Winter vegetable garden — cold frames, polytunnels

Winter vegetable garden — cold frames, polytunnels

Winter growing is less about making plants grow in the cold and more about keeping hardy plants alive and harvestable through the dark months. The crops were established in autumn; winter protection holds them in suspended animation so you walk out and pick fresh greens in January. This guide covers the three protection structures, how much each one buys you, and exactly what to grow under them in the US and UK.

Plan winter cover in Growli: Tell Growli your location and Growli flags which winter crops your zone supports outdoors, with and without protection, and schedules the autumn sowing dates that have to be hit for a winter harvest.

The principle — you are extending the larder, not the growing season

A critical mental shift: in deep winter, daylength and low light — not just cold — limit growth. Below roughly 10 hours of daylight (the "Persephone period," Eliot Coleman's term for the weeks either side of midwinter), even protected crops barely grow. Protection does not make them grow through that window; it keeps them alive, unfrozen, and harvestable so the autumn-grown plants act as a living, refrigerated larder you draw down through winter, with active regrowth resuming as light returns in late winter.

This is why timing is everything: winter crops must reach near-maturity in autumn, before the Persephone window. Sow too late and the plant is too small to hold through winter and too slow to grow once cold sets in. The autumn sowing dates in the fall vegetable garden guide are the foundation winter growing builds on.

The three structures — and how much each one buys

Each layer of protection traps daytime solar gain and slows nighttime heat loss, effectively shifting the plot's hardiness microclimate. Eliot Coleman's widely cited rule of thumb: a single layer of cover is worth roughly one USDA zone, and stacking layers compounds it.

Floating row cover / fleece (baseline)

Spun fabric laid over crops or on hoops. Gains roughly 2–4°C / 4–8°F — useful for autumn extension and protecting the hardiest greens, but on its own it is not a true winter solution in cold zones.

Cold frame (≈ 1.5 zone shift)

A passive solar box: a bottomless frame with a sloped, clear, hinged lid (glass or twin-wall polycarbonate) facing the winter sun. No power, no heating. Coleman describes the cold-frame microclimate as roughly 1.5 zones — about 500 miles — to the south, with interior night temperatures commonly 4–11°C / 7–20°F above outside air. The single most important cold-frame skill is ventilation: a sealed cold frame on a sunny winter day can exceed 30°C / 86°F and cook the crop. Prop or vent the lid above roughly 5°C / 40°F outside.

• Best for: salad leaves, spinach, mache, claytonia, winter lettuce, overwintering young brassicas and seedlings
• Zone reach: brings zone 5–6 conditions toward zone 7 inside the box

Low tunnel (≈ 1 zone shift)

Hoops (PVC, metal conduit, or wire) covered with fleece or clear polythene over a bed. Cheaper and faster than a cold frame, scales to whole rows. Gains roughly one zone. Coleman's double low tunnel — a layer of fabric row cover inside an outer layer of greenhouse plastic — takes hardy crops through hard winters in cold zones, effectively stacking two layers for close to a two-zone shift. Vent the ends on mild sunny days.

• Best for: spinach, kale, leeks, carrots and parsnips held in the ground, hardy salad rows
• Zone reach: roughly one zone; double-layer roughly two zones

Polytunnel (UK) / high tunnel (US) (≈ 1–2 zone shift)

A walk-in plastic-covered structure on a steel or timber frame. Unheated, it gains one to two zones and extends the full growing season at both ends — earlier spring sowing, later autumn cropping, and a protected winter larder. Adding an inner low tunnel inside the polytunnel stacks protection for the coldest weeks. This is the serious winter-growing structure for committed gardeners and the UK allotment standard.

• Best for: the full winter range plus early-spring and late-autumn production
• Zone reach: one to two zones; with an inner cover, more

These zone-shift figures align with Eliot Coleman's Four-Season Harvest methodology and Johnny's Selected Seeds' winter-growing guidance (Coleman's commercial collaborator on low-tunnel systems).

The step-by-step winter garden protocol

Step 1 — Establish in autumn, before the Persephone window

Use the fall vegetable garden backward-counting maths to hit autumn sowing dates. Winter crops should be near-mature before daylength drops under ~10 hours. In most temperate zones that means sowing late summer to early autumn.

Step 2 — Match the structure to your zone

• Zone 7+ / most of the UK: many crops survive outdoors with only fleece on the hardest nights; a cold frame or low tunnel makes the harvest reliable and comfortable.
• Zones 5–6: cold frame or double low tunnel for a real winter harvest.
• Zones 3–4: polytunnel/high tunnel with an inner low tunnel, and expect a holding larder rather than active growth.

Step 3 — Site for winter sun

Winter sun is low and weak. Orient cold frames and tunnels for maximum south-facing exposure (north-facing in the southern hemisphere), away from shadows cast by buildings and evergreens, ideally on free-draining ground — winter waterlogging kills more overwintering crops than cold does.

Step 4 — Ventilate obsessively

The number-one winter-structure killer is not frost — it is cooking crops on a sunny day inside a sealed frame, and fungal rot in stagnant humid air. Vent whenever the day is mild and sunny; close before nightfall to trap the day's heat. Automatic cold-frame vent arms (wax-piston openers) remove the need to be home at midday.

Step 5 — Water sparingly, in the morning

Cold wet soil and stagnant air breed botrytis and downy mildew. Water only when needed, in the morning so foliage dries before night, and at the base, not over the leaves.

Step 6 — Harvest as a larder, expect little regrowth

Through the darkest weeks, pick what you need and accept slow or no regrowth. Active growth resumes from late winter as light returns — many crops then surge for an extra-early spring harvest weeks ahead of spring-sown ones.

Crop-by-crop winter guidance

The most cold-hardy (survive hard frost, often unprotected in zone 7+/UK)

• Spinach — the premier winter green; overwinters down to severe cold under cover, surges in late winter
• Kale — extremely hardy; flavour sweetens through frost; Cavolo Nero / Tuscan and curly types reliable
• Mache / corn salad — possibly the single hardiest salad leaf; thrives in cold, almost unkillable under fleece
• Claytonia (winter purslane / miner's lettuce) — very hardy, productive salad leaf for cold frames
• Leeks — stand in the ground all winter in most temperate zones; harvest as needed
• Parsnips — left in the ground; flavour improves markedly after hard frost converts starch to sugar

Hardy with structure

• Winter lettuce — cold-tolerant varieties (Winter Density, Arctic King, Rouge d'Hiver) under cold frame or tunnel
• Chard, mustard, mizuna, tatsoi, winter cress — hardy salad/cooking greens under cover
• Spring cabbage, sprouting broccoli, Brussels sprouts — stand through UK winters outdoors; sweeten after frost
• Carrots — hold in the ground under a deep straw or leaf mulch in milder zones; lift before deep ground freeze in cold zones
• Spring onions / overwintering onion sets and garlic — autumn-planted, overwinter dormant, surge in spring (see when to plant garlic)

Will not overwinter

Tender summer crops (tomatoes, peppers, beans, cucumbers, squash, basil) do not survive winter even under a polytunnel without heat. Do not attempt them — the structure's job is hardy crops only.

Pair compatible cool-season crops in the limited winter space using the companion planting guide, and size the dense plantings with the plant spacing calculator.

Common winter vegetable garden mistakes

• Sowing too late. The single biggest failure. Plants must be near-mature before the Persephone low-light window; a small plant going into winter will not grow and will not survive.
• Expecting growth in midwinter. Below ~10 hours daylight, crops hold rather than grow. This is normal — harvest the larder, do not panic.
• Sealing the structure on a sunny day. A closed cold frame can cook the crop in an hour. Vent obsessively.
• Overwatering. Cold, wet, stagnant conditions cause more winter losses (botrytis, root rot) than cold itself.
• Siting in winter shade. Low winter sun is easily blocked by walls and evergreens — the structure must catch what little there is.
• Attempting tender crops. No unheated structure overwinters tomatoes or beans. Hardy crops only.
• Forgetting waterlogging. Free-draining ground or raised beds under the cover; winter wet rots roots.
• No autumn establishment plan. Winter growing is decided in August–September, not December.

UK + US notes

UK

• The UK's mild maritime winters make it one of the best winter-vegetable climates in the temperate world — hard sustained ground freezes are rare across most of the country.
• UK winter staples: winter brassicas (kale, January King and Savoy cabbage, purple sprouting broccoli, Brussels sprouts), leeks standing in the ground, parsnips left in situ, overwintering spinach and chard, autumn-planted garlic and onion sets, and broad beans for the earliest spring crop.
• The polytunnel is the UK gardener's and allotment-holder's standard winter structure; cold frames and cloches handle smaller salad areas. Slug pressure persists even in winter under cover — keep ventilation up and use ferric phosphate pellets.

US

• Zones 7+: spinach, kale, mache, claytonia, overwintering garlic, and winter lettuce produce reliably outdoors with light protection; a cold frame makes it comfortable and dependable.
• Zones 5–6: Eliot Coleman's double-low-tunnel system (fabric inside plastic) or a cold frame brings a genuine winter harvest of spinach, mache, and hardy leaves.
• Zones 3–4: a high tunnel with an inner low tunnel gives a holding larder of the hardiest greens; expect minimal midwinter growth and a strong late-winter resurgence.
• Mild-winter zones (9–11): "winter" is effectively the prime cool-season growing window — most cool-season crops grow actively outdoors with no protection while summer is the off-season.

For establishing the crops that overwinter, see seed starting indoors, the fall vegetable garden guide, and the broader vegetable garden layout.

Related

• Fall vegetable garden — the autumn establishment that winter builds on
• Succession planting — year-round continuous-harvest framework
• When to plant garlic — the classic overwintering crop
• Seed starting indoors — raising hardy transplants
• Vegetable garden layout — bed plan and rotation
• Frost date calculator — set the autumn sowing deadline
• Plant spacing calculator — dense winter plantings under cover
• Companion planting hub — pairing cool-season crops in tight space

Sources: Eliot Coleman, Four-Season Harvest and The Winter Harvest Handbook (cold-frame and low-tunnel zone-shift methodology); Johnny's Selected Seeds winter-growing and Quick Hoops low-tunnel growing library; Maine Organic Farmers and Gardeners Association season-extension resources; RHS winter vegetable and polytunnel guidance.