This is a great time of year for cutting back overgrown shrubs, especially spring-flowering ones. Lilac is a prime target, as well as evergreen ceanothus, berberis, pyracantha… pretty much anything that doesn’t flower later this year, really.
I spent a couple of hours laying into a client’s lilac last week. It must have been 20 years old, and about that long since it was last looked at: it was heavily congested in the centre and the 4″ thick branches were splaying outward and endangering his shed. And it was about 25-30ft tall, which would have been fine in a big garden but his is a small-ish suburban strip.
I like to think of this kind of pruning as rejuvenating pruning – though it also counts as common-or-garden hacking back. What you do is remove about a third to a half of the wood – taking out the thickest, oldest branches first, and then any which are pointing in the wrong direction, as well as any which are diseased and/or dead, of course. If you’re me, you find it hard to know when to stop, especially with lilacs: when they’re overgrown, they tend to develop long, leggy stems with the leaves and flowers right at the very end, and that means once you’ve taken out the bulk of the shrub, you’re left with some rather sad-looking whippy lengths of wood waving around in the breeze. It all looks very odd.
Fear not: this is quite normal. The thing to do is to leave it looking a bit wierd this year, and wait until the stump starts pushing up new growth, which it will do very quickly. Then next year you can cut out another third of the long whippy old wood; and by the third year, when the new growth has got to a decent height and will be flowering well, you can take out the last third to be left with a bush that’s all the same height, hopefully a nice shape and full of strapping young growth which looks far nicer and is much more healthy.