Research: High tunnels show promise
Growing method expensive, best for high-value crops
By STEVE BROWN
VANCOUVER, Wash. -- Protecting high-value crops from inclement weather is critical in much of Western Washington and Oregon, and high tunnels are a good way to do that, researchers say.
Carol Miles, vegetable specialist at Washington State University's Northwestern Washington Research and Extension Center in Mount Vernon, described her research, which included both successes and failures.
The protected environment inside a high tunnel proved surprisingly beneficial, she said. In comparing tomato crops, Mount Vernon researchers saw an 8- to 38-fold increase in yield by using the tunnels.
One early lesson was discovering how susceptible a tunnel is to the wind, she said. Just a few days after the first open-ended structure was installed at Mount Vernon, a windstorm from an unexpected direction shredded the canopy.
Aligning the tunnel with prevailing winds is important, but so is reacting quickly to changing conditions, she said. End braces add security, and metal hoops outlast PVC. Buttoning down the side walls and enclosing the ends helps the structure survive winds.
Beyond the design and structural integrity, what grows in the tunnel is what makes it pay off.
"I look at high tunnels as expensive real estate," Miles said, "so you need high-value crops. The tunnel adds heat, but it doesn't raise nighttime temperatures. Whatever you put in it has to withstand those lows."
Tomatoes are the primary high-tunnel crop in Washington, the U.S. and in the world, she said, but the practice is also useful for strawberries, melons, eggplants, basil and chili peppers -- whatever likes heat.
A four-season structure, built to withstand wind and snow loads, can also be used to grow salad mixes and leafy greens during the cool of winter.
Closed ends capture more heat, but good ventilation keeps diseases in check, she said. Researchers found that the open-ended tunnels had healthier tomato plants, but the tomatoes were higher quality in the enclosed tunnels.
Chris Guntermann, who sells high tunnel and greenhouse structures at Horticultural Services Inc. in Oregon City, Ore., said there are 115 different variations in size of structures, features and thicknesses of plastic covering.
"A greenhouse is never big enough," he said. "You'll always want bigger and more."
He offered tips in building and maintaining a tunnel:
* Infrared polyvinyl acts like a cloud cover, keeping plants warm at night.
* A roof with a 6-in-12 pitch will shed water; a 3-in-12 pitch won't.
* Washing the plastic will make it last longer and allow more light transmission.
* In choosing a tunnel, consider economics, seasons of use, ventilation, risk tolerance, weather threats, labor efficiency and expandability.
"Get the strongest greenhouse you can afford," Guntermann said.
Miles said growers learn as they go along that "what works for one crop may not work for another."
WSU Extension horticulturist Charles Brun told growers they have a lot of decisions to make. "There's not just one company out there," he said. "It's like buying a car."