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Covering cherry trees with tunnels could open all kinds of opportunities for growers, especially those who contend with rainfall and humidity. In addition to being able to protect fruit from rain, weather-induced diseases, and insects, the tunnels may expand the list of cherry cultivars that can be grown in some regions.

For the last three years, Dr. Gregory Lang, horticulturist at Michigan State University, has studied cherry trees grown under tunnels at MSU’s Clarksville Horticultural Experiment Station and the Southwest Michigan Research and Extension Center in Benton Harbor. At Clarksville, tunnels were placed over mature Rainier cherry trees, while at Benton Harbor, more than 35 cherry varieties were planted under tunnels. Many of the varieties planted are ones that have not performed well under high disease and ­rain-cracking pressures in Michigan.

Thus far, tree growth data has come from Benton Harbor, with fruit data coming from Clarksville. Data will also be generated from the Benton Harbor tunnels starting in 2008, when the trees produce their first crop.

Preliminary data showed that the tunnels produced larger fruit than uncovered trees, but color and yield were lacking, said Lang. “There was less red blush on the Rainiers grown in the tunnel because of the shade,” he said, adding that cherries need ultraviolet light for coloring. Yield was also a problem because of poor pollination.

To overcome the low yield, Lang used bumblebees last year for pollination instead of honeybees. Yields inside the tunnels nearly doubled, going from 5.4 for Rainer on Gisela 5 and 4.0 tons per acre for Rainier on G.6 in 2006, to 8.8 and 8.4 tons per acre, respectively, in 2007. The control treatment of Rainiers on Gisela 5 and 6 grown outside the tunnels produced 8.0 and 8.6 tons per acre, respectively.

“There was statistically no difference in yield between being inside or outside the tunnels,” Lang explained. “If anything, inside the tunnel was slightly higher yield, which might have been due to a slight frost during bloom that impacted trees outside.”

Fruit size in the tunnels in 2007 was still large, but because yield was higher, fruit size was smaller than the previous year. Tunnel fruit size averaged 10 to 11 grams in 2007, he said. “But even with equal yields between the inside and outside of the tunnels, fruit size was still better under the tunnels than outside,” Lang said at the International Fruit Tree Association annual ­meeting.

To improve color of the fruit grown inside tunnels, the reflective material Extenday was used under the tree rows after bloom. Also, trees inside the tunnel were pruned to improve light interception, and the sides were opened to let in more light. The extra steps worked, he said, as the Rainiers inside the tunnel developed a “nice blush color” in 2007.

Tree growth

Growing trees under the tunnels has its challenges, with shade being one of the biggest.

“The tunnels automatically impose shade over the trees,” Lang said. “We’re growing the trees with about 25 percent less light.” Light measurements showed that the three-year-old plastic sheeting used for the tunnels had a 75 percent light transmittance.

Leaves on trees inside the tunnels were about 20 percent larger than the outside treatment, he noted, due to the shade response. “The outer leaves are happy with 75 percent of light, but leaves on the interior are getting an extra dose of shade,” he said. Also, lateral shoot growth was greater for trees inside the tunnel compared to shoot growth on trees outside the tunnel.

But while the added shade can be a negative, on the plus side, the tunnels decrease winds by 10 to 15 percent, reducing stress on the trees.

Data on tree growth showed that the tunnel trees were 24 percent taller than trees outside. “This is not necessarily a great thing if you’re growing trees under a tunnel structure,” Lang said. “We really don’t want to grow tall trees quite so fast. Tree growth could be challenging in the future.”

Lang and his colleagues also found that tunnel trees with Extenday grew larger than the outside trees with Extenday.

In 2006, the trunk girth of trees inside the tunnel was smaller than trees outside. However, by 2007, the trunk girth of the tunnel trees had increased 35 percent from the previous year, and they were 22 percent larger than the outside trees.

Lang thinks the tree size difference relates to the health of the trees. The outside trees are under more stress and the leaves respire more, he explained. “The tunnel trees are living under luxurious conditions. The extra steps of venting, using Extenday, and pruning for light are probably secondary contributions to improved tree growth,” he commented.

High-value crops

“The concept of tunnels looks sound,” Lang concluded. “It appears that we can grow good quantities of high-quality fruit.”

His research team still needs to learn how to optimize all the factors involved in the tunnel production system, from the number of trees per acre to learning how quickly the trees come into production, he said.

“When you put plastic over the tree, you’re changing everything. When you make one adjustment in the system, it can affect other issues.”

Lang said he hasn’t yet analyzed the economics of the system because they are still refining best ­tunnel practices.

But he stressed that the tunnel system comes with a caveat. “It’s not for everyone. You have to know that you have a high-value market for your crop—be it a Whole Foods type of natural or organic market or your own farm market. It has to be a high value market where you can receive a premium.”

The tunnels are also being tested on apricots, another high-value crop but one that struggles with fruit set in the Midwest because of frost issues. “Apricots fit the high-value crop definition even for growers with their own farm market,” Lang said, but added that the lack of dwarfing rootstocks for apricots will add additional challenges to growing the trees under tunnels.