If you want to propagate a lilac without digging up new shoots you can always take cuttings from a healthy lilac and start each cutting as a new plant. With a little care you can root lilac cuttings in a prepared pot in the late summer to early fall and it will be ready for planting out by the middle to end of spring. Remember, however, that lilacs live for 100 years, so only take as many cuttings as you have permanent locations in which to plant them.
Fill a 4-inch pot with equal parts sand, vermiculite, and peat and wet it to make it moist but still light and not overly soggy. Push the pencil almost all of the way down through the soil mixture and create four evenly spaced holes.
Cut the top off a two liter bottle about 3 inches down from the top to form a tube shape with a closed off end to create a mini-greenhouse when placed over the soil in your pot.
Clip a stem from the lilac with pruners that is as thick as a pencil and has a total of five buds with the cut along the bottom preceding a bud and the cut along the top being just above a bud. Collect up to three more cuttings.
Remove the leaves which grow out from the three buds along the bottom end of the cutting by clipping them off as close to the base as possible. Cut the leaves at the top two bud sets with a clean width-wise cut halfway down the leaf.
Dip each bottom end of your cuttings into the rooting compound and immediately slip the cutting into one of the pencil holes in the soil mixture of the pot. Your third set of buds should be level with the top of the soil mixture. Add up to three more cuttings to the pot.
Place the 2 liter bottle upside down over the cuttings and adjust it so the bottom is dug into the soil slightly. Place the pot in a sunny, warm window and leave it for eight to 12 weeks. During this time the roots of the cutting should begin to grow.
Transplant your cuttings to their own individual pots when you see new growth start. Fill the bottom of these pots with pea gravel followed by potting soil, place a cutting in the pot and water weekly until late spring when the cutting can be planted outdoors.