Pruning Techniques for Hydrangeas
The best approach is to remove the thickest, oldest stems at ground
level, rather than shearing them. That way, you will remove the
flowers on those stems but not all of the flowers. You may be able
to keep it in bounds that way. Shearing tends to cause a thicket of
twigs at the ends of the stems, which block sunlight from the
interior of the plant and reduce air circulation through it. This
can create favorable conditions for diseases to develop, and it is
generally stressful for the shrub.
Pruning is not a substitute for proper plant selection. If you have
to constantly prune a shrub (or tree) to keep it in bounds, you may
have chosen the wrong plant for the site. It does a disservice to
both of you -- It creates more work for you and the plant cannot
grow to its genetic potential. Expect most oakleaf hydrangea
cultivars to grow 6 to 10 feet tall and wide, if not slightly
larger. There are dwarf cultivars of oakleaf hydrangea that may suit
your needs better, including 'Pee Wee,' 'Little Honey,' 'Shannon,'
'Sikes Dwarf' and 'Summit.' Most of these are 3 to 4 feet tall with
a slightly wider spread.