Japanese lilac (Syringa reticulata) is a large shrub or small tree that blooms with fragrant white flowers. Japanese lilac trees are the perfect solution for a small garden space because they grow 15 to 20 feet in height with a 15-foot spread. Unless your Japanese lilac is overgrown, it won't require heavy pruning. Japanese lilac branches don't bloom until they are three years of age, so use caution when cutting old wood. Prune the Japanese lilac tree immediately after flowering.
Trim large stems from the center of the Japanese lilac tree by cutting them back to a main branch or the trunk. This will open up the tree for better ventilation.
Remove suckers from the soil around the tree or from the trunk near the soil. Use pruning shears to clip them off completely.
Prune the tree to the shape desired. Prune from the sides to create a slimmer profile, and trim from the top of the tree to remove height. When you cut back a branch, cut it to 1/4 inch above a bud (swollen part of the stem or branch).