DEVELOPMENTS IN HIGH DENSITY CHERRIES IN THE USA

Cherry growers in the United States have been slow to adopt new high density systems with plantings of over 500 trees per ha. Early results with dwarfing rootstocks showed a clear trend towards a reduction of fruit size with many of the dwarfing rootstocks. Since large fruit size is very desirable in the fresh sweet cherry market, many growers in the Pacific Northwest and California preferred to work with the full vigor Mazzard rootstock, recognizing its limitations, rather than risk small fruit. These growers began to look for ways to overcome the disadvantages of full vigor rootstocks, the most important of which are large tree size and low precocity. To improve the early yields, new orchards on Mazzard rootstock are being planted at a moderately high density of 4 m x 5.5 m. To improve precocity, limb bending and minimal pruning are used. Branching is enhanced by scoring and, in some cases, the use of Promalin. Often, trees are summer-pruned to increase precocity and reduce vigor. The training system of choice in the Pacific Northwest is the Steep Leader system. It is an adaptation of the open vase system used by growers throughout the U.S. for many years. It has three or four nearly vertical scaffold branches with secondary fruiting branches on the leaders. Each leader is a mini central leader tree with a lower permanent scaffold branch. The system is best suited for low to moderate density orchards on full vigor rootstocks, although it has been adapted successfully to dwarfing rootstocks as well. A detailed pruning plan is presented. Growers believe that this training system will be able to maintain tree productivity on Mazzard rootstock at this spacing.