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Workers wrap a rolled-up rain cover in black plastic to protect it through the winter. It remains attached to a cable above the tree row.

Workers wrap a rolled-up rain cover in black plastic to protect it through the winter. It remains attached to a cable above the tree row.

Geraldine Warner

To grow cherries in Europe without rain covers is difficult because of the high risk of fruit cracking and bird damage. Covers are expensive, but it’s an investment growers have to make in order to grow cherries in wet climates, observes Will ­Sibley, chair of East Malling Trust in the United Kingdom.

Sibley and a group of members of the International Fruit Tree Association visited the orchard of Jaco Elenbaas at Lewedorp in the Zeeland area of The Netherlands, where Elenbaas and his son Johann have installed Voen covers over young cherry blocks.

The family has 86 acres of tree fruits, including 12 acres of cherries, and will double their cherry acreage in the next few years. Their older cherries are on Colt rootstocks while their more recent plantings are on the more productive Gisela 6, 5, and 3. Trees on Colt rootstocks are root-pruned each year on alternate sides.

Posts

Ferdinand Sailer, representative for Voen GmbH & Co.KG of Germany, said most of the covers in The ­Netherlands and United Kingdom are supported by steel posts. In Scandinavia, growers use almost exclusively wooden posts, and in Italy growers use concrete posts. In Germany, some posts are wooden and some steel.

One advantage of galvanized steel posts is durability. Sailer said the posts could last 60 years and can be moved to another orchard after the first one is removed.

Each post is bolted to a three-foot long anchor, which is drilled into the ground leaving about 16 inches above the soil level. Posts are almost 15 feet tall and can be installed up to 30 feet apart. The cover system can be used with row spacings from 10 feet to 20 feet.

The woven polyethelene cover is produced in Germany and is UV stabilized to withstand 500 kilolangleys of exposure. It is made of overlapping strips, similar to a tile roof, allowing air to pass through. “The covers work with the wind, not against the wind,” Sailer said.

The covers are clipped to cables running over the tree rows and are fastened together with clips between the rows. All the cables are above the covers so they don’t interfere with machinery used in the orchard.

Most growers unfurl the covers when the cherries start to turn color, although some open them early in the season for frost protection. Sailer said the temperature under the cover can be 1 to 3 degrees Celsius higher than outside at that time of year. If there’s no wind, it can work well for frost protection.

If the orchard is covered at bloom, bees need to be placed inside. Usually, bumble bees are brought in as well as honeybees because they wake up earlier and work when the weather is cold. “They’re a little slower than honeybees, but they work consistently,” Sailer said.

In the Netherlands, covered orchards are usually enclosed around the sides to keep birds out.

Sailer estimates the lifespan of the cover at between 5 and 8 years, depending on the amount of wind it has to withstand, how long the covers are open, and how well the material is cared for. Branches that hit the cover can cause damage.

“When we discuss the covering with growers, they say they will care for the materials, but if they have seasonal workers who don’t care, then it’s a problem,” Sailer said. “Seasonal workers have to care for the material as if it is their own.”

Hibernation

In preparation for winter, the cover is rolled up on each side and fastened with bungie cords to the cable running down the tree row. Black plastic sheeting is then fastened around the cover and secured with a bigger bungie cord. Last year, the company tested a prototype “hibernation” machine that is designed to speed up the process of rolling up and storing the cover.

Early the following season, the black plastic can be removed, so the cover is ready to be quickly opened up when needed.

Sailer said a permanent black sheeting is recommended so it doesn’t have to be recycled and replaced each year. The sheeting can be stored on the top cable during the summer.

He estimates the total cost of the covering system at 40,000 to 50,000 euros per hectare (US$22,000 to 28,000 per acre), depending on whether steel or wooden posts are used. A good team can install the cover in 120 to 150 hours, he said.