A polytunnel is a tunnel-shaped structure used in horticulture to create a protected growing environment for plants. It consists of a framework covered with a durable plastic film, resembling a large, elongated hoop house or high tunnel. This structure allows gardeners and commercial growers to modify the outdoor climate, safeguarding crops from unpredictable weather conditions and temperature fluctuations. Polytunnels establish a customized microclimate that supports the cultivation of various fruits, vegetables, and flowers, extending the growing season and improving crop yields.
Structural Components and Materials
The physical foundation of a polytunnel is its frame, typically constructed from galvanized steel tubing bent into semi-circular arches or hoops. This steel is treated with a protective zinc coating to prevent rust and corrosion, ensuring the frame’s integrity. The hoops are anchored directly into the ground using heavy-duty ground stakes or foundation tubes, providing stability against wind and weather. The structure is further stabilized by ridge tubes, crop bars, and diagonal bracing mechanisms that run the length of the tunnel.
The transparent covering is typically a UV-stabilized polyethylene (PE) film, tensioned tightly over the frame. This material is lightweight, flexible, and allows for significant light transmission; thickness varies from 140 to 250 microns depending on use and climate. A primary advantage of this plastic is its ability to diffuse sunlight, scattering the light evenly across the plants inside. This diffusion prevents scorching and promotes balanced growth. The polyethylene cover generally requires replacement every 5 to 10 years due to degradation from ultraviolet radiation.
Primary Function in Horticulture
The fundamental operation of a polytunnel relies on the greenhouse effect to create a warmer environment than the outside air. The polyethylene film allows short-wave solar radiation to pass through, warming the soil, plants, and air inside. The tunnel then traps the resulting long-wave thermal energy, retaining heat that would otherwise escape. This solar heat retention elevates the internal temperature, often by 5 to 15 degrees Celsius above the ambient outdoor temperature.
This controlled microclimate provides protection from damaging weather events, including heavy rain, strong winds, and frost, which can physically damage vulnerable plants. By shielding crops from these elements, growers can reliably start seedlings earlier in the spring and continue harvesting later into the autumn or winter. This extension of the growing season can add several weeks to the productive calendar. Furthermore, the physical barrier helps exclude many common pests and diseases, reducing the need for chemical intervention and fostering healthier plant growth.
Polytunnels vs. Traditional Greenhouses
A primary difference between a polytunnel and a traditional glass greenhouse lies in their initial purchase price and construction requirements. Polytunnels are significantly more cost-effective, often costing only a fraction of a comparable-sized greenhouse. Unlike a greenhouse, which typically requires a level, solid foundation, a polytunnel can be installed directly onto slightly uneven ground with minimal site preparation. This flexibility simplifies the installation process and reduces the labor and time involved in setting up the structure.
The covering materials also present a clear contrast in performance and maintenance. Glass in a greenhouse is durable but expensive to replace if broken and may focus sunlight, causing plant scorching. The polyethylene film of a polytunnel, while needing periodic replacement, offers superior light diffusion, which is better for plant health. The film is also much easier and cheaper to repair or replace. Polytunnels are inherently more portable; they can be dismantled and relocated if necessary, whereas a glass greenhouse is considered a permanent fixture.