In October, it rained 9 inches where I live in eastern New York, near the Vermont and Massachusetts borders. Even before the temperature dropped low enough to destroy fruit quality, these frequent, sometimes torrential, rains ruined the fall raspberry crop. In a good year the berries are picked from early September to mid-October, but more typically, hard frost or rain devastate production and quality even though the canes are otherwise poised to continue yielding.

On November 11, in an experimental high tunnel on the Cornell University campus, the raspberries were fruiting as if there were no tomorrow. Marvin Pritts, Cornell’s horticulture professor who specializes in small fruit, is pleased with the considerably higher yields they have achieved from the second year of the planting. “We don’t yet know when the harvest will stop,” he said, delightedly. As it turned out, their 2005 berry harvest would continue until almost the end of November.

For several years, Pritts has been experimenting with production systems that would allow northern growers to harvest raspberries during an increasing number of months in the calendar year. In October, he unveiled outstanding preliminary results for the first harvest of the raspberry high tunnel trials. Yields have been extremely high, averaging nearly 2 pounds. per foot of row in control plots of Heritage after only nine weeks of harvest. Fruit appearance was excellent although sugar content dropped during the three weeks of almost constant rain.

Musing on the system’s success, Pritts said: “With such an unusual fall, the raspberry harvest went later than I expected. Only one question remains in my mind: Is this weather a fluke that really benefited us?”

Typically, northern growers prefer fall-bearing types that fruit early to avoid frost. Pritts’ objective was to delay fruiting until late in the fall when the availability of fresh raspberries is low and the price is high.

Mid- and late-fall raspberry production is especially appealing to apple growers who market through their own farm stands. “Most of the people who have followed up with me are direct marketers who are trying to keep their markets going,” Pritts said. High tunnel berries would be very attractive for pick your own in bad weather or later in the season as it gets cooler. Otherwise, growers who already have a harvest crew available will be more inclined to be seriously interested.

Extending the fall season
In the Cornell system, an unheated high tunnel is used to enhance the performance of fall raspberries well beyond the time when they would typically wane. Most of the planting is in Heritage, a northeast fall favorite which, Pritts said, “in a normal season goes great guns in September and then after a hard frost is done.” Other varieties were also trialed.

The canes were planted in April 2004, mowed to the ground that fall, and then the framework of a 96 by 30 ft. high tunnel was erected over the planting in spring 2005. The tunnel was covered with plastic film in mid-September, just prior to harvest. Native bumble bees provided all necessary pollination. After the berry plants began bearing, a spun polyester row cover was flung over the canes whenever frost was expected.

In an attempt to delay flowering and fruiting and thus extend the season beyond that of field-grown berries, the Pritts team tried several experimental treatments which they compared to control plots. In one treatment, in late February after a period of cold weather, they applied straw over the mowed rows at the rate of 6 tons/acre. In other plots, they mowed canes to the ground in early June shortly after they emerged. There were also two pinching treatments in which they removed the top 4 to 6 inches of the primocanes when they reached a height of about 2.5 feet, and 3.5 feet respectively. Each of these four treatments delayed flowering.

Pinching the canes at 3.5 feet tall most successfully delayed fruiting, while pinching earlier created a canopy so thick that they had to thin it out to allow bees to get in. Mowing would need to be done earlier than June, as those canes were just beginning to fruit in mid-November. Finally, the straw mulch effectively delayed flowering and berry production.

The second part of the experiment examined several high-quality varieties that often cannot be completely harvested in this region due to frost. The varieties are Caroline, Josephine, Autumn Britten, NY01.63, NY01.64, and NY01.65. These numbered selections were made by fruit breeder Courtney Weber who suspects that they may have traits that allow them to perform well in high tunnels. So far, the researchers have been especially happy with Caroline, which produced very well, and they have identified Josephine, which has an excellent taste, as one of the promising varieties that normally fruits too late outdoors for the climate.

The price tag for a purchased 96 x 30 ft. high tunnel (steel, lumber, and plastic) is about $5,000. Such a sum could easily be spread out over the 10 or more years of life expectancy for a tunnel. Also, because plastic film only stays on these raspberry high tunnels from September until the end of the fall harvest, expenditures for plastic will be lower than for high tunnels kept covered year round. (The berries require a cold period to fruit and also prefer cool weather to enhanced summer heat).

Drawing from his observations in these trials, Marvin Pritts explained how a grower could spread out the fall raspberry season further. He suggested treating each of a high tunnel’s three beds differently. To accelerate some berries, use row cover over one bed in early spring when the canes are just emerging. Leave alone one bed, and then use one of the delaying treatments in the third bed. The high tunnel could be covered with plastic as early as late August or early September.

Winter production
Pritts’ first attempt to enable growers to produce berries off-season established a system for getting a prolific harvest of “incredibly sweet” raspberries from mid-February till mid-April. His starting point was a quest to find a use for greenhouses sitting empty in the cold of winter.

Pritts grew varieties that normally cannot be grown in the north as they don’t tolerate temperatures below 10 degrees Fahrenheit. The raspberries are grown in pots kept outside until their leaves drop and the required chilling has occurred. When they are moved into a minimally to moderately heated greenhouse in mid-December, flowering is induced and bumblebees are brought in.

The greenhouse is maintained at 55 degrees at night and 70 degrees during the day. Warmer temperatures would slow down photosynthesis and production.
The beauty of this system, according to Pritts, is that “most of the growing takes place outside where the light is free.” The canes are already grown by the time the pots are moved to the greenhouse.

Taste tests with chefs demonstrated a demand for these delicious berries. Growers scattered around the northern half of the U.S. from Washington to New Jersey have adopted this profitable system on both a small and large scale.

One of Pritts’ future plans is to develop a way for northeastern growers to produce spring raspberries. He plans to build another high tunnel and test various manipulations of appropriate varieties. He is also eager to create a high tunnel growing system for accelerating late season primocane-fruiting blackberries developed in Arkansas so they will bear in more northern climates.

For more information, Marvin Pritts will have the season’s full data available in several months, after analysis of the fall-bearing high tunnel harvest. The experiment will continue during the 2006 season. Pritts can be reached by phone at 607-255-1778 or by email at mpp3@cornell.edu.
Several publications about high tunnel production of raspberries and strawberries are available on the Cornell website: http://www.hort.cornell.edu/extension/commercial/fruit/berry.html.ï ¹

Tracy Frisch was the founder of the Regional Farm & Food Project, an eastern NY-based sustainable agriculture organization known for its farmer-to-farmer workshops and networking and its local food system initiatives. She left RFFP after nine years to build her own energy-efficient, solar-powered house. Her current focus is writing while finding time to care for her land and work for social justice. Tracy can be contacted at tracy@farmandfood.org or 518/692-8242.