One of the most common questions I get from passersby each spring is “why are your Lilacs so full of flowers and mine aren’t”? This question typically has a very simple answer which is “it’s all about the pruning”! I inevitably find out that they decided their Lilac needed to be pruned to keep a certain shape and this trimming was done during the summer when the shrub was growing quickly. 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. Perhaps the most important thing to keep in mind is the timing of when you trim a Lilac. To maximize the number of blooms you need to prune the plant immediately after it finishes blooming in the spring. Once a Lilac starts growing after its flowering has ended it is already beginning the process of forming next year’s flower buds. If you wait too long to trim the plant you are literally cutting next year’s blooms off. If not routinely trimmed a Lilac can grow to be 15 to 20 feet tall so if you want to keep it at a reasonable height of say 7 to 8 feet tall then do the trimming immediately after it finishes blooming.
Ok, so you’ve done your trimming right after flowering that means you’re finished with this task, right? With many shrubs, 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 necessary as Lilac flowers appear on new growth and thinning promotes growth that will be needed for future flowers.
When thinning a Lilac you want to be sure you take no more than 1/3 of the stems as pruning too many could impair the plant's 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 and 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.
Finally, if you are taking on the task of rejuvenating a neglected Lilac 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 plant's 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!