Protecting the vegetable garden from cold: fleece, mulch and cold frames in numbers

Floating row cover, mulch, cold frame: each protection gains between 2 and 8 °C depending on the technique and fabric weight. Here are the exact thresholds, the materials table, and the move half of gardeners get backwards.

The row cover: how many degrees do you really gain?

A cover does not heat — it traps the soil's warmth. And fabric weight makes all the difference:

Weight	Frost protection	Light transmission	Use
Light ~17 g/m² (0.45 oz/yd²)	None	90 %	Insect barrier only — not a frost cover
Medium ~19 g/m² (0.55 oz)	down to -2 °C	85 %	The standard cover, can stay on day and night
Heavy ~50 g/m² (1.5 oz)	-2 to -4 °C	30-50 % only	Remove by day, or double a light cover instead

Doubled, a medium cover protects as long as temperatures stay above -3 to -4 °C.

The move half of gardeners get backwards: put the cover on in late afternoon, while the soil is still warm — you are trapping the day's accumulated soil heat, not the night's. And the cover must not touch the foliage: wherever it touches, frost gets through.

Mulch: the thicknesses that matter

The effect is measurable with a thermometer: at sunrise, soil 8 cm under mulch can be 4 °C warmer than bare ground. Above all, mulch cancels the repeated freeze-thaw cycles that heave shallow-rooted young plants out of the ground.

Thickness	Use
10 cm (4 in)	Effective minimum on beds; settles to ~7 cm — perennials, herbs
15 cm	Straw or hay, lasts a season; straw around beetroot and celery left in the ground
20 cm	Dead leaves/grass clippings mix — field measurement: -6/-7 °C at night, only the top 10 cm frozen, loose soil beneath
25-30 cm	Root vegetables overwintering in open ground (carrots, parsnips) across the whole bed + 50 cm margin — keeps the soil at 2-4 °C

Two non-negotiable rules: keep 7-10 cm clear around the crown to prevent rot, and feed before mulching with bark or wood chips (nitrogen hunger risk). The honest counter-argument: in orchards the soil is sometimes left bare — naked soil releases the day's heat at night, keeping the air 1-2 °C warmer than over mulched ground.

Cold frames and tunnels: the next level up

When a cover is no longer enough (-4 °C and below), you move up a tier: a single layer of plastic creates enough warmth to lift indoor temperatures by one to one-and-a-half hardiness zones — set hoops up as soon as temperatures drop below 7 °C. A typical low tunnel: a 1.80 × 0.90 m bed, hoops every 60 cm.

Golden rule: ventilate as soon as it passes 10 °C outside — under plastic it overheats fast. In truly cold zones, combine a tunnel with an unheated greenhouse to overwinter tender crops like chillies.

What to protect in the vegetable garden — the plant-by-plant list

• Stay in the ground under mulch: carrots, parsnips, beetroot, celeriac, leeks — mulch turns them into a living root cellar.
• Under medium/heavy cover: lamb's lettuce, spinach, winter lettuces, young autumn shoots — laid a week or two before sowing, the cover also pre-warms the soil.
• Bring in or shelter frost-free: potted citrus (the overwintering guide), lemon verbena, oleander, geraniums — a frost-free shelter is mandatory.
• No protection needed: Welsh onion, chives, lemon balm and mints need nothing; planted garlic and shallots don't care about frost.

Dead leaves: this season's free mulch

The October-November resource: a 20 cm mix of leaves and clippings protects down to -6/-7 °C at night while only the top 10 cm freeze. Gather them now, store them in a wire-mesh silo — they will mulch all winter, then serve as surface wood chips (BRF) in spring.

The Germie protocol

1. Before the first frosts: bring in tender pots (citrus, verbena), harvest squashes and green tomatoes.
2. As soon as it drops below 7 °C: hoops + medium cover over winter crops; lay it in late afternoon on warm soil, without touching foliage.
3. After the first real frost: mulch 10-15 cm around perennials (7-10 cm from the crown), 25-30 cm over roots overwintering in the ground.
4. On mild sunny days: ventilate tunnels and cold frames from 10 °C outside.

And while the garden sleeps, it is winter indoor sowing season — the LED grow light and the heating mat stand in for sun and greenhouse until spring.