Answer: It is either a late frost or winter injury that's causing the dieback, says Bill MacKentley, owner of S. Lawrence Nurseries, specialists in hardy fruits in Potsdam, New York. Your cherries could be in a frost pocket that consistently gets late frosts each year. In that case, mulch the cherries with aged compost six inches thick in fall to slow the onset of flowering and growth in spring, he says. Another possibility is that your bush cherries weren't properly hardened in fall, so the tender shoots were killed in the cold winter weather, MacKentley says. Even though Hansen's bush cherry is hardy to zone 2, if it was still actively growing in fall, it might not have had time to harden off before the cold weather set in, he explains. To help the plants harden off in time, stop fertilizing after mid June, he says. The compost added the previous fall as a mulch should serve the dual purpose of a mild fertilizer in spring.
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