Answer: Transplanting a mature lilac is a huge job, like moving a tree, and is best done in the fall or very early spring. You would need to dig as much of the root system as possible and replant at the same depth as it grew before, keep well watered, and trim back the top significantly to compensate for root loss and transplant shock. The pruning would cost a year of bloom because lilac blooms on wood that grew the year before.
It might be easier to transplant a root sucker or to root a tip cutting than to move the entire plant. To transplant a sucker, locate a smaller shoot coming from the ground and dig that up, taking as much root as possible with it. Replant and keep very well watered until it begins to grow, then water enough to keep the soil evenly moist until it becomes well established.
A tip cutting can be rooted in early summer and planted out in the early fall. Take a four to six inch non flowering shoot and remove leaves from the lower half. Dip cut end in rooting hormone and insert about three inches into dampened soilless potting mix. Cover with clear plastic and set in a bright location out of direct sun. Open occasionally to allow for air exchange and make sure the soil stays damp. Once roots form, gradually remove the plastic wrap and then move to gradually into sun all morning. Plant out in early September, water well to keep the soil evenly moist but not soggy until the ground freezes, mulch well for the first winter.
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