Lilacs are sentimental favourites with most gardeners—their blowsy form and sweet fragrance conjures up cherished memories of days gone by. But nostalgia aside, these vigorous shrubs can become overgrown in just a few years, which can result in plants that produce fewer blooms and are increasingly susceptible to disease.
Ideally, lilacs should be deadheaded annually, as soon as their flowers begin to fade. Also, as much as one-third of the shrub can be pruned out to increase light levels and air circulation. But the reality is that most of us find ourselves busy with myriad other garden-related chores at this busy time of year, and quite often, the lilacs get overlooked.
So, what should you do if you find yourself faced with an overgrown catastrophe? Clearly, drastic measures are called for. One option is just to dig up the offending plant and replace it. As an alternative, you can try to rejuvenate the shrub by giving it a severe pruning. Luckily, lilacs have amazing regenerative powers and can revert to their former glory in a relatively short period of time.
Hard pruning takes a certain amount of chutzpah on the gardener's part (and you'll have to sacrifice blooms for a year or two), but it's an easy process and the results are well worth overcoming any trepidation you may feel.
In late winter or very early spring (while the plant is still dormant), cut down all dead or weak stems to ground level, then saw back the main woody stems to within 20 to 30 centimetres of the base of the shrub. (You can identify dead stems by scratching off a small section of outer bark with your thumbnail: if the wood below is green, then the stem is alive; if it's grey or brown, the stem is dead.) Make cuts on an angle whenever possible to discourage water from collecting, and pare back any rough sections of bark with a sharp knife to aid healing.
As the weather gets warmer (mid-spring), it's essential to provide plenty of food for the lilac, so mulch the area above the visible roots with a good layer (seven to 10 centimetres deep, one metre in diameter) of compost or composted manure; keep the area well watered. For the next two years, the mulch should be replenished in the spring, as soon as the plant breaks dormancy.