes at -1 °C, according to the University of Maine. It is the one to protect. The RHS advises a double layer on strawberries in flower, open during the day. The guide on strawberries goes further.
Potatoes. The first leaves freeze from 0 °C, according to NC State. A 17 g gains up to 2 °C. That is enough for a light frost, not for a hard freeze. Take it off when the risk of frost has passed. The full calendar is in the guide Planting, earthing up and harvesting potatoes.
Winter salads and spinach. Spinach holds down to about -18 °C, winter cut-and-come-again lettuce to about -9 °C, according to the Oregon sheet. The fleece is therefore not there to save them. It makes them grow a little in winter. A 17 g can stay for weeks. The RHS points out that a fleece can even stay all summer.
Carrots and broad beans. Carrot left in the ground holds down to about -15 °C, broad bean to about -9 to -7 °C (Oregon sheet). For roots in the ground, the same sheet notes that a mulch reinforces the protection the soil already gives them. Keep the fleece for nights that fall below these thresholds.
Without hoops, the fleece can stick to the leaves if it freezes after rain, the Oregon sheet warns.
In your region
Fleece is mostly for spring, between planting and the end of the frosts. The Vegiwise catalogue gives these dates for two crops that benefit.
Fleece is useful until the average date of your last frost. After that, it is only for a cold night that has been forecast.
The test to do at home
The published figures come from American fields and French vineyards. Your garden has its own climate. Put two min-max thermometers at ground level, one under the fleece, the other uncovered, a metre apart. Read them after a clear night. The gap between the two minimums tells you how much your fleece gains where you live.
For the thresholds of each vegetable and the other moves against the cold, read the guide Frost in the vegetable garden. To go further than a fleece, Greenhouse, tunnel, cold frame or cloche compares the shelters.
When you plant your potatoes or your strawberries, the app gives you the window for your climate. Start here.
FAQ
Can you leave the fleece on all winter?
Yes, on vegetables that hardly grow, like leeks or cabbages. The Oregon sheet uses it that way all winter. Open it on sunny days to avoid the heat.
Does fleece protect from insects?
Yes. The RHS cites leek moth and cabbage caterpillars. For these pests, also read Leek moth and leek fly and Cabbage white and cabbage caterpillars.
Fleece or plastic film?
Fleece. According to the RHS, an unperforated plastic holds too much humidity and overheats in the sun. Fleece lets air and water through.
Does a fleece protect from -5 °C?
A single layer, rarely. It gains 1 to 4 °C depending on its thickness. A double layer protected strawberry plants down to -6 °C according to Penn State.
How long does a fleece last?
The RHS writes that a 17 g fleece may only last a year. Woven polyethylene covers last at least five years, according to the same page.
Photo: Josef Schlaghecken, Wikimedia Commons, licence CC BY-SA 4.0
Sources
- 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 (voiles moyens et lourds : degrés gagnés et lumière transmise)
- RHS (Royal Horticultural Society), Fleece and crop covers (voile de 17 g et de 35 g, environ 2 °C, double couche, durée de vie, insectes)
- RHS, Fruit: protecting from frost (fraisiers : cloche ou double couche de voile, ouverte le jour pour les pollinisateurs)
- University of Maine Cooperative Extension, Highmoor Farm, Frost Protection Tips For Strawberries, 16 mai 2023 (voiles fins et épais, seuils des fleurs de fraisier)
- Rutgers Cooperative Extension, H. Cooper, Spring Row Cover Management for Plasticulture Strawberries, 30 mars 2026 (environ 4 °F sous un voile moyen, double couche, floraison avancée, pollinisateurs)
- Vitisphere, Wrap your vines in a fleece to protect them against the frost, 4 juin 2021 (voile P30, simple et double couche, gel de printemps 2021)
- NC State Extension, C. Gunter, Frost and Freeze Protection for Vegetable Crops, mis à jour le 12 mai 2023 (retirer les voiles avant le coup de chaleur)
- IRM, Dico météo : température minimale (refroidissement du sol par rayonnement les nuits claires)
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.
