When to Prune Hydrangeas so they bloom! Mophead and Lacecap Hydrangeas (pink, blue or purple) bloom on old wood meaning that their buds start to form in the fall for next summer. If they are trimmed in the fall, winter or spring, all of the buds will be removed and your hydrangeas will be flowerless. ONLY PRUNE IN JUNE AND JULY! Or if shrubs are too large for their space. Remove dead stems or flowers anytime of the year – this should be done annually and will not affect regrowth.