Polytunnel vs Greenhouse for UK Plots

Polytunnel vs Greenhouse for UK Allotments: The Real Dirt on Cost, Warmth, and British Weather

You’ve cleared the couch grass, built your raised beds, and finally got your first decent crop of spuds. Now you’re eyeing that bare patch of soil and thinking: “I need to extend the season. I want to grow tomatoes that actually ripen. I need somewhere to overwinter my chillies without them turning into a mushy mess.”

Welcome to the polytunnel vs greenhouse dilemma. It’s a decision that splits plot-holders like blight splits a potato. And for a UK allotment, the stakes are high – you’re dealing with wind that can shred polythene, winters that freeze glass, and a budget that’s already been stretched by seed compost and slug pellets.

I’ve been through this rodeo. On my 20-plot community allotment in Yorkshire, I’ve seen both structures fail spectacularly and succeed brilliantly. I’ve lost a polytunnel to a February storm that sounded like a freight train, and I’ve watched a second-hand greenhouse turn a cold frame into a tomato factory. Let’s strip away the marketing fluff and look at the science, the costs, and the real-world performance for UK conditions.

The Cost Clash: Upfront Spend vs Long-Term Value

Let’s start with the wallet. A basic 8ft x 6ft greenhouse from a major UK retailer (think B&Q or Wilko) starts around £200-350. A comparable polytunnel of the same footprint often costs less – sometimes as low as £100-150 from a specialist like First Tunnels or Northern Polytunnels.

But here’s where it gets tricky. That cheap polytunnel uses 150-micron polythene. On a Yorkshire plot, exposed to westerly winds and hail, that polythene will start to degrade in 18 months. You’ll be patching it with tape by year two, and replacing the cover by year three. A decent polytunnel with a 200-micron UV-stabilised cover costs £250-400 for a 10ft x 12ft model, plus a ground anchor kit (add £40). For a greenhouse, the glass or polycarbonate panels, if properly fixed, can last a decade with minimal maintenance.

My real-world verdict: Polytunnels are cheaper upfront, but you must factor in cover replacement every 3-5 years. Greenhouses are a higher initial investment, but lower long-term running costs. For a 5-year plan, a mid-range polytunnel actually costs about the same as a budget greenhouse once you replace the cover twice.

Warmth and Growing Windows: Which Structure Wins for Tomatoes?

This is where the science gets interesting. The growing tomatoes question is the acid test. I’ve trialled both structures with blight-resistant varieties like ‘Crimson Crush’ and ‘Mountain Magic’. Here’s what I found.

A greenhouse retains heat better overnight because glass is a better thermal conductor – it absorbs solar radiation during the day and releases it gradually at night. A polytunnel, made of single-skin polyethylene, loses heat faster. On a clear April night in Yorkshire, I’ve measured a 3-4°C temperature difference between inside my greenhouse and inside my polytunnel. That means your tomatoes in a greenhouse will get a head start of about 2 weeks in spring.

But there’s a caveat. Polytunnels diffuse light better. The milky or clear polythene scatters sunlight, reducing leaf scorch and creating a more even growing environment for leafy crops like spinach and lettuce. A greenhouse with clear glass can create hotspots that actually stress tomato plants on sunny August days, leading to blossom end rot (calcium deficiency caused by fluctuating water uptake).

For winter protection, a greenhouse wins again. With a layer of bubble wrap and a heater set to 5°C, you can overwinter tender perennials like pelargoniums and even citrus. A polytunnel, even with a heater, will struggle to stay above freezing during a prolonged cold snap. The RHS has data showing that polytunnels lose 30% more heat than a double-glazed greenhouse during a -5°C night.

Wind Damage: The UK’s Silent Plot Killer

If you’ve ever watched a polytunnel lift off like a plastic balloon in a 50mph gust, you know the terror. Wind damage is the single biggest threat to a polytunnel on an exposed UK allotment. I lost a 10×14 tunnel in 2020 when Storm Ciara hit. The ground anchors pulled straight out of the clay, and the frame twisted into a metallic pretzel.

Greenhouses are heavier and more rigid. A well-anchored greenhouse on a concrete base is far more resistant to wind uplift. However, greenhouses can be shattered by flying debris during a storm. A single hailstorm in 2021 cracked three panes on my neighbour’s greenhouse, costing him £90 to replace.

My rule of thumb: If your plot is in a wind-exposed area (near the coast, on an open hill, or in a valley that funnels gusts), a polytunnel is a risk unless you invest in a heavy-duty frame and bury the skirt in a trench. The Gardening Which? surveys consistently show that tunnels with a 32mm galvanised steel frame and a windbreak hedge survive twice as long as budget models. For a sheltered plot, a polytunnel is fine. For exposed sites, a greenhouse with toughened glass or twin-wall polycarbonate is safer.

Durability: What Actually Breaks First?

Let’s get honest. Nothing on a UK allotment lasts forever. Here’s the typical lifespan I’ve observed on my site:
– Budget polytunnel (£100-200): Cover lasts 2 years. Frame bends in high wind. Total lifespan: 4-5 years with cover replacement.
– Mid-range polytunnel (£250-400): Cover lasts 4-5 years. Frame is solid. Total lifespan: 8-10 years with one cover change.
– Budget greenhouse (£200-350): Glass breaks easily. Aluminium frame is thin. Total lifespan: 5-7 years if you fix panels.
– Quality greenhouse (£500+): Polycarbonate panels resist hail. Heavy-duty frame. Total lifespan: 15-20 years.

The weak point on a greenhouse is almost always the glazing clips. They rust or snap. On a polytunnel, it’s the zipper doors. They jam, rip, or fail after 2 years. I now recommend buying a polytunnel with a roll-up door instead of a zipper door – it’s more robust.

Ventilation and Pest Control: The Hidden Factor

Here’s something I rarely see in other guides: ventilation is the difference between a productive structure and a fungal factory. Both structures suffer from high humidity, which encourages botrytis and powdery mildew.

A greenhouse with an automatic vent opener (worth the £35 investment) can keep airflow consistent. Polytunnels rely on you manually rolling up the sides. If you miss a day in July, the temperature inside can hit 50°C, cooking your tomatoes and encouraging red spider mite.

For pest management, both structures offer similar protection from carrot fly and cabbage root fly. But a greenhouse with a solid base is better at excluding slugs – they can’t crawl up glass or polycarbonate as easily as they can climb damp polythene. I use Vitax Slug Gone wool pellets around the base of my greenhouse; it works perfectly. For a polytunnel, you need to bury the skirt and apply nematodes every 6 weeks from April.

Which Structure is Right for You? A Decision Tree

Let’s make this simple. Read the scenarios below and find your match.

*Start here:* What’s your budget?
– Under £250? Go polytunnel. Accept that you’ll replace the cover in 3 years.
– Over £400? Consider a greenhouse if you want long-term durability and better winter protection.

*Next:* Where is your plot?
– Exposed to wind (no shelter, coastal, high ground)? Choose a greenhouse with polycarbonate panels.
– Sheltered (between hedges, in a valley)? A polytunnel works well.

*Finally:* What do you grow most?
– Growing tomatoes, peppers, and aubergines? Greenhouse gives you 2 weeks head start.
– Salad leaves, spinach, and chard? Polytunnel’s diffused light reduces bolting.
– Overwintering crops (kale, leeks)? Polytunnel with a fleece row cover is fine.
– Tender perennials or citrus? You need a heated greenhouse.

The Verdict from a Yorkshire Plot

After 15 years of trialling both, here’s my honest recommendation for the average UK allotment holder:

For most people, a 10ft x 8ft greenhouse from a reputable brand (like Halls or Elite) with polycarbonate panels is the best investment. It costs more upfront (£400-600), but it will last 15+ years, handle wind better, and give you the best environment for tomatoes and winter protection. Buy it second-hand from Gumtree or Facebook Marketplace – I’ve seen solid 6×8 greenhouses go for £50-100.

If your budget is tight, or you need a larger space quickly (say 12ft x 20ft for a family), a polytunnel is your friend. Just buy a model with a 32mm frame, 200-micron cover, and a roll-up door. Anchor it with ground screws, not budget pegs. And plant a windbreak hedge of hawthorn or hazel on the prevailing wind side – it will extend your polytunnel’s life by 3 years.

Final Practical Tips Before You Buy

1. Check your allotment rules. Some associations ban polytunnels for aesthetic reasons. Others require a concrete base for greenhouses. Get written permission.
2. Orient for sunlight. Both structures should run north-south to maximise light exposure on both sides. But if you’re in a windy area, orient the shorter side into the prevailing wind.
3. Invest in a thermometer with max/min recording. I use a cheap digital one. It tells you if you’re cooking your plants.
4. Don’t forget the ground. A polytunnel on bare soil needs a membrane to stop weeds. A greenhouse on a concrete slab gives you a clean, warm floor for potting.
5. Plan for water. Both structures dry out fast. Install a water butt with a gutter kit (polytunnel kits are available from First Tunnels).

Further Reading

For more on UK-specific tunnel performance, check the RHS greenhouse advice page. The Garden Organic website has field trials on polytunnel crop rotation. And for wind data, the Met Office provides local wind averages – essential before you buy.