I know I should have pruned around about July but didn't get around to it. It's huge and making a corner of my garden very dark. Is it too late or should I now wait for the spring? Not sure if it's ever been properly pruned.
...I should of course added 'my mock orange' !
Philadelphus responds well to being cut back quite hard and you could cut it all back to the base, but then you'd miss out on the flowers for a couple of years. I think I'd work through it starting this autumn, cutting back one third of the shoots each autumn for 3 years, that way you'll still get some flowers but after 3 years you'll have a totally rejuvenated shrub.
As Philadelphus blooms best on 2nd year wood, after you've done the 3 year pruning, I'd leave it a couple of years before pruning again, then repeat the 3 step process but at 2 year intervals from then on.
That sounds quite complicated but I think I've explained it right. A scattering of Fish, Blood and Bone would be good for it after pruning or in the early spring.
Thank you Dovefromabove and Verdun. Most helpful.