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Not all plastics for high tunnel use are created equal

By RICHARD SKELLY
AFP Correspondent

ATLANTIC CITY, N.J. (March 21, 2017) — While the layman might argue that plastic is plastic, there’s the difference. Kathy Demchak, a senior agricultural Extension agent and other researchers at Penn State have identified more than 130 types and brands of plastic available for use on high tunnels and a multitude of other uses.
Demchak has 15 years of experience with berry crop production in tunnels and gave a detailed talk on various types of plastics that can be used for high tunnels at the New Jersey Agricultural Convention and Trade Show.
“There are synthetic plastics out there, but most of them are designed for shorter term uses,” Demchak said. “If you want a plastic that lasts longer than four years, there are other products out there you can buy. There are other types of materials out there and plastics that are modestly reinforced with cotton or another fabric material that’s weaved through the plastic itself.”
In her research at Penn State,  Demchak said a primary goal was to identify what was available in the marketplace for farmers.
“We figured we’d find maybe 20 or 30 different plastics out there. Instead we found 132 different types of plastics available out there. We realized this was going to be more complicated than we first thought, and at least 51 of those are available in North America, so we got samples of all of these plastics and got some additional ones and we measured their characteristics for letting light transmit through, so we looked at UV characteristics and diffusion of light.”
Demchak and her team at Penn State found out that the best plastics for high tunnel use include sheets and rolls that have some additives to prevent the plastic from breaking down in various ways.
“We also looked at characteristics for letting light transmit, so we looked at transmittance from UV, visible and infrared light forms,” she said, noting her team also looked at diffusion, noting “some of the plastics that were very clear allowed more heat in.
“If you’re growing tomatoes or raspberries, a diffusing plastic can have some advantages,” she said. “Cucumbers are another crop where it might be useful to have a better diffusing plastic.”
Plastics that tend to diffuse the light more are more difficult to see through.
“If the light is being diffused, it feels cooler to you and it’s coming at the plant from different angles,” she said. The type of plastic used for the roof of a high tunnel can also affect the breakdown of pesticides, she added.
“Plants can’t put on sunscreen, but they can produce antioxidants so the results is healthier plants,” Demchak said, adding that farmers and backyard gardeners with their own high tunnels need to be cognizant that the side of the plastic with the coating on it will be less reflective of sunlight, “so when you put the plastic on high tunnels, you want to point the less reflective side facing up,” in order to maximize sunlight exposure.
All of the results of Demchak’s studies with light diffusion and plastic types for high tunnels have been posted on a website affiliated with the research conducted from Penn State, www.tunnelberries.org.