I am often asked when is the best time to prune back lilacs? The answer is right after they have finished blooming for the season. Lilacs develop new buds for next year’s flowers a few weeks after they have finished blooming. Wait too long and you will be pruning off next year’s flowers, resulting in a season with very few blooms.
When the flowers heads begin to turn brown, they are turning into seeds; deadheading the flowers at this point will let the lilac put its energy into the buds for next year's blooms instead of seeds. This is not necessary; however, it will improve the overall abundance of blooms. To deadhead, follow the flower's stem down to the next set of leaves and snip off with pruners. Besides deadheading, this is the time to cut back any growth that is cumbersome to your landscape. This year I trimmed my lilac tree back a good deal and removed branches that were touching the house and blocking part of the walkways. Mature lilacs need one third of the old stems removed; this may seem drastic, but this step will keep your lilacs from becoming a mass of green leaves with little to no blooms. For this, a garden hand saw works great!
Removing these thick branches will make way for young shoots to thrive and produce blooms. This is also a good time to remove suckers and zig zagging branches from inside the lilac. This will open up the lilac to allow more airflow and light to reach inside allowing for young branches to produce more leaves and blooms. Lastly, step back and look at the shape of your lilac and give any overgrown parts a haircut to keep your lilac looking healthy in the landscape.
From my little garden to yours,