Lilac trees explode with colorful, fragrant flowers in the spring months. After long, cold winters, they are a sure sign that spring is on its way. However, as years go by, your lilac tree may begin to look less than wonderful. When this happens, it's time for a trim. Before you start trimming it, though, you must know how to best trim a lilac. Luckily, it's not hard to learn.
Remove all flower heads before they develop into flowers the first year. In the second year, remove 75 percent of flower heads before they develop into flowers. Use pruning shears for this work, and complete it in mid-May.
Prune off old flowers once they are done blooming. Doing this prevents the lilac tree from putting energy into seeds. Early June is typically when the flowers will be done blooming.
Prune away the suckers that sprout from the base of the lilac tree in August. These suckers will take energy away from the main tree and can also make the tree look unsightly.
Trim off any diseased or weak branches in the autumn. October is the accepted month to do this pruning. This is also the time to shape your tree as you see fit.