There are 4 different classifications of hydrangea: Annabelle, PeeGee, Oakleaf, Mophead. The best time to prune Annabelle and PeeGee is during the fall, so anytime between September and November. You can trim them down low to the ground because they will grow back with new buds on the new wood every spring. Since Oakleaf and Mophead produce buds on old wood, the best time to prune them is after their summer bloom, so from mid-June to late-July.