A lilac, which can live for over a century, is an easy plant to maintain: Only minor care is needed annually after springtime blooming. Lilacs need pruning only every three to five years. Pruning is primarily done to increase flower production and improve air circulation, which can reduce the risk of potential diseases that thrive when stalks are tightly spaced.
Cut away only spent flowers (blooms die off approximately three weeks after blooming begins) by using the shears to snip right behind each bloom. Any lower on the stem risks cutting the area where next year's buds are already growing. Take this step as soon as blooming for the entire lilac is complete no matter when the bloom time is.
Thin old branches every three to five years at the same time you remove old blooms. Cut the branches horizontally just above ground level or where they separate from the main trunk. Remove branches, as desired, from the interior center to help increase air flow through the lilac.
Leave six to eight strong stalks each time you trim or prune to support new, full growth. By getting rid of the old growth (which needs moisture, but does not bloom) and the spent buds (which would use energy to produce seeds), you allow these select stalks to use all of the plant's energy for their growth and blooms, increasing the number of blooms you see the following year.
Shape the lilac depending on individual taste. To create a single, tall specimen plant, trim old growth and new suckers at ground level by cutting straight across, leaving only a few strong plants. To form a lilac that looks hedge-like, trim dead or old growth as before, but cut away only suckers on the front or back. Allow new shoots to grow and mature on each side to make the plant appear longer.
Resist buzzing the lilac across the top to shorten it. Lilacs are meant to be as tall as their variety will allow. Because they produce blooms on the previous year's growth, cutting the entire plant across the top may preclude any blooms the following year.