In February I posted some information about one way to rejuvenate an overgrown shrub - cut it down to the ground and let it regrow. This is great for shrubs that just get out of control, or shrubs that have more colourful bark on young wood (eg. dogwoods).
One potential problem is that for very old, vigorous shrubs such as these lilacs, the large root system will have lots of energy to send up a gazillion new shoots in spring. If you don't prune some of them back, they will all be skinny and weak. So don't just chop your shrub to the ground and then forget about it, as these people seem to have done!
You want to prune at least half to two-thirds of those new shoots out (probably more) to give space and energy for the other half to become thick and strong. And, if you do it too late, then the shoots that are left will be too long and too weak and will flop over once they lose the support of the shoots you just removed.
So, this is a great and easy technique but stay on top of the new growth by pruning out at least half of it for the first year, until some good strong branches develop.
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