How do I prune my Hydrangea paniculata that blooms late in the season?
It is not strictly necessary to prune Hydrange paniculata, however if it has grown too large then you can certainly prune it to control its size. You will also get larger flowers if you reduce the number of branches on the plant. This species of hydrangea flowers on new wood i. e on branches that have grown in the current year of summer. If you want your plant to stay large then just cut back a few old branches each year to keep it under control. If you want to keep your plant small then cut back every stem to leave two or three buds on each stem. This could be done in late fall but it best to do the pruning in late winter or early spring before new growth has started in the warm season following winter.
Courtesy of the NYBG Plant Information Service