Answer: Where you prune depends upon how large your lilac is and whether you need to reduce its size. If it's a small plant, you can simply cut off the spent flowering stem, cutting it back to the stem where it originates.
Pruning is not mandatory every year on main stems but cutting off faded blooms a week after they fade will help the plant concentrate on preparing more flower buds and not seeds. As in any landscape plant, pruning should be done immediately after flowering is finished.
As your lilac matures, remove one-fourth (1/4) of the plant starting the fifth year after planting to encourage new basal shoots. Remove old wood at ground level.
Pruning encourages better flowering and reduces insect problems such as lilac borers which attack thicker, older wood.
Pruning also allows for better air circulation, limiting diseases such as powdery mildew.
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