Forsythia are a spring-flowering, cane shrub. That means it blooms on old wood, and should be pruned 4-6 weeks after it finished flowering. If you wait longer (until summer or fall) to prune, you will be removing some of next year's buds. And as a cane shrub, it should never be trimmed with hedge shears!
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Prune Like a Pro_Forsythia.pdf
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