Organic Broadcaster

Taste of sweet success: Strawberry season extended using low-tunnel production

By Steve Poppe and Esther Jordan

Berries-e1381261210198Availability of locally grown strawberries is extremely limited in the Upper Midwest, primarily due to the short growing season. Fruit is an important part of a healthy diet, and while there is an expressed interest in having greater access to locally grown straw-berries, lack of suitable varieties and production systems has prohibited growers from being able to fulfill this need in our region.

With part of a 2013 USDA Minnesota Specialty Crop Block grant and funding provided by the North American Strawberry Growers Association, researchers at the University of Minnesota (UMN) Department of Horticultural Sciences and the West Central Research and Outreach Center (WCROC) in Morris embarked on a new project, one that would not only study newer day-neutral strawberry varieties in an organic low tunnel system to extend the season, but also improve fruit quality and reduce inputs.

Comparative field trials were established on the University’s certified organic research land and on grower-cooperator land. If successful, this new method of growing long-season strawberries may help increase the number of strawberry growers in the Upper Midwest, increase yields and availability of locally grown strawberries from June through October.

Goals of the Project

We had three goals for this study:

1. Determine if newer day-neutral cultivars grown under organic management on raised beds differ in yield and plant growth characteristics when grown under low tunnels compared to open field.

2. Use our research and innovative growing techniques to contribute to an increase in the number of strawberry growers in the Upper Midwest.

3. Contribute to improved nutrition among consumers by offering fresh strawberries during a non-traditional time.

Why day-neutral strawberries?

Traditionally, the most successful varieties for field production in our region are June-bearing types. Newer day-neutral strawberry varieties, coupled with novel production methods, may offer growers the option of a longer harvest sea­son using environmentally responsible methods.

Day-neutral strawberry varieties produce flowers and fruit continuously when temperatures are optimal for plant growth. Recent USDA research on day-neutral strawberry varieties grown under low tunnels has resulted in increased yields of high quality fruit when compared to open-field-grown plants, with reduced incidence of bacterial and fungal diseases, fewer weeds and reduced water use.

Construction and Maintenance

strawberry tunnelsThe first step in creating our low-tunnel system was to construct a series of raised beds six inches high and two feet wide, allowing for 17,500 plants per acre. Dormant day-neutral cultivars were planted in white on black plastic mulch in a staggered row with drip irrigation. Using steel rods, black poly stoppers, clear plastic and twine, we constructed a low tunnel system for each of the raised beds, resembling a “covered wagon.”

Throughout the early part of the season, flowers and runners were removed from the plants. This allows the plant to establish and have leaf surface to support later fruit production. We also closely monitored strawberry plants for insects, specifically, the tarnished plant bug (TPB) and spotted wing drosophila (SWD). Using an integrated pest management approach, we routinely checked for the TPB, since it is one of the more prevalent insects to affect strawberry fruit. One of the newest pests to threaten quality strawberry fruit production is the SWD. As a precautionary measure, we placed several SWD traps near our strawberry plants. While the SWD has been identified in numerous areas in Minnesota, we did not detect any SWD females in our strawberry plots at our Morris site.

Harvesting and Yield Data

Harvesting traditional June-bearing strawber­ries in Minnesota typically begins in mid-June, and usually is completed by early July. Our day-neutral low tunnel strawberry plants began producing berries beginning the third week of July 2013, and continued until mid-October 2013.

Traditional June-bearing strawberry varieties in Minnesota have a baseline yield of 5,500 pounds per acre (lb/A). As shown below, lb/A for each of the six cultivars in the low tunnel and non-low tunnel surpassed this baseline. Data from both trial sites (WCROC and St. Paul) are included. Preliminary data from USDA low tunnel trials in Beltsville, Maryland calculated yields for day-neutral varieties varying between 8,600 lb/A to 19,000 lb/A. Yield data from the Morris site for the first year under low tunnels exceeds this benchmark for all cultivars and all systems.

Pounds per Acre

LT = Low Tunnel, raised bed with plastic; PL = open field, raised bed with plastic; SM = straw mulch, raised beds without plastic

LT = Low Tunnel, raised bed with plastic; PL = open field, raised bed with plastic; SM = straw mulch, raised beds without plastic

Data also was calculated to determine the aver­age number of lbs. per plant over the duration of the harvest season. The range of commercially acceptable lbs./plant for day-neutral cultivars is 1 to 1 1/2 lbs. Day-neutral cultivars grown under low tunnels averaged 1.45 lbs. per plant at the Morris site and .90 lbs. at the St. Paul site, while on plastic in the open field 1.02 lbs. per plant were recorded at the Morris site and .89 lbs. at the St. Paul site.

During the late summer/fall picking season at the WCROC site, we tasted a noticeably sweeter strawberry. We randomly chose berries to ana­lyze sugar content by measuring brix levels. The average brix level was 7.6 between late July and early October in both low and non-low tunnel treatments. For comparison, we randomly took brix readings in the June-bearing variety trial between late June and early July; the average brix level was 7.7. In 2013, day-neutral cultivars were just as sweet as the June-bearing cultivars commonly grown in Minnesota.

Looking Ahead

Since the hardiness of these new day-neutral cul­tivars has not been determined for Minnesota, we are growing the day neutral varieties as annuals. We removed all plants and the low tunnel system plastic from our certified organic land to maintain the integrity of organic certification. This project is scheduled for another year of production.

For step-by-step instructions on constructing a low tunnel system for strawberry use, or for more information on the project, please visit our low tunnel strawberry blog at the UMN Commercial Fruit website, strawberries/low-tunnel-strawberry/.

Cold Climate Strawberry Farming, a UMN interactive eBook designed to offer commercial strawberry growers with successful growing practices in the Upper Midwest, will be released in June 2014. This eBook offers information on innovative marketing techniques, cultivar recom­mendations, and video segments of particular techniques. It will be free of charge and may be accessed on any computer or mobile device. Its release will be announced on the blog, on Facebook (, and Twitter (@coldclimestrawb).

Steve Poppe ( and Esther Jordan ( work for the University of Minnesota West Central Research and Outreach Center in Morris, Minn.

From the March | April 2014 Issue


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