2018 ASHS Annual Conference

Plastic films used to cover protective structures such as high tunnels and greenhouses vary in their light-transmitting characteristics. These films are manufactured to transmit a high proportion (often 90% or greater) of visible light (and thus PAR), but depending on the film, may block a significant portion of UV-A, UV-B, and/or infrared (IR) radiation entering or leaving the structure. Different plastic films also diffuse light to varying degrees, affecting the extent to which light is scattered as it passes through the film. An ongoing multi-state project is testing three commonly used films with a wide range of light-transmitting characteristics, along with two experimental films. Among these commercially available films, transmission of UV-A and UV-B ranges from roughly 5 to 90% depending on plastic film type and wavelength, transmission of visible light ranges from 70 to 95%, and transmission of near IR radiation ranges from 50 to 95%. The experimental films either block nearly all UV, or allow about 80-90% transmission of UV, while allowing transmission of over 90% of wavelengths longer than UV. In an experiment on day-neutral strawberries in single-bay high tunnels in 2016 and 2017, yields with all films were greatly increased compared to no covering, but differences due to film type were minimal, perhaps because plants were at ground level and exposed to similar temperatures when tunnels were vented. With two cultivars of primocane-fruiting raspberries in PA in 2017, the first year in which effects were due primarily to plastics rather than previous growth in the nursery, there was an interaction between cultivar and film type. The film that transmitted high light levels across the spectrum, diffused light the least, and resulted in the highest foliage temperature produced the highest yields with one cultivar, and tended to produce the highest yields with the other one. When used on low tunnels for day-neutral strawberry production on raised beds covered with either white or black plastic mulch, films that transmitted higher amounts of UV tended to result in a brighter red color, presumably due to increased production of anthocyanins as reported in other research. Film type effects were noted for certain pests, especially Japanese beetles, whose numbers were reduced to the greatest extent by the films that most markedly reduced UV light transmission.