Experts and BHG readers answer.
Planting lilac cuttings
A simpler way to propogate a lilac is to "layer" it. In spring, take a flexible outer stem and bend it carefully until the tip touches the ground. Mark that spot in the soil, and work in compost. Make a diagonal slit in the stem you chose, about 12 inches back from the tip, with a sharp knife. Dip the cut in rooting hormone. Put rocks or landscape staples on either side of the cut to hold down the stem. Pile soil on top of the cut part. Mulch and water through the summer. By next spring, the new plant will have put down roots. Sever it from the parent, then keep it where it is for a few weeks, until you are sure it is big enough to move.