Posted: 24/10/2014 at 17:26
Philadelphus sometime rejuvenate quite well. I'd take a third of the branches out to a low level this winter, then another third after flowering next summer, and the third third after flowering the following summer. Removing the oldest looking growth first.
In February give it a good sprinkling of Fish Blood and Bone and mulch well and fingers crossed in a couple of years you'll have a more reasonably sized shrub which blooms well.
I'd take some hardwood cuttings this winter as well, as that shrub has been around a while and it would be good to grow a replacement or two as the rejuvenation may not be totally as you'd wish. Even with a bit of plastic surgery we usually show our age somewhere