High tunnels are widely used for raspberry production in much of the world, and it is easy to understand why. The longer growing season, increased yields and improved quality, and the ability to schedule tasks without concerns, for example, the occurrence of rain-changing plans, make tunnel production appealing. Growers also often cite improved fruit appearance as a reason for growing raspberries in high tunnels.
From 2015 to 2019, a trial was conducted at Penn State as part of a large SCRI project, “Optimizing the Protected Culture Environment for Berry Crops,” led by Eric Hanson at MSU. This part of the project looked at the high tunnel production of raspberries and strawberries with the goal of determining whether plastic-type affected yield and pest complexes.
Plastic films used on high tunnels are typically 6-mil thick and are categorized as “clear” or “diffuse.” Clear plastics scatter light less and diffusing plastics scatter light more. In specification sheets for plastics, this is often referred to as “clarity” vs. “haze.” With a diffusing plastic film, the light should be spread more evenly throughout the tunnel and the plant canopy, theoretically making better use of available sunlight and decreasing sunburn (Figure 3a and Figure 3b). You also may see references to “light transmission” or “light transmissivity.” This describes how much visible light (which also covers the range of wavelengths that plants use for photosynthesis) passes through the plastic film.
With a new plastic cover, 85-90% transmission is common for the majority of clear and diffusing plastics. You’ll also see references made to the plastic films being either “UV-blocking” or “UV-stabilized.” UV light breaks plastic down without additives to protect it. If a UV-blocker is used, UV light is prevented from passing through the plastic, much like a UV-blocking sunscreen would work. If the plastic is UV-stabilized, the plastic contains an additive that prevents the plastic’s chemical bonds from being broken down, but most of the UV light still passes through. This is what allows a high tunnel film to typically have a 4-year life, rather than becoming brittle and falling apart. Then there are “IR” films, which partially prevent the wavelengths that we feel as heat from entering the tunnel during the day and being emitted at night.
Read the complete article at www.smallfruits.org.