Propagating a lilac bush through stem cuttings give best results in early spring prior to full leaf emergence. According to the North Dakota extension service, rooting lilac cuttings may prove slightly difficult if using older hardwood from the bush. Best results are seen by using smaller diameter softwoods before full sized leaves are realized. It may take up to two growing seasons before the new seedling is transplanted to a permanent location.
Fill a 6-inch diameter pot with coarse sand or vermiculite as the soil medium.
Take cuttings from the lilac bush that are 1/4 inch to 3/8 inch in diameter, and are from 6 inches to 8 inches in overall length. The leaves should not be full size. Preferable leaf size is a small green bud.
Dip the cut end of the lilac stem into the rooting hormone. The white powder should cover at least the bottom 2 inches of the stem.
Insert the cut end of the lilac stem into the soil medium, at least 2 inches to 3 inches deep. You can place up to five cuttings per pot evenly spaced around the container.
Water the cuttings using the mist setting on the spray bottle. Thoroughly soak the cuttings. Allow the water to drip into the soil around the stem.
Cut the bottom off of the 2-liter soda bottle with the scissors. Keep the plastic cap in place. Set the soda bottle over the cuttings like a miniature greenhouse. The 2-liter bottle should fit snugly inside the 6-inch diameter pot.
Keep the lilac cutting moist. Watch the interior of the soda bottle. When moisture ceases to collect on the interior of the bottle, remove and mist the cuttings. Keep the container from direct sunlight.
Check the cuttings for root growth after eight to 10 weeks under the soda bottle. Carefully pull a cutting from the soil medium. The roots should be at least 1 inch in length before repotting to another container. Transplant one seedling per pot.
Transplant the new lilac seedling to its permanent location the following spring. According the Missouri extension service, some seedlings may benefit by staying in the container for up to two full growing seasons. Consult your local agricultural extension service for your particular climate and lilac growing conditions.