The best time to trim a lilac bush is determined by a few variables. Great variation can be seen in lilac bush varieties when it comes to growing patterns, height and width at maturity, and other factors that can funnel in to decisions about how, when, and where to trim away growth for the healthiest, most vibrant lilac bush. Some lilac bushes will reach a mere 8 feet high at maturity if left untrimmed, while others can grow in excess of 30 feet high as they mature. While light maintenance trimming can be done as needed to remove dead, damaged, or diseased growth, a single annual trimming should be carefully timed so as not to adversely impact the health and vitality of next year's blossoms and the overall health of the lilac bush as it grows.
Experts recommend leaving lilac bushes untrimmed until they reach a minimum height of six feet tall. This serves as protective measure to ensure that the tree produces enough healthy new growth to sustain it throughout its lifespan. Excepting light maintenance pruning to remove damaged or diseased limbs or foliage, the best time to trim a lilac bush is always immediately after the bloom season has completed. Trimming at any other time creates a risk for the health of the lilac bush, either from cutting away next year's developing flower buds or exposing healing foliage to the harsh temperatures of winter. It is recommended that lilac bushes be extensively trimmed once annually to assist with maintaining shape and adequate air circulation to the interior of the bush.
To begin, gather clippers and any other trimming implements and spray them lightly with a fungicide spray to disinfect them prior to use. Next, examine the entire bush for signs of failing or diseased foliage and trim that away first. Examine the base of the lilac bush for weeds, debris and fallen foliage, and carefully clean away all debris to discourage pests or fungus. Since lilac bushes are enthusiastic growers, it may be necessary to prune away entire limbs to open up sufficient air circulation to the interior of the bush. This prevents fungus and pests from going unnoticed until damage is more extensive. After trimming away branches to open up the center of the bush, examine the overall shape and height of the lilac bush and strategically trim away unruly ends that distract from the plant's overall visual effect. Be sure to deadhead or trim away faded blossoms to discourage the lilac bush from self seeding new growth. In extreme cases, if the lilac bush has overgrown its environment to the extent that it has taken over the garden bed, experts recommend pruning it back to six or eight inches from the ground. In these extreme cases, it will take approximately three years for the lilac bush to create enough new regrowth to bloom. It is never recommended to "top" or lop off the height of the bush, but rather go for an overall rounded effect when shaping a lilac bush.