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Lilia Caldeira, data technician for Dr. Roberto Nuñez-Elisea, measures a summer-pruned cherry tree planted next to the control (at right) that is dormant-pruned. Photo courtesy of OSU

Lilia Caldeira, data technician for Dr. Roberto Nuñez-Elisea, measures a summer-pruned cherry tree planted next to the control (at right) that is dormant-pruned. Photo courtesy of OSU

Is there another way to keep sweet cherry trees small—but still productive and yielding high quality fruit—without using dwarfing rootstocks? An Oregon State University researcher thinks so and is collecting data he hopes will demonstrate it.

Cherry growers throughout the United States are interested in keeping trees small to reduce the need for ladders and create more pedestrian-style orchards. But there are orchard management challenges to cherry trees grown on both dwarfing and vigorous rootstocks.

Trees on vigorous rootstocks, like Mazzard, may take six years to generate a crop and provide a return on investment, explains Dr. Roberto Núñez-Elisea, an Oregon State University researcher based at the Mid-Columbia Agricultural Research and Extension Center in Hood River. Also, Mazzard trees allowed to grow without serious constraints are large, and not conducive to pedestrian orchards. They tend to produce moderate yields instead of heavy crop loads, but do produce large fruit of excellent quality.

While dwarfing, precocious rootstocks produce fruit in the third or fourth year, trees tend to crop excessively, he said, adding that they require skillful crop load and canopy management to maintain adequate fruit size, particularly after the fifth year. Spur, bloom, or fruit thinning is often necessary.

“The challenges of managing sweet cherry trees on vigorous rootstocks include controlling excessive vigor, stimulating production in earlier years, and increasing yields,” he stated in a research report. “On the other hand, the challenges of using dwarfing rootstocks include maintaining adequate wood vigor and leaf-to-fruit ratios to achieve large fruit.”

Middle ground

But what would happen if trees with intermediate characteristics between dwarfing and vigorous rootstocks could be grown?

Núñez-Elisea is studying the use of summer pruning of trees on Mazzard rootstocks to learn if the technique can make a Mazzard tree compact and small, increase precocity, but still produce fruit of high quality. Previous work on young Regina trees on Gisela 6 rootstock showed that summer-pruned shoots produced less vigorous growth and more spurs the following year than shoots that were not pruned.

Initial results show that summer pruning can keep the trees about eight to ten feet tall—50 to 75 percent the size of a normal Mazzard tree. The number of flowers was also increased, he reported.

“There is an effect on precocity,” he said. “With summer pruning, we can produce more flowers.”

But there is a big if that he hopes his research will answer this season.

“Can we make big fruit?” he asked. “Based on the potential crop load and leaf area, my guess is that there is an adequate balance to achieve good fruit size.”

The summer pruning study began two years ago on two-year-old Sweetheart/Mazzard trees planted on a 12 foot by 18 foot spacing, with 202 trees per acre. The trial is located in Hood River at grower-cooperator Tim Annala’s orchard. Núñez-Elisea is comparing growth and precocity of trees managed by different summer pruning treatments that vary timing and severity of pruning cuts. The control trees are managed by the grower-cooperator as traditional steep leaders and pruned during the dormant period. The summer-pruned trees do not receive any dormant pruning.

Results

Núñez-Elisea has already learned that tree size can be managed effectively with the summer pruning, which also encourages precocity. The control trees trained as steep leaders are an average of 11.2 feet tall with a canopy diameter of 9.7 feet, but the summer-pruned trees are half to three-quarters the size of the controls. He also observed that pruning cuts made by heading as compared to tipping produced smaller trees.

Heading is a more severe cut that leaves about one foot of wood after the cut is made. Tipping is considered a lighter pruning cut and removes only eight to ten inches of wood.

Summer pruning also reduced trunk size. The mean trunk cross-sectional area of the control trees was 73 cm2, whereas the summer-pruned trees were between 77 percent and 88 percent of the control size.

Núñez-Elisea also found that the type of pruning cut influences growth. Almost no new growth occurred after heading cuts made in August. Late summer tipping also resulted in no new growth in the same year.

The type of cut and pruning time also influences spur production. Tipping in early summer increased spur production by nearly 50 percent compared to controls and heading treatments, he noted. Heading in late summer produced fewer spurs than the controls.

Fruit quality

The big test now is to see if the shorter Mazzard trees are still productive and yield large fruit.

This season, he will collect yield and quality data from the study, as it is the first year that the trees will bear fruit.

Summer pruning has an added advantage of being done during dry weather, reducing the risk of bacterial canker that can occur from pruning wounds made during wet weather.

Núñez-Elisea added that growers are excited about the potential development of finding a way to use Mazzard rootstock, yet keep the trees small. Dwarfing rootstock can be difficult to obtain from nurseries and is not inexpensive due to royalties. Dwarfing rootstocks also require intensive management.

He envisions planting densities between 300 to 400 trees per acre, with yields of 30 to 40 pounds of fruit per tree in the fourth year.

Small trees also offer the benefit of being easier to net and cover for rain and bird protection, and there is potential to reduce pitting in the fruit because the shorter branches have less movement.

“There is a huge area regarding summer pruning that we don’t know much about in sweet cherries,” he said, adding that there are many questions about renewal wood, timing, rootstock and cultivar combination, canopy architecture, and such. “But I know that summer pruning works in keeping tree height down and promoting precocity.”