--- title: "What to Grow in an Unheated Polytunnel This Winter" canonical: "https://brittlebloom.com/what-to-grow-in-an-unheated-polytunnel-in-winter/" author: "Nora Vance" published: "2026-10-24T15:00:00+00:00" modified: "2026-10-15T14:17:31+00:00" language: "en-US" site: "Brittle Bloom" description: "If you’re asking what to grow in an unheated polytunnel in winter, the short answer is a lot more than most people think. With the right crops and timing…" categories: "Greenhouse & Polytunnel" attribution: "Brittle Bloom (https://brittlebloom.com/)" --- # What to Grow in an Unheated Polytunnel This Winter If you’re asking what to grow in an unheated polytunnel in winter, the short answer is a lot more than most people think. With the right crops and timing, you can eat homegrown salads, roots, and alliums all the way through January and February. It just takes a little planning before the season turns. Inside an unheated polytunnel, temps hover roughly 2 to 5°C above outside air. Add a layer of fleece and hardy crops can shrug off frosts down to -10°C or cooler. As of 2026, that simple setup keeps serious kitchen gardeners in zones 7 to 9 harvesting through winter. So don’t let the tunnel go empty. ## Quick Answer Grow winter-hardy leafy greens, root crops, alliums, and brassicas in an unheated polytunnel. Mâche, kale, spinach, leeks, carrots, and pak choi all thrive in the cold. Sowing between late August and mid-September is the secret. Add fleece for deep frosts and ventilate on mild days. ## Know Your Tunnel: The Real Conditions Inside That Decide Your Crops Before you pick seeds, understand the three numbers that matter: temperature lift, light, and airflow. Your tunnel changes each one, and those changes decide what survives. **Temperature lift.** A polytunnel acts like a blanket, not a furnace. Nighttime lows run 2 to 5°C warmer than outside air. That difference turns a hard killing frost into a mild nip. University extension research across the US confirms this passive boost is enough for a wide range of winter crops. **Winter light.** Cold is not the real limit. Short daylight hours are. Growth slows below 10°C and nearly stops below 4°C. So you want crops that can hold in a dormant state, then surge when days lengthen in February. The RHS describes this as “overwintering,” and it’s the single biggest shift in thinking for new tunnel growers. **Airflow and humidity.** Closed tunnels trap moisture. Stagnant, damp air breeds botrytis and grey mould on leaves. On sunny mild days, open the vents. A little cold air beats a blanket of rot. One more thing matters: your site. A south-facing, sheltered tunnel with good drainage gives you far more options than a wind-battered one in a frost pocket. Match your crop list to what your tunnel actually offers, not what the seed catalogue promises. ## The Winter Crop Decision Guide: Match Crops to Your Climate, Timing, and Tunnel Work through these four branches in order. By the end, you’ll have a clear crop list, not a vague wish list. ![Winter salads](https://brittlebloom.com/wp-content/uploads/2026/10/winter-salads-muy9fxbg.webp) Image source: Wikimedia Commons / myself (User:Piotrus) (CC BY-SA) ### Branch 1: Your Local Winter Climate If your winters are mild, in zone 7 or warmer, you can grow winter lettuce, mizuna, pak choi, broad beans, and even hardy peas. If hard freezes hit you regularly, in zones 5 to 6, lean on mâche, kale, spinach, leeks, carrots, and winter radish. These crops handle -10°C or colder once established. The frost line sets the boundary. ### Branch 2: Your Sowing Timing If you sow in late August to mid-September, you get fast growth and real harvests by November. If you miss that window, sow hardy greens in modules until early November and transplant them. They won’t size up until spring, but they’ll overwinter beautifully. Timing separates a winter harvest from a winter holding pattern. ### Branch 3: Your Tunnel’s Condition If your soil drains well and the polythene is intact, grow root crops and leafy greens together. If water puddles on the surface, skip roots and focus on raised beds or deep straw mulch. Wet feet kill more winter crops than frost does. ### Branch 4: Your Growing Goal If you want fresh salads all winter, choose mâche, claytonia, winter purslane, land cress, and corn salad. If you need meals with substance, grow kale, chard, leeks, and parsnips. Most growers do a mix: something to pick daily and something hearty for the pot. ## Step-by-Step: Growing Winter Crops from Soil Prep to Fresh Harvest Follow this order and you’ll dodge most of the problems that empty winter tunnels. 1. **Clear summer crops** once they finish. Pull old plants, pick out debris, and compost healthy residue. Diseased plants go in the bin, not the compost. 2. **Clean the tunnel.** Wash trays and scrub the polythene if needed. A clean tunnel carries far fewer pests and diseases into the cold season. 3. **Check the frame and cover.** Repair any tears, tighten anchors, and make sure vents open freely. Snow and wind find weak spots fast. 4. **Feed the soil.** Spread 2 to 3 cm of compost over the beds. Winter crops grow slowly, but they still need nutrition when active. 5. **Sow in modules** for salads and brassicas. That gives you strong, transplantable seedlings instead of seed that sits cold and damp. 6. **Transplant at 4 to 6 true leaves.** Firm them in well. Water once, then let the soil stay slightly drier than you would in summer. 7. **Mulch thickly** with straw or compost around the plants. Mulch protects roots, suppresses weeds, and keeps soil from splashing onto leaves. 8. **Ventilate on any sunny day above 10°C.** Open vents for a few hours. This dries the air and hardens the plants against frost. 9. **Harvest regularly.** Cut-and-come-again leaves keep producing when you keep picking. Remove yellowed leaves to stop mould before it starts. ## Common Mistakes That Leave a Winter Tunnel Bare (and How to Avoid Them) **Sowing too late.** This is the biggest one. Winter crops need time to root before light and warmth fade. If you sow in November instead of September, you’ll spend winter watching tiny seedlings and slapping yourself. **Leaving the vents shut.** We get it, it’s cold. But sealed tunnels stay wet, and wet tunnels breed botrytis. Vent on mild days, even in January. Air movement matters more than chasing every degree of warmth. **Overwatering.** Winter plants drink very little. Cold, saturated soil sits there, stunts roots, and invites rot. Check the top 3 cm of soil. If it’s damp, skip the watering can. **Choosing tender varieties.** Regular lettuce dies at -4°C. Winter lettuce, mâche, and claytonia shrug off far colder nights. Read the packet and look for “winter hardy” or “overwintering” labels. The variety is everything. **Ignoring extra frost protection.** A single layer of horticultural fleece inside the tunnel adds another 2 to 3°C of protection. It costs little and takes seconds to drape over beds on cold nights. Use it. **Forgetting that polytunnels cook on sunny days.** Clear skies in February can push the inside to 20°C while it’s freezing outside. No ventilation means heat stroke for your winter salads. Crack those vents. ## Winter Tunnel Care: Ventilation, Watering, and Frost Protection Your winter routine is simpler than summer, but it’s just as important. Three things need your attention, and they’re easy to remember: air, water, and a layer of cover. ![Horticultural fleece](https://brittlebloom.com/wp-content/uploads/2026/10/horticultural-fleece-muy9fz52.webp) Image source: Wikimedia Commons / Simon Mortimer (CC BY-SA) **Ventilation.** A sealed polytunnel sounds cosy, but it’s a trap. Still, damp air settles on leaves and starts botrytis within days. Open the vents whenever the inside temperature passes 10°C, even on frosty days, as long as the sun is out. Fifteen minutes of airflow beats an afternoon of stagnant fog. **Watering.** Winter plants barely drink. Check the top 3 cm of soil before you reach for the can. If it feels damp, leave it. Most beds need water every 7 to 10 days, and always in the morning so leaves have time to dry. Cold, wet roots invite rot and fungal diseases. **Frost protection.** The tunnel gives you one layer of defence. A sheet of horticultural fleece gives you a second. Drape it over crops on nights when temperatures are forecast to drop hard, then pull it off the next morning so light reaches the leaves. Water-filled barrels along the aisle also store daytime warmth and release it slowly overnight. **Long-term maintenance.** Check for tears after every storm. A ripped panel lets wind whistle through and undoes all your protection. Patch small holes with polytunnel tape, and keep an eye on the frame’s anchors so plastic doesn’t rub against metal hoops. ## Frequently Asked Questions ### Can I grow tomatoes in an unheated polytunnel in winter? No. Tomatoes need heat and long daylight hours. They’ll sit there sulking and then rot. Focus on hardy greens, roots, and alliums instead. They’re bred for cold weather, and they’ll reward you without any heat source. ### What is the easiest crop to grow in a winter polytunnel? Mâche, also called corn salad, is the easiest by a long shot. It handles -15°C, grows in low light, and doesn’t bolt. Sow it in late summer, thin it once, and pick it all winter. You can’t mess it up. ### Do I really need a heater? No. An unheated polytunnel already runs 2 to 5°C warmer than outside air. That small lift pushes hardy crops through frosts that would kill them outdoors. Add fleece on the coldest nights and the extra 2 to 3°C makes a huge difference. ### When is the best time to sow winter crops? Late August to mid-September. That window gives plants time to establish roots while the soil is still warm and the days are long enough to grow. If you miss it, sow hardy greens in modules by early November and transplant them for spring growth. ### How often should I water crops in winter? Every 7 to 10 days, and only if the soil is dry at the surface. Overwatering kills more winter plants than cold does. Water in the morning so any moisture on leaves evaporates by nightfall. Damp leaves are how mould gets started.