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This Dutch cherry orchard has a rain cover and is enclosed on the sides with netting to keep out insects and birds.

This Dutch cherry orchard has a rain cover and is enclosed on the sides with netting to keep out insects and birds.

Photo by Geraldine Warner

So-called rain covers for cherries provide benefits far beyond just protecting the fruit from damage by rain or hail, says Reinhard Vöhringer, a German orchardist who developed the Voen covering system, which is being used on more than 2,500 acres around the world.

The Voen cover system consists of overlapping strips of plastic film sewn to a base fabric. Vents between the strips allow the air to flow through so that high temperatures do not build up beneath the cover. The cover creates about 20 percent shade, or more if it is not kept clean.

High density

Vöhringer said covers are most suitable for high-density orchards where trees are spaced 2 to 3 meters (6 to 10 feet) apart with 4 or 5 meters (13 to 16 feet) between rows. He recommends having space between the top of the trees and the cover. In Europe, Gisela 5 is the standard rootstock for high-density cherry plantings, with the more dwarfing G.3 used for Regina cherries. Krymsk rootstocks are being tested.

Covers are extremely important in parts of Europe where rainfall is high, such as southern Germany, Vöhringer told growers at the B.C. Tree Fruit Horticultural Symposium in Kelowna, British Columbia, Canada, earlier this year. As well as preventing cracking of the fruit, covers allow harvest to proceed uninterrupted by rain. They permit harvest timing to be based exclusively on fruit maturity, rather than being dictated by the whims of the weather.

The predictability of harvest enables growers to better organize their labor force and also has marketing advantages. Producers can contract with buyers to supply cherries of a known quantity and quality, Vöhringer said. That means that retailers can advertise cherries with the confidence that they’ll have fruit to sell, and the result is better returns to the growers.

At the same time, growers’ costs can be reduced. Covered cherries don’t incur the high sorting costs at the warehouses as cherries that have been exposed to rain. “Good fruit saves a lot of money in sorting,” Vöhringer remarked.

Temperature

Covers have other benefits unrelated to rain. For example, by keeping the orchard dry, a cover can minimize the need to spray for brown rot during bloom. The humidity underneath a cover can be slightly higher than outside, Vöhringer said, but not high enough to be a problem.

The cover can also help protect cherries from spring frosts. Generally, the temperature under a Voen cover is 1 to 3 degrees higher than the outside temperature, and heaters or sprinkler systems can be used under the cover to raise the temperature by another 4 degrees. A grower who can protect cherries from frost in a year when other growers have frost damage stands to make more money than when everyone has a good crop, Vöhringer pointed out.

Pests

To exclude pests, such as cherry fruit fly, spotted wing drosophila, or late-season wasps, and avoid the need to spray insecticides, the cover can be converted into a cage by installing insect netting around the sides. Research in Germany and Switzerland has shown that this is an effective way to exclude pests. For cherry fruit fly, the mesh size should be 1.35 by 1.35 mm. Growers in the Netherlands use covers and side netting to exclude birds.

Fruit size

Daytime temperatures under the Voen cover are slightly lower than the external temperature, but night time temperatures are slightly higher. Vöhringer said cherries thrive in the more moderate climate, and both the leaves and fruit tend to grow bigger. “I am still ­surprised how big can be cherries under the cover,” he ­commented.

Research by Dr. Manfred Büchele at the KOB research center in Bavendorf, Germany, showed that 42 percent of cherries from trees that were not covered were larger than 30 mm (1.2 inches) compared with 71 percent of those from covered trees. Larger cherries mean not just higher prices, but lower picking costs, too.

A basic cherry orchard in Europe produces about 13 tons of fruit per hectare (5.4 tons per acre). Using figures from Europe, Vöhringer calculates that a covered orchard would yield 15 percent more (making 15 tons per hectare), whereas an uncovered orchard might lose 30 percent of the yield to birds and rain (leaving 9.1 tons per hectare).

The larger fruit from the covered orchard could return 3.50 euros per kilo ($2.09 a pound), compared with 2.50 euros per kilo ($1.50 per pound) for the uncovered orchard. Returns would total 52,325 euros per hectare ($27,771 per acre)  for the covered orchard versus 22,750 euros per hectare ($12,074 per acre) for the uncovered, which means the cover would increase returns per hectare by 29,575 euros per hectare ($15,696 per acre).

Therefore, it would take less than two crops to recoup the cover’s cost of about 40,000 euros per hectare ($21,123 per acre).