One of the most frequent questions that I’ve received from neighbors over the years is “why are your lilacs so full of flowers and mine aren’t”? This common question usually has a very simple answer, “it’s all about the pruning”! As the conversation progresses I inevitably seem to find out that they decided that the lilac needed to be pruned to keep a certain shape and this trimming was done during the summer because the shrub was growing so fast. Unfortunately, this haircut took off a lot more than just a few out of place branches.
If you want your lilac to have a lot of flowers the most important thing you need to know is how to prune it. The first thing to keep in mind is the timing of when you trim the plant. To maximize the number of blooms you need to prune the plant immediately after it flowers. Once a lilac starts growing after the flowering has ended it is already beginning the process of setting next year’s flower buds. If you wait too long to trim the plant you are literally cutting next year’s blooms right off. Lilacs can grow to be 15 to 20 feet tall if they are not routinely trimmed so if you want to shape the plant and keep it at a reasonable height of say 7 to 8 feet tall than do your trimming in the spring right after flowering.
Ok, so you’ve done your trimming right after flowering that means you’re finished with this task, right? With many plants you would be correct but in the case of lilacs and a few other shrubs such as forsythia, the plant benefits from what is known as thinning. The process of thinning involves removing roughly 1/3 of the stems each year to encourage the plant to continue to produce new ones. This is needed as the lilac flowers appear on new growth and without this the flowering will not be what it could and should be.
You want to be sure you take no more than 1/3 of the stems as taking too much could impair the plants ability to maintain its strength. When thinning a lilac I look for woody stems that are 2” in diameter or greater and remove those first. You should also remove any dead or diseased stems, I once found a stem with a huge hole in it from who knows what bug. Finally remove stems that cross in front of and rub each other as this can damage the bark and allow a foothold for insects. While the lilac may look a little “thin in the skin” after pruning in a few weeks you should see a lot of new growth coming up from the bottom of the plant.
One final note for you is if you are taking on the task of rejuvenating a lilac that has been neglected over the years a lot of patience is in order. You can return this plant to its former glory but it is likely to take 3 years of thinning and pruning (remember you only take up to 1/3 of the plant stems each year) to restore the plants strength and vigor. It’s a lot of work and takes time but when you smell that lilac bloom in the spring it makes it all worthwhile!