A polytunnel is a semi-circular growing structure made from steel or PVC hoops covered in polyethylene plastic film. It works like a greenhouse but costs significantly less, creating a warmer, sheltered environment that lets you grow crops earlier in spring, later into autumn, and with noticeably higher yields than open-field gardening.
How a Polytunnel Works
Sunlight passes through the plastic covering and hits the soil, plants, and surfaces inside. About 45% of that incoming solar energy is visible light that plants absorb for photosynthesis. The remaining roughly 50% is near-infrared radiation, which doesn’t do much for plant growth directly but is absorbed by the soil and internal structures. That absorbed energy is then released as heat into the enclosed air. Because the plastic film traps this warm air and blocks wind, interior temperatures typically run 5 to 15°C (9 to 27°F) above outdoor conditions.
That temperature bump sounds modest, but combined with wind protection, it’s enough to keep many plant varieties growing slowly through cold months that would otherwise kill them. In warmer seasons, the effect accelerates growth and extends your productive window by weeks on either end.
What They’re Made Of
The frame is usually galvanized steel hoops for larger or commercial tunnels, or PVC pipes for smaller backyard versions. Treated lumber sometimes forms the base or end walls. The covering is polyethylene film, chosen because it’s lightweight, lets through plenty of light, and costs a fraction of glass or rigid polycarbonate panels.
Modern horticultural-grade polyethylene isn’t just plain plastic. It contains a UV stabilizer that slows degradation from sun exposure, and these films are typically warranted for four years or more. Many also include condensate control (sometimes called anti-drip), a wetting agent that lets moisture flow down the film in sheets rather than forming heavy droplets that fall on plants and promote disease. Some films add anti-fogging agents to prevent visibility and light-blocking fog from forming inside during early morning and late afternoon. UV-blocking versions go a step further, reducing populations of whiteflies, thrips, and aphids by filtering out the ultraviolet wavelengths these insects use to navigate.
What You Can Grow
Polytunnels are used for everything from tomatoes, peppers, and cucumbers to strawberries, salad greens, herbs, and cut flowers. The sheltered environment is especially valuable for crops that are sensitive to rain damage, wind, or temperature swings. Tomatoes and lettuce are among the most popular polytunnel crops, and the yield difference compared to open-field growing is substantial. Research published by the American Society for Horticultural Science found that lettuce grown under UV-filtered coverings produced 63% greater fresh weight than lettuce grown in full-spectrum outdoor conditions. Further UV filtering pushed that advantage even higher.
Beyond raw yield, polytunnels improve the percentage of your harvest that’s actually sellable or usable. Rain-splashed soil, wind scarring, and pest damage all drop significantly under cover, so more of what you grow ends up on the plate rather than in the compost bin.
Polytunnel vs. Glass Greenhouse
The most common comparison people make is between a polytunnel and a traditional glass (or polycarbonate) greenhouse. The differences come down to cost, durability, and flexibility.
- Cost: Polytunnels are substantially cheaper to buy, build, and maintain. A glass or polycarbonate greenhouse of the same footprint can cost several times more.
- Setup time: A polytunnel can be assembled in a day or a weekend without professional help. Greenhouses are more permanent structures that take considerably longer to plan and build.
- Durability: Glass greenhouses last decades with minimal upkeep. Polytunnel covers need replacing every four to six years, though the frame itself lasts much longer.
- Flexibility: Polytunnels are easier to relocate or resize. If your growing plans change, you can extend a tunnel or move it to fresh soil.
- Aesthetics: Glass greenhouses look more polished in a garden setting. Polytunnels are purely functional in appearance.
For most home growers and small-scale farmers, the lower upfront cost and ease of installation make polytunnels the practical choice. Commercial operations and serious hobbyists with larger budgets often prefer the longevity and appearance of glass.
Ventilation and Temperature Control
The same heat-trapping effect that helps in cool weather becomes a problem in summer. Without ventilation, a polytunnel can overheat quickly and stress or kill plants. There are three main ways air moves through a tunnel: doors at one or both ends, roof vents along the ridge, and side vents. Roof vents are the most effective because hot air rises naturally and escapes from the highest point. Side vents, often louvered panels or roll-up sides that let you open the lower walls, are less effective on their own but work well in combination with ridge ventilation. Many growers simply open both end doors on warm days for a through-draft.
Some larger tunnels use shade cloth draped over or inside the structure during peak summer to reduce the heat load. Since near-infrared radiation is the main source of excess heat, anything that reflects or blocks it before it reaches the soil makes a measurable difference in keeping temperatures manageable.
Common Sizes and Uses
Backyard polytunnels typically range from about 3 meters (10 feet) wide and 6 meters (20 feet) long up to around 4 by 10 meters. Commercial tunnels can stretch 30 meters or more in length and 8 to 10 meters in width. The semi-circular shape isn’t just for aesthetics. It sheds rain and wind efficiently, maximizes the volume of warm air inside relative to the surface area of the covering, and keeps the structure stable without heavy internal framing.
Home gardeners use them to grow warm-season vegetables in cooler climates, overwinter tender plants, and start seedlings weeks before the last frost. Small farms use them to produce high-value crops like strawberries and salad mixes for local markets, where the extended season and improved quality justify the modest investment. In milder climates, unheated polytunnels can support year-round growing with careful crop selection.