UV Light - Hoop House / Poly Tunnel
The amount of UV light that passes through a polytunnel depends largely on the type and quality of the plastic cover used. Polytunnels, also known as hoop houses, are typically covered with polyethylene film or other types of plastic sheeting, which can either block or allow certain wavelengths of light, including ultraviolet (UV) light, to pass through.
Types of UV Light:
UV-A (320-400 nm): This is the longest wavelength of UV light and generally the least harmful. Some of this light can pass through many types of greenhouse plastics.
UV-B (290-320 nm): This light can cause sunburn in plants and humans. Most greenhouse plastics block a significant portion of UV-B light.
UV-C (<290 nm): This is the most harmful type of UV light, but it is usually absorbed by the Earth’s atmosphere and doesn't reach the ground.
Factors Affecting UV Transmission Through a Polytunnel:
Type of Plastic Covering:
Standard Polyethylene (PE): Most polyethylene covers block a substantial amount of UV light, especially UV-B, while allowing UV-A to pass through. The exact amount of UV transmission can vary, but most standard films transmit between 70% and 90% of visible light, with reduced UV transmission.
UV-Stabilized Polyethylene: This type of polyethylene is treated with UV inhibitors that prevent the plastic from degrading under sunlight. While it still allows some UV-A to pass, it generally blocks a greater proportion of UV-B.
Anti-UV or UV-Blocking Films: Some polytunnel covers are designed specifically to block most or all UV light (both UV-A and UV-B). These are often used to protect sensitive plants or to create a cooler environment inside the tunnel. These films can block up to 95-99% of UV radiation.
Clear Polyethylene: Clear plastic films generally allow more UV transmission than opaque or white diffused films. They let through more than 80% of UV-A, but still block most UV-B light.
Thickness of the Plastic:
Thicker plastic films tend to block more UV light. Common polytunnel films range from 100 to 200 micronsin thickness. A thicker film will block more UV light than a thinner one, but it can also reduce the transmission of visible light, which is crucial for photosynthesis.
Film Age and Condition:
Over time, plastic films degrade due to exposure to sunlight, heat, and weather conditions. As the plastic ages, it may let more UV light through if the film becomes thin or damaged. Regular replacement of polytunnel coverings (typically every 3-5 years) is important to maintain optimal light and UV conditions.
Treated vs. Untreated Films:
Some polytunnel covers are treated to allow UV light through, especially in cases where UV light can be beneficial for certain crops (such as strawberries or tomatoes) in enhancing flavor, color, and disease resistance.
Practical UV Transmission Estimates for Polytunnels:
UV-A Transmission: 60-80% depending on the type of plastic and its clarity.
UV-B Transmission: Typically 10% or less, as most polytunnel plastics block a significant portion of UV-B light.
UV-C Transmission: Almost no transmission, as both the plastic and the atmosphere block UV-C.
Summary
In general, most polytunnel coverings allow UV-A light to pass through while blocking UV-B and UV-C radiation. Depending on the type of plastic you choose (standard, UV-stabilized, or UV-blocking), you can expect between 60% and 80% of UV-A to pass through, while most UV-B will be blocked. If you're looking for more precise UV transmission rates, checking the manufacturer's specifications for the plastic cover is advisable.
