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"When and how should I prune my butterfly bush ?
We suggest cutting butterfly bushes back to about 4 feet high for the winter (so that heavy winter snow won't crush the bush to the ground). And don't cut the butterfly bushes back too early; November is our recommendation.
Depending upon the severity of winter, your butterfly bush may appear dead in spring. Don't despair. Even when a butterfly bush doesn't develop leaf buds on the previous year's branches, it will usually sprout new growth from the root system. Be patient though. It might be late May before your butterfly bush shows signs of new growth.
In early June, cut off any branches which died over the winter. This is also a good time to trim the butterfly bush to the shape you'd like it to be.
During its blooming period, usually mid July through September, you might deadhead (remove spend flowers) every two weeks or so. This assures that the bush will put its energy into producing more flowers rather than seed."