Should I deadhead my lilac flowers after they have finished blooming?
Deadheading (removing the spent flowers) lilac flowers after they have bloomed is recommended for a couple of reasons. First of all, the spent flowers are not very attractive. More importantly, deadheading prevents seed pods from forming, thereby diverting the plant’s energy to be used for flower bud development for the following season. Lilacs that have been deadheaded just after flowering often flower more heavily the following spring. To deadhead, cut at the base of the flower cluster and just above the uppermost leaves.
Most lilacs bloom on the previous year’s growth.If the lilac needs some pruning, this would be the time to do so; if done later in the season, the lilac flower buds risk being removed which would significantly reduce the amount of flowering the subsequent spring.