Cracking under tunnels
Keeping rainwater out of the root system will be necessary to prevent fruit cracking.
A solid set canopy delivery spray system is under study to provide chemicals and nutrients in Michigan State University's cherry tunnel project. The white reflective Extenday material is also under study.
One of the main reasons Dr. Greg Lang began experimenting several years ago with growing cherry trees under plastic tunnels was to protect the tender fruit from rainfall. But the results after rain fell on the tunnels last year were not what he expected.
Since 2005, the Michigan State University horticulturist has been covering cherry trees with high tunnels at two locations in Michigan. At MSU's Clarksville Horticultural Experiment Station, mature Rainier cherry trees have been covered with tunnels, while a range of cherry varieties have been planted under cover at the Southwest Michigan Research and Extension Center in Benton Harbor. Lang is now evaluating more than 40 varieties planted under the tunnels. Many are varieties not typically planted in Michigan because of the region's potential for disease and fruit cracking from rain and humidity.
From year to year as the research has progressed, Lang has made adjustments for the complexities of growing trees under an unnatural environment. One of the first challenges to overcome was pollination and fruit set. By switching from honeybees to bumblebees for pollination, yields inside the tunnels are now equal to trees not covered, with larger fruit from the covered trees due to less stress on the trees.
"There are environmental impacts from the tunnels," Lang said during a seminar on fruit production in tunnels at the Great Lakes Fruit and Vegetable Expo in Grand Rapids, Michigan. Managing light interception is very important under tunnels. His research found that the tunnels increase the growing degree-day accumulation by 10 percent compared to no tunnels. Additionally, the plastic tunnel sheeting reduces photosynthetic active radiation (PAR) by 25 percent and filters out nearly 60 percent of the infrared light waves.
"You have to work hard to increase the light interception at the lower levels of the trees inside the tunnels," Lang said. With trees kept to about nine feet tall to fit inside the tunnels, the fruiting zone is low, around five feet off the ground. He's had to pay close attention to fruit-to-leaf ratios to ensure there are enough leaves to support a crop.
He found that color in Rainiers was improved greatly by using the reflective material Extenday under tree rows after bloom and by pruning for better light interception. Lang continues to study white and silver reflective material and is also evaluating a black weed-barrier fabric for the benefits of weed control, conserving soil moisture, warming soil up quicker in the spring, and serving as a barrier to soil insects.
Other important aspects to tunnel management include managing temperatures, managing irrigation and humidity, and managing fertility. Switching from microsprinkler to drip irrigation and raising the sides of the tunnel have helped reduce humidity. Fertigation is being used to spoon-feed nutrients to the dwarfing rootstocks. He is also studying the use of a stationary sprayer, with microsprinkler lines running overhead, to apply nutrients and chemicals. If chemicals can successfully be applied overhead, tractors will be unnecessary, and another row of trees could be planted in place of a tractor alley.
Lang is especially pleased with tree growth under the tunnels. Nursery trees planted under tunnels last April were nicely branched and already filling the orchard space by June, he noted, adding that a crop will be ready for harvesting in the second or third year.
"With the precision bud manipulations that we are doing, we're putting the branches exactly where we want them," he said. "That's critical because we're dealing with very expensive space."
The dwarfing trees grown under the tunnels are an average of nearly 25 percent taller, with 20 percent larger leaves, Lang reported. Lateral shoot length is also greater under the tunnels. Also, the white reflective material has helped to increase tree growth by about a third compared to outside trees without the Extenday material.
But while there are many advantages to growing cherries under cover, Lang found that protection from cracking last year was not one of them.
"For the first three years of the study, we had no rain and no cracking," he commented. But in early June 2008, when fruit were most susceptible, significant rain fell in a short time period. "Rain cracking is the big issue with tunnels, and here's where we had some real eye-opening results."
For Rainier cherries inside the tunnel, about 60 percent of the fruit was cracked; outside fruit showed 89 percent cracking. Lapins showed 32 percent cracking under the tunnels; outside was 91 percent.
"What's going on?" he asked. "We kept the rain off the fruit. But we still had cracked fruit."
Lang said he recalls at least one report from New Zealand where cracking was observed under rain covers. "But in all my years—15 years worth—of cherry research, I've never seen that happen."
He believes that while tunnels kept rain directly off the fruit, the four inches that fell in a week's time poured off the plastic into the soil and was taken up the trees. "With the right warm temperatures and high humidity, and all that water coming into the tree, there was no transpiration going on to get the water back out of the leaves," he explains. "The fruit continue to swell. They are full of sugar, they take up water, and they eventually do crack.
"This impressed upon me that if you're going to grow cherries under high tunnels, you need to deal with the water coming off the tunnel canopy and get it out of the root system."
In Europe, plastic gutters are often used at the top of the tunnels to run rainwater out of the orchard before it even hits the ground. However, with the need to vent the sides for temperature and humidity control in Michigan, gutters won't work, he said.
He believes that ground gutters or trenches will work in sandy soils, but may not be the best solution in heavy soils that hold water. For heavy soils, subsurface drainage tile lines may be a solution for managing excess soil water and getting rainwater out of covered orchards.
Lang still has many issues to address as he fine-tunes cherry production in high tunnels. In the coming seasons, he will be working on netting the tunnel openings to keep birds out; developing a protectant strategy for certain insects and diseases; and evaluating different tree training systems for the best fruiting wall. Further work is needed to learn the best methods of moving rainwater out of the tunnel area to prevent cracking.
Thus far, tunnels have eliminated the need for multiple sprays for Japanese beetle and cherry leaf spot disease, but Lang found that copper sprays are still needed for protection from bacterial canker. When the tunnels are removed for the winter, to allow trees to go dormant, the trees are still exposed to canker infections.
He is convinced the high tunnels have potential for organic production of sweet cherries in Michigan, although work is still needed to assess effective organic controls for insects like aphids, mites, and plum curculio.