I have an old lilac bush which was cut down to rejuvenate it about 2 years ago. It grew back with numerous shoots. I removed a lot of them and kept just a few. Those few have grown very tall, with mop tops and are very skinny. They bend from the weight of their tops. Should I prune them in some way so they will be less top heavy?
Pruning back your lilac bush (Syringa vulgaris L) to give it a boost was a great idea. If you are rejuvenating a lilac bush that has started to produce only small clusters of flowers, pruning back to the ground (inches from the soil) in the early spring will give a welcome boost. As you probably observed, it took a few years to see those fragrant blooms again but it will be worth it.
Lilacs can grow immensely tall – 30 ft for some varieties – if let unpruned. As lilac flowers grow on old wood the best time to prune is right after they finish flowering, usually early June (again depending on the variety). Aim to maintain their natural shape with plenty of air space (which means pruning out older branches in the interior of the bush). Pruning your bush to eye level will make maintenance manageable as well as prevent them from getting too top heavy. A good rule of thumb is never to remove more than a third of a bush in any given year (unless your are going that hard prune to the ground). Pruning just a third will help to maintain a natural shape and to avoid stressing the plant too much.
What I suspect here is that your bush just got too tall – doing another hard prune down to the ground on the tallest branches that are just too top heavy with help to correct. It may take time before you have your ideal lilac shape but with some careful pruning you’ll get those lovely flowers in the next few seasons.
Here are some excellent links that discuss pruning and care of lilacs in more detail: