Apr 13, 2017

Low tunnels show good results for strawberries

June-bearing (short-day) strawberries are a high-value crop, but their brief harvest season severely limits the window of opportunity for making a profit.

At most, with a combination of cultivar and production methods, the harvest season might last six weeks, but for most growers it is only about half this long. Rainy weather during these three weeks, especially if it occurs on weekends, can have a significant financial impact on growers, particularly if they market through pick-your-own.

“It would greatly benefit growers if strawberries could be produced over a longer season, into the summer and fall, as this would extend the season and open up new markets,” Marvin Pritts, horticulture professor at Cornell University, said at the recent Great Lakes Fruit, Vegetable and Farm Market EXPO in Grand Rapids, Michigan.

He covered the topic along with Kathy Demchak, senior Extension associate at Penn State University.

“In the 1980s, varieties of strawberries (day-neutral) with the capacity to produce flowers during all day lengths (spring, summer and fall) were released to the public,” Pritts said. “While there was initial excitement with these new varieties and their flavor was excellent, grower interest waned.”

He said growers recognized yields were low, fruit size was small, berries were expensive to pick and tarnished plant bugs (TPB) damaged the ripening fruit.

However, a new generation of day-neutral varieties was released in 2004.

“Although these originated from California, they were relatively well adapted to the Northeast, producing much larger fruits and higher yields than earlier releases,” he said. “They produce fruit the year of planting and continue fruiting into the fall. After overwintering, they produce another flush of fruit in spring.

“The fall crop and the second-year spring crop can be protected from rain and cold temperatures by covering rows with plastic on metal hoops – a technology called low tunnels,” Pritts said.

The tunnel plastics not only exclude rain but decrease the amount of ultraviolet light and infrared radiation – reducing spore germination and heat load on the plants.

“The combination of day-neutrals and low tunnels has the capacity to extend the strawberry season from three weeks to five months,” he said.

Pritts made some recommendations:

Growers should establish raised beds (18 inches or wider) in late fall or early spring so they can be planted as soon as possible in spring. Delaying planting until May or June will significantly decrease yields.

Each bed should have a trickle irrigation line attached to a fertilizer injection system. Cover each bed with white plastic and plant Albion in a staggered double row, with plants 9-12 inches apart in each row.

Use a tool that will insert roots into the bed while disturbing the plastic as little as possible.

“Albion is the variety that has the best flavor and performs consistently well in our climate,” he said.

Remove the flowers for the first three weeks or until vigorous new leaves appear from the crown. Plant grass seed between the rows, or at a minimum place straw mulch along the edges of the plastic beds to prevent mud from splashing on the berries.

Fertilize the planting with 1-2 pounds of actual nitrogen per planted acre per week. Install tunnels when plants begin to throw new flower trusses. Cover the tunnels with 4 to 6 mil plastic.

Pritts said Dubois Agrinovation sells kits with plastic that has predrilled holes for ventilation when the plastic is lowered.

“This cost is recovered in the first year,” he said.

At least one side of the plastic should remain up under normal weather conditions to allow for pollination and to prevent heat build-up. Lower the sides when the weather is cold or stormy. A benefit of the plastic is the near elimination of common diseases such as botrytis (gray mold) and fruit anthracnose. Fertilize with nitrogen according to local recommendations.

“In New York, it is recommended to increase the nitrogen to 5 pounds an acre per week once the plants begin to set fruit.”

Harvest the fruit at least twice a week. Peak yields will occur in late August to early September, with production occurring through October.

Once the temperature falls below 40˚ F, lower the tunnel sides. If the temperature falls below 30˚ F in mid-October, cover the entire field with row covers for the night to continue fruiting.

“This will extend the harvest season should the weather warm again,” he said.

Once harvest is over, lower or remove the plastic and cover the beds with straw.

“Albion does not overwinter well in cold weather,” she said. “Remove the straw in late March/early April and allow these plants to fruit again.”

The tunnel can be used to protect from late spring frost.

“Over the course of the first year with an April planting date, we harvested 20,000 pounds per acre, which is as much as a good June-bearing cultivar will produce in one season. Average berry size of Albion was 15 grams, which is the size of a medium king fruit on a June-bearer. Flavor is excellent.”

He said production peaked in early September with two quarts (four pints) of berries per 10 feet of row, but in October plants consistently produced about a quart of berries every 10 feet of row until a hard frost.

In spring of the second year, a large flush of fruit is produced about the same time as that of early June-bearers. Tunnels can be used to accelerate flowering if desired. Spring yields can be almost as much as the previous year’s yield.

“We have not found it to be economical to hold over these plants into a second summer and fall,” he said. “Rather, we grow them for about 15 months and then remove them.”

Pritts said this past summer, in particular, with 26 days above 90˚ F was not conducive for second-year production.
“We found that, while attractive, growers may not be able to ‘fit’ such a crop into their farm operation since day neutrals require constant attention,” he said.

“Plastic has to be raised and lowered, plants have to be fertilized weekly, and once harvest begins it lasts for months,” he said. “However, the rewards can be great.”

Pritts said growers have reported gross sales of $50,000 per acre from Albion in New York state. Given that the cost of materials for an acre is about $44,000, sales can pay for the materials in the first year. In the second year, costs include plants, fertilizer, labor and harvest. Conservatively, this can be $20,000, but with sales approaching $30,000 or more, the margins are quite good.

Pritts said spotted wing drosophila damage has been minimal in trials, provided that fruit is harvested regularly and not left rotting in the field.

— Gary Pullano, associate editor