High Tunnels Vary Greatly

Strawberries In High Tunnels

Editor’s Note: The story’s author, Thomas Walters (twwalters@wsu.edu) is a small fruit horticulturist at Washington State University (WSU)-Mount Vernon. Others working on this project include Debra Inglis, also of WSU-Mount Vernon; Annette Wszelaki and Jeffrey Martin of the University of Tennessee, Knoxville TN; and Russ Wallace of Texas A&M University, Lubbock TX.

Many consumers want locally-produced, organic strawberries, but growers know they are not easy to farm. Organic strawberry growers must contend with erratic seasonal production, weeds, fruit molds, and other diseases.

A team of scientists funded by USDA’s Specialty Crop Research Initiative has been addressing these problems in three very different locations: Knoxville, TN (subtropical climate with hot, humid summers), Lubbock, TX (semiarid climate with dry summers), and Mount Vernon, WA (dry summer subtropical climate with cool, humid summers). At each location, both June-bearing and day-neutral strawberry cultivars were grown using an annual production system, with raised beds, black plastic mulch, and drip tape. We compared production in high tunnels and in the open field at each location, choosing tunnels appropriate to each location.

Four-season tunnels were used at Knoxville and Lubbock, and remained in place for the duration of the experiments. The polyethylene film covers of the three-season tunnels used at Mount Vernon were taken down in late October each year, and were not replaced until the following April. Organic production practices were utilized in Knoxville and Mount Vernon, but some conventional fungicides, insecticides, and fertilizers were used at Lubbock.

Higher Yields

High tunnels dramatically increased marketable yields at Knoxville and at Lubbock. In fact, tunnels were essential to growing marketable strawberries at Lubbock, where seasonal high winds severely damaged plants and fruits in the open field, and rendered them unmarketable. In contrast, high tunnels did not significantly affect yields in Mount Vernon, although they did substantially decrease losses
due to gray mold.

High tunnels were marred by the weather in all three locations. In Knoxville, open field plants were severely damaged by a hailstorm, rendering the fruits unmarketable; tunnels protected the fruit, but the tunnel covers were damaged by the hail and had to be replaced.

At Lubbock and Mount Vernon, tunnels sustained damage from windstorms and had to be repaired or replaced. At Knoxville, the best yields came from the June-bearing cultivar Strawberry Festival, the day-neutral cultivar Albion, and Chandler, often described as a weak day-neutral. At Lubbock, Strawberry Festival was also the best producer, followed by LCN and Chandler. The day-neutral cultivars stopped producing once it got hot in Knoxville and Lubbock, generally by June. In contrast, the day-neutral cultivars San Andreas and Albion were the most productive in the relatively cool environment of Mount Vernon, where they produced fruit from June through October both years.

Plugs Vs. Bare Roots

We also compared plug plants with their bare root counterparts. Plug plants of Albion produced higher marketable yields compared with bare root plants, more than compensating for the higher cost of the plugs. It was also easier to plant plugs through the plastic mulch than to plant bare root plants. Strawberry plug plants are already widely used in the southeastern U.S., but are not commonplace in the rest of the country, so availability may be an issue.

High tunnels and annual growing systems contributed to producing good quality organic strawberries in three widely divergent climates. At Lubbock, they were essential to growing strawberries at all. However, to be successful, appropriate cultivar and tunnel construction choices need to be made according to location.

Tunnels Can Be Tricky

One of the toughest challenges in producing organic strawberries is managing gray mold, commonly caused by botrytis. Rotted fruit is of course unmarketable, and infected fruit can appear fine at harvest but develop symptoms just a day or two later, dramatically shortening shelf life.

In Mount Vernon, WA, where gray mold is prevalent, high tunnels reduced gray mold incidence to about 10% to 20% of that in the open field. However, incidence of verticillium wilt was higher in the high tunnels in 2010, possibly as a result of higher soil temperatures under the tunnels.

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