Extreme weather wreaks havoc on greenhouse poly film

With 15 acres of greenhouses under his management, Jeremy Dewar at North American Plants in McMinnville, Oregon, replaces polyethylene (poly) film on a recurring basis. Lately, however, he is replacing it at a higher rate than usual.

Like other greenhouse operations in Oregon, the greenhouses at North American Plants have been battered by extreme weather events over the past three years.

Between the ice storm of 2021 and the heat dome of 2020, where temperatures reached 115 degrees in parts of the Willamette Valley, poly film greenhouses in Oregon have taken a beating.

The heat dome, particularly, was damaging at North American Plants, said Dewar, maintenance manager at the micropropagation nursery.

“Right where the fasteners are, where the plastic touches the metal, the plastic got so hot it was ripping apart,” Dewar said. “And a lot of it was only about a year, year-and-a-half old at the time. That is not natural for plastic. Maybe for really old plastic, but not for new plastic.”

Dewar added that the nursery’s greenhouses weathered the ice storm that brought a foot of ice to some parts of the valley in February of 2021 without incident. And, he said, ice and snow typically don’t cause problems at the nursery.

“Since we put air between our plastic, it provides insulation that keeps our greenhouses warmer, and when the ice and rain hit, that helps it slide off instead of getting stuck in the curves and arches. Also, our fasteners are pretty flush with the top of the plastic, so it shoots off pretty well,” Dewar said.

“It’s just the heat that seems to be the biggest issue,” he said. “Our summers are getting to be more like California summers, and it is putting extra stress on the plastic.”

UV protection

Extreme heat, according to Michelle Moore, CEO of Adapt8, can break down the ultraviolet (UV) layer in poly film, causing it to crack and sag, and lowering the plastic’s life expectancy.

“The UV coat on film that protects it from sun degradation can be impacted by not just the direct rays of the sun, but also the heat, because it can chemically tie up the UV,” Moore said.

“The sagging caused by the heat would make it more susceptible to other damage as well. So, if there is a snow after that, you’ve already got the stretch going,” she said.

Poly film is made to withstand ice and snow, Moore said, but as it stretches, ice and snow tend to be disproportionally distributed on a greenhouse, which can stress greenhouse piping to the point of collapse.

Multiple greenhouses were lost in the ice storm of 2021, Moore said. “And in 2008, there were $84 million of greenhouses that collapsed in that snowstorm. That is not because the plastic failed. It stretched and held. But the frame couldn’t handle the disproportional weight.”

Heating greenhouses can mitigate the effects of snow and ice on poly film and help prevent it from accumulating on plastic, according to Melanie Miller-Gonzalez, president of OBC Northwest, which manufactures greenhouses in Canby, Oregon.

Growers also can push ice and snow off greenhouses with a broom or other implement to ease the pressure on greenhouse piping.

“You can manually push it off,” Miller-Gonzalez said. “But sometimes, there might not be enough manpower at a nursery, and if we get a big snow event, a grower might have to make a decision to cut through the poly film to save the pipe structure of the greenhouse, because all that weight will demolish a greenhouse.”

In general, Dewar said North American Plants likes to get four years out of the 6-millimeter poly film it installs on its greenhouses, an expectation in line with the industry standard, according to Miller-Gonzalez.

“If it is applied correctly, the 6-mil does have a four-year expected life span if it is used as a double-layer material with an inflation fan that creates an air layer in between for insulation,” Miller-Gonzalez said.

This past year, however, the nursery was replacing a lot of three-year-old poly film that was damaged in the heat dome of 2020. “We actually are replacing the plastic on a ton of houses,” Dewar said.

In addition to the cost of purchasing the poly film, Dewar said the time and expense involved in installing it can be substantial.

“Installing is not easy,” he said. “We’ve come up with a system of our own that makes it a little easier, but it is still a lot of work, and I’m paying four guys for three hours just to get the plastic off and put it back on. And when you are doing ten, 15 houses, that can add up.”

Other steps

While there is little a grower can do to preserve poly film in the face of 110-degree heat, there are steps growers can take to extend the life of their film, according to Miller-Gonzalez, including avoiding using certain substances in greenhouses.

“There are a lot of substances you shouldn’t be using in your greenhouse,” Miller-Gonzalez said, “and I’m not sure all growers are aware of what is actually deleterious to the poly’s longevity.”

According to a brochure from Berry Plastics, chlorinating solutions or household bleaches should be avoided, as well as copper sulfate, sulfur and certain other crop protection products. “Herbicides, insecticides, fungicides and fumigants commonly used in greenhouses may affect some greenhouse films and cause them to resist UV sunlight for a shorter time,” the brochure states. “Try to avoid direct contact with the film.”

“It can break down the poly,” Miller-Gonzalez said. “You can have a 6-millimeter, four-year poly that doesn’t even last a year.”

Opting for a 6-millimeter or even 8-millimeter poly film, rather than a 3- or 4-millimeter film, also can help a grower get more out of plastic, Miller-Gonzalez said. The price difference between a 4-millimeter poly film and a 6-millimeter poly film can be substantial, typically around 30%, Miller-Gonzalez said, but a grower often can get twice the longevity out of a 6-millimeter poly film.

Miller-Gonzalez added that greenhouse poly film comes in many thicknesses, with 3, 4, 5, 6 and 8 millimeter being the most common options, and 4 and 6 millimeter being the predominate thicknesses used in the industry. Also, she said films are available in clear, white, white/black and black.

Going with a lower-cost, thinner film, such as 4-millimeter poly film, can be a viable option in certain situations, Miller-Gonzalez said. “It depends on the exterior environmental conditions, as well as interior environmental conditions,” she said. “You might get four years out of 4-mil poly film, but typically it is one to two years.

“If a grower has the means, a 6-mil product in double layer that is properly inflated with an inflation fan is the way to go,” Miller-Gonzalez added. “You will get a lot more life out of your film.”

There are also different quality 6-millimeter films, according to Dewar, who recently decided to go to a higher grade in hopes of getting more out of it.

“It is still the same company that we have been using, but we are using a different line of theirs,” he said. “It costs a little more, but we are hoping to get a little longer life out of it.”

Double-walled material

Solexx, a double-walled insulated material with a ten-year warranty manufactured by Adapt8, is another option for greenhouse covering. While two to four times the cost of poly film, Adapt8 CEO Moore said that in certain situations, the material can be worth the extra cost.

“If you are a grower who is growing nursery stock and just want a frost cover and aren’t adding any heat, film can make a lot of sense, especially if you are getting four years out of it,” Moore said. “Where it really makes a difference is where somebody is growing a high-value crop and adding heat.”

The material’s more substantial structure can provide better protection for plants in extreme weather events, she said. And it can save on heating costs.

“It used to be natural gas was $2 a thermal unit,” Moore said. “It is $8 right now, and it is going up to $10. So, the cost of heating has gone up substantially.”

Moore noted that rebates available from Energy Trust of Oregon also can significantly lower the cost of Solexx.

Mitch Lies is a freelance writer covering agricultural issues based in Salem, Oregon. He can be reached at [email protected].

From the February 2023 issue of Digger magazine | Download PDF