A polytunnel, often referred to as a “hoop house” or “high tunnel,” is a cost-effective, semi-circular structure designed for protected crop cultivation. It functions as an agricultural shelter that provides a controlled environment, making it a popular alternative to more expensive, traditional glass greenhouses. This structure allows growers to manage external climatic variables, creating a localized microclimate that favors plant growth and improves crop yield potential.
Core Components and Structure
A polytunnel is defined by its two main structural elements: a robust frame and a specialized covering material. The framework that forms the tunnel’s characteristic arch is typically constructed from galvanized steel tubing, which is favored for its strength and resistance to corrosion in humid conditions. For smaller, more temporary structures, materials like PVC piping or treated wood may also be used to create the arched supports.
These arches are connected by various bracing components, including ridgepoles and crop bars, which provide lateral stability and prevent structural collapse under wind or snow load. The entire framework must be securely anchored into the ground, often by burying the base rails or using specialized foundation tubes. The covering material is a UV-stabilized polyethylene plastic film, which is stretched taut over the frame and secured around the base.
This polyethylene film is engineered to be durable, often available in thicknesses like 200 microns for commercial use, and is treated to resist degradation from sunlight. Secure attachment to the frame is achieved using specialized locking systems or by burying the edges in a trench. Access is provided by simple doors at one or both ends, which also double as a means for ventilation.
How Polytunnels Function
The polytunnel creates a favorable growing environment by utilizing a principle known as the “greenhouse effect” or passive solar gain. Solar radiation, primarily in the form of short-wave light energy, passes easily through the polyethylene film and is absorbed by the soil, plants, and internal surfaces. These surfaces then re-radiate the energy as longer-wave infrared heat.
Because the plastic film is largely opaque to this longer-wave infrared radiation, the heat becomes trapped inside the structure, causing a rise in the internal air and soil temperature. This effect can raise the internal temperature by 5 to 15 °C above the ambient outdoor temperature, which is significant for plant metabolic processes. The physical barrier of the covering also dramatically reduces wind speed, which minimizes the convective cooling effect known as wind chill on the plants.
The enclosed nature of the polytunnel also helps to maintain higher humidity levels within the structure compared to the outside air. This stable, moist microclimate reduces the rate of water loss through plant transpiration, which is beneficial for plant health and growth. Growers manage the internal environment by opening and closing the end doors or using roll-up side panels to control ventilation, allowing them to balance temperature and humidity for optimal conditions.
Key Advantages for Cultivation
Season extension is a primary benefit, allowing for earlier planting in the spring and later harvesting into the autumn or even winter months. This is achieved because the elevated soil temperatures and protection from frost allow seeds to germinate faster and young plants to establish themselves before outdoor conditions are favorable.
The structure provides substantial crop protection against adverse weather conditions that can damage delicate foliage and fruit. Plants are shielded from heavy rainfall, which can cause soil splash and disease spread, as well as strong winds and hail. The physical barrier also helps to reduce the incidence of certain common pests and diseases by separating the plants from the external environment, although specific pest management is still necessary.
Furthermore, the creation of a warmer, more stable microclimate enables a wider crop selection than would otherwise be possible in a local climate. Growers can successfully cultivate more sensitive or warmth-loving plants, such as sweet peppers, aubergines, or certain varieties of tomatoes, that require consistently higher temperatures to produce a viable yield. The ability to control the environment allows for the commercial growth of high-value crops.