Rainier cherries from trees on Gisela 5 and 6 rootstocks were significantly larger when grown under tunnels than when grown without tunnels. Preliminary data showed that 73 percent of the tunnel cherries were 9 row, compared with only 9 percent of the no-
High-density sweet cherry trees grown in tunnels produce larger fruit than trees outside tunnels, preliminary data from Michigan State University trials suggest. The trials also showed that establishment costs for the tunnel system, which are significant, may be justified under cherry growing conditions in the Midwest because of the higher crop values that would be received for the premium quality fruit.
Tunnel-grown Rainier cherries on Gisela 5 and 6 rootstocks at the Clarksville Horticultural Experiment Station, Clarksville, Michigan, weighed 12.5 grams, on average. In comparison, no-tunnel fruit grown in the same block and of the same variety weighed an average of 8.3 and 9.3 grams, depending on the rootstock.
But more impressive was the fruit size distribution in the tunnel cherries, said Dr. Gregory Lang, lead MSU horticulturist on the project (see "Rainier cherry yield and quality"). Of the total crop, 97 percent of the Rainier cherries grown under tunnels on G.5 were 9-row to 10-row, compared with 42 percent in those size ranges for cherries grown without tunnels. For Rainier on G.6 under the tunnels, 98 percent of the cherries were 9-row to 10-row, compared with 71 percent for cherries grown without tunnels. No more than 4 percent of the tunneled cherries were 10.5 row or smaller, while in the nontunneled cherries, up to 58 percent were 10.5 row or smaller.
The net crop value per acre (gross value less harvest costs) for the Rainier on G.5 in the tunnels was estimated at $19,100, compared with $14,850 for the Rainier cherries on G.5 grown with no tunnels, although with Rainier on G.6, the nontunneled fruit had a higher crop value than fruit grown under the tunnels.
Yields achieved in the tunneled cherries were 5.4 to 4 tons per acre, explained Lang. "That’s not bad yield for us in the Midwest, especially when you consider there was no cracking, no bird pecks, and very little cullage."
He thinks that target yields of 5 to 7.5 tons per acre are achievable and would make the tunnel production system economical. Labor costs are lowered when fewer, but larger fruit are picked, he added.
The Michigan State researchers have also noticed that new shoot leaves on the tunnel trees are about 20 percent larger than on trees grown without tunnels.
Pollination and fruit set
More work is needed, however, to improve pollination and fruit set of trees grown under the tunnel system.
"Yields in the tunnel were half of what they were outside," Lang said, noting that honeybees need polarized light for navigation. and the tunnels interfered with light requirements of honeybees. A related tunnel project in raspberries also demonstrated interference in the honeybees’ light requirements; fewer honeybees were found inside the raspberry tunnels.
"But when we brought in bumblebees, activity picked up inside and outside the tunnels," he said. "Bumblebees don’t need ultraviolet or polarized light."
Another pollinator option would be blue orchard or hornfaced bees. He believes that they may prove to be better pollinators than honeybees inside the tunnels.
"I think we can solve the pollination problem with bumblebees. It’s more an issue of activity than pollen source."
Pollination options that will be explored this year include putting bumblebee and honeybee hives inside the tunnels and placing hornfaced bee straws inside each tunnel.
Lang also suspects that the tunnel’s plastic sheeting is impacting bud fruitfulness and contributing to poor yields. He likens the plastic to having another set of leaves on the tree canopy that would further shade fruiting buds. To compensate for the shading from the tunnels, Lang will experiment this year with rigorously venting the tunnel sides to get more incidental lighting on the buds.
His plan is to put black plastic weed barrier material down the tunnel rows to heat up the soil in the spring during bloom and then follow the black with white Extenday reflective material to bounce light into the canopy.
The Extenday and venting should also help develop a better blush on Rainier and other blush varieties. Data showed that blush cherries in the tunnels were not as well colored as those outside. Lang is hopeful that bringing the tunnel sides up five to six feet from the ground will improve bud fruitfulness, blush color, and may eliminate some humidity within the tunnels.
Lang said that in recent months, midwestern growers have shown considerable interest in the tunnel research, particularly growers using U-pick and direct farm sales to market their crop.
"An advantage of the tunnels that the U-pick growers pointed out is that their customers could be there to pick whether it’s raining or not," he said, adding that much of the success of U-pick farms correlates with weekend weather during harvest time.
The tunnels also have shown noticeable reductions in spring frost damage, rain-induced cracking, and postharvest diseases. Additionally, there is potential for using integrated pest management and organic practices in the tunnel production system.
And while much research remains to be done regarding climatic issues, fruit set, yields, fruit quality, and tree architecture, the preliminary results thus far have been dramatic. The data have revealed potential advantages under Great Lakes growing conditions for growers who want to produce premium cherries under high density, dwarfing rootstock production systems, Lang concluded.