Jul 27, 2007

Branches Become Mini-Trunks for Sweet Cherries

In a new experimental orchard in Prosser, Wash., and in cooperating orchards throughout the state, sweet cherry trees are being planted at a slant and then bent near horizontal and fastened tight to a trellis wire. At planting, dormant buds are rubbed off the bottom and those left on top become not permanent limbs but renewable mini-trunks in a solid fruiting wall.

These are the first steps in a novel training concept designed to produce a cherry orchard that is precocious, productive, compatible with automation and, potentially, mechanical harvesting.

The Prosser orchard is a research project of Washington State University (WSU) horticulturist Matthew Whiting, who showed it off in June during the annual sweet cherry field day at the university research station. The field day was attended by a group of about 200 fruit growers, some on a summer tour with the International Fruit Tree Association.

Normally, in establishing new orchards, growers “head back the unbranched whips and start over,” using the heading cuts to establish buds for scaffold limbs, Whiting said. With the new system, “you take advantage of the wood you pay for.”

In this orchard, the trees were planted as tall whips and put into the ground at an angle of 20 to 30 degrees. The whips were then bent down and tied to a trellis wire about 2 feet off the ground.

Along the whip, all buds were removed from the underside, leaving upright growing buds 12 to 18 inches apart. This approach will yield what Whiting calls “upright fruiting offshoots.”

At the end of the first year, the strongest uprights are not headed. Only weak uprights are pruned during the dormant period, leaving a short stub to encourage strong new growth. The idea is to make all of the uprights equally strong and add about 2.5 feet of new growth each year.

Sweet cherries are strongly upright and want to grow.

“Let them,” Whiting said.

Other than using size-controlling Gisela rootstocks, the upright shoots are encouraged to grow vertically. Laterals that form on the uprights are removed. Whiting believes the precocity of the Gisela rootstocks will be important for early returns and for moderating vigor as trees come into early production.

“We will add wires and clip the uprights to them, but we don’t know how tall they’ll grow – we are learning as we go with various cultivar/rootstock combinations, in this orchard and those of our grower cooperators,” Whiting said.

A key to this approach is the systematic renewal of fruiting wood, accomplished with aggressive dormant heading cuts into the upright fruiting offshoots. Moreover, the potential to incorporate automation and future mechanization will be critical. Creating a compact fruiting wall is key, he said.

The WSU orchard was planted with 10 feet between rows and 8 and 6 feet between trees. The idea is to fill the space in the rows with fruiting wood and leave alley space for workers to prune, tie and harvest from platforms.

“Improving labor efficiency will be important if cherry growers in the Pacific Northwest are to remain profitable,” Whiting said. “Our vision for the next generation cherry orchard is of superior cultivars on precocious, productive and size-controlling rootstocks trained to vertical or angled fruiting walls comprised of repeated upright fruiting offshoots. The architecture, we propose, must be readily compatible with automation and mechanization and take advantage of the natural sweet cherry growth habit.”