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Growth Regulators
Young trees
Early flowering in a newly planted cherry orchard can delay vegetative development and lower long-term productivity. The use of gibberellic acid (Pro-Gibb 4%) beginning the year after planting can prevent overflowering in young trees (see Table 1-22).
Applications should be made as dilute sprays with a handgun, or airblast sprayer, in 25 to 50 gallons of spray per acre. Apply the spray 2 to 4 weeks after bloom, with 3 weeks the optimal timing. Apply 50 to 100 ppm (20 to 40 oz Pro-Gibb/100 gal) as a fine mist. Use higher rates for trees growing in poorer soils and lower rates for trees in high-vigor soils. Promalin may also be used to increase branching in young sweet cherry trees. Use it as you would for apples (see Growth Regulators to Aid in Apple Tree Training). Application can only be made, however, in a mixture with latex paint. Apply the mixture to nodes where branches are desired. Do not apply after buds have broken, or young shoots may be damaged (Table 1-22).
Bearing or mature trees
Foliar applications of Pro-Gibb 4% can help reduce blind wood and increase fruiting of tart cherries. The material works by affecting lateral bud differentiation, which is apparent the year after application. Therefore, changes in shoot, spur, and flower production will not be evident until 2 or 3 years after the program is begun. Once this period is satisfied, response will be yearly provided annual applications have been made.
Apply Pro-Gibb 4% as a foliar spray containing 4 to 8 fluid ounces in 100 gallons finished spray from 14 to 28 days after bloom (or up to 14 days after shuck split). Use full coverage sprays of 50 to 150 gallons per acre on medium to large bearing trees. Be sure entire trees receive good coverage. A good horticultural wetting agent will aid in foliar wetting.
Ethephon (Ethrel) may be used to loosen red tart cherries for mechanical harvest. Caution: Ethephon has caused severe gummosis in some Pennsylvania tart cherry orchards, affecting low-vigor trees most severely. Apply ethephon only to trees making optimum growth. Do not use more than 2.5 pints ethephon or less than 50 gallons of water per acre. Apply 14 to 7 days before anticipated harvest. Do not apply when the temperature is below 60°F or above 85°F. Be sure to leave some trees unsprayed for comparative purposes.