Taking raspberries inside for higher yields

Photos Courtesy of Eric Hanson.
These Michigan State University raspberries will be up to 30 percent larger than field-grown berries by the time of harvest.

Raspberries are an established and proven crop in Michigan. Although there are some 200 growers in the state, there are only about 500 acres of raspberries grown, so hardly anyone is growing more than 10 acres.Eric Hanson thinks that a new development in raspberry cultivation, the use of high plastic tunnels, could suit those growers and improve profitability. High tunnels are expensive to install, but they reap huge benefits in yield, quality and pest protection. Small growers, he says, are ideally suited to trying a small plot of raspberries under plastic and giving their farm an economic boost.

That’s just what high tunnels have done in his tests, says Hanson, a professor and fruit extension specialist at Michigan State University. Since he and his colleagues began studying the technique in 2005, tests have shown some remarkable results in this cool climate. Configured more like a miniature greenhouse than a plastic row covering, a high tunnel has a significant effect on yields.

Raspberry plantings under high tunnels are in narrow rows, but high yields result from an extended season, larger fruit and less damage by pests.

“It usually doubles the yield,” Hanson says, and results on specific varieties in some years show even more than that. One of the primary reasons for the yield increase is that the plastic covering extends the raspberry season, sometimes pushing berry production earlier than usual and then extending the season a week or two beyond that of field-grown raspberries.

This is true of both summer-bearing fruit such as the Canby and Nova varieties, as well as the fall-bearing varieties such as Heritage and Caroline. The summer bearing season is extended about a week, he says, to the end of July. The fall bearing season can be extended two weeks. This extension is more important, because it can add time right into the frost season, up to mid-October.

Hanson notes that season extension is due to some unknown factors, but in general the fall extension comes about less because of freeze prevention than because the tunnels keep plants vigorous. “It’s just a matter of keeping the temperature higher during the day so the plants don’t go into dormancy,” Hanson says. In general, temperatures are moderated around the clock. He points out that there may also be more yield in the first year of production under tunnels, which helps the system pay off quickly.

After the Haygrove tunnel was erected at Michigan State, it was estimated that it would take 240 man–hours to set up such a system.

Another effect of the tunnels is that raspberries often grow 20 to 30 percent larger than normal, Hanson says. In addition to the ideal climatic conditions under the plastic, he has measured a decrease in mold and rot in the Michigan dew and rain. There is a “fraction” of the Botrytis gray mold on protected tunneled plants in comparison to field plantings, and anthracnose on canes was a mild problem in field plantings, but absent from the tunnels in 2007, the first year this data was compiled. The same was true of leaf spot.

Hansen has 1/8 acre of test raspberries under high tunnel, and he says that quality fruit is a result of the physical barrier presented to insect pests. Field plantings can be severely damaged by potato leafhoppers and Japanese beetles, in particular, and there is little of this under the plastic.

High tunnels have been used for many different crops and are commonly used in England. Several universities, including Cornell and Oregon State, are testing them and finding them beneficial in crops ranging from strawberries and sweet cherries to cut flowers. Tomatoes have been shown to be one of the vegetables most responsive to the technique. Hanson has tested strawberries under his system and found it equally beneficial.

Hanson says that his tunnel system, and many of the others being tested across the country, are from the Haygrove Tunnels company in the United Kingdom. They have several basic types, which can be seen at their Web site, www.haygrove.co.uk, and Michigan State is using the multibay type. There is also a round type and a peaked-roof type designed to shed winter snow, which allows it to be used year-round.

All types use a steel structural system that supports plastic film that can feature different thicknesses and abilities to diffuse light and retain heat at night. Crops can be planted in the ground, as raspberries are, or suspended in elevated substrates. They can have various bay widths and leg heights. The MSU tunnels are 24 feet wide and 200 feet long, with raspberry rows 8 feet apart. That’s closer than the 10-foot spacing in the field, in order to take advantage of the improved growing conditions. Plants are drip irrigated.

Phil Prillwitz, who farms 180 acres of fruit in Berrien County with wife Terri, has 2 acres of raspberries under tunnels and agrees that yields are boosted and seasons extended for both summer and fall crops. In fact, although there is a definite learning curve in the management, he has virtually no raspberries outdoors anymore.

“Raspberries love it under tunnels,” Prillwitz says. Growing both red and yellow berries under plastic has enhanced his marketing season and given him “the whole crop,” without losses from weather or pests. He uses them primarily as a shelter, even leaving the ends off some tunnels, and the main benefits are keeping out rain, mold and insects. He also grows some currants under plastic and is testing strawberries and vegetables, such as tomatoes.

Oak Hill Fruit Farm’s first small tunnel was an experiment five years ago, but it was so successful that Prillwitz put in an acre of tunnel the following year and gradually expanded to 2 acres. He uses a system built by ClearSpan comprised of a series of connected 25-by-85-foot tunnels, 16 per acre.

He plants raspberries in narrow rows, from 5 to 7 feet apart, for maximum production. The houses have a round roof with greenhouse plastic designed to last four years. Prillwitz rolls up the plastic after the season and stores it either inside or outside. One drawback is that labor costs are higher for indoor raspberries, because plastic must be managed, and in these confined quarters, mechanical pruning can’t be used. Once the season is over, he uses a narrow-row tractor to cultivate. Drip irrigation is used.

Prillwitz points out that although yields are high because of the extended season, what he really counts on is getting the maximum yield of the raspberry plant at all times, and he has a higher–quality product that hasn’t been rained on or chewed on by insects.

The MSU tunnels are 24 feet wide and 200 feet long, with raspberry rows 8 feet apart.

The most important element to him is that in a spot market for a highly perishable product, he can take an order and know that the fruit will be there. He can harvest in rain or hail and still get a premium crop on any given day. Cost of tunnel installation is high, though he can’t give a current estimate because his equipment was purchased so long ago, but he figures he is making a profit on the berries harvested under this system.

Hanson verifies that investment costs will be high. His preliminary cost estimate for the system he has tested reveals that about $33,000 per acre total will be spent to get the Haygrove tunnels installed. About $26,000 of that is for the steel structure alone, with the rest going for plastic, labor and other costs. That doesn’t include planting the raspberries, of course.

“They should last at least 15 years,” Hanson says of the steel structure. Amortized out, the cost for plastic, labor and capital investment in future years amounts to about $5,000 per acre per year. Plastic lasts three to four years. Based on the MSU farm crew’s experience in erecting Hanson’s experimental tunnels, it would take 240 man-hours per acre to set it up.

Hanson estimates that raspberries would yield an extra 10,000 pounds per acre annually under tunnels. Depending on the price for each year’s berry crop, the initial investment could be paid off in three or four years, and there could be extra savings in inputs such as pesticides.

There are other potential drawbacks, however, including the management learning curve. One of the foremost threats is wind damage in gusty regions. Although the Haygrove systems can be fitted with anti-wind devices, such as stabilizing braces, and vented to allow wind to pass through (Prillwitz uses nylon ropes to tie his plastic down), Hanson says a storm could wreak some damage. He notes that a test at Kansas State University was abandoned due to wind damage.

“My recommendation is that growers try this in a small way,” Hanson says. It is a viable system with benefits that offset high initial cost, and other North American research has verified this. It is especially attractive for growers selling premium fruit at high prices to local markets such as U-pick or farmer’s markets, but not as attractive for raspberries destined for the frozen food market. He predicts that in the future, this might be an excellent system for growing organic produce, because higher crop prices could pay off the investment more quickly.

Don Dale is a freelance writer and a frequent contributor. He resides in Altadena, Calif.