Lilacs are deciduous perennials that produce fragrant white, purple or pink flowers in the spring. They are native to Europe but are found commonly in North America, particularly the northeastern United States. Lilacs technically are classified as shrubs, but because they can grow up to 16 feet tall, you'll need to approach the job of removing a lilac plant with the right method.

Timing

Lilacs may be removed at any time of the year if you do not intend to transplant them. Remove the plant in the spring if you can, however, after the ground is free from frost but before the lilac plant has started to bud. This way you'll be able to get the shovel into the ground easily but won't have to deal with quite so much growth or worry about the flowers getting in your way.

Trimming

Because lilac plants can be very large, use a chainsaw to hack off the upper growth. Leave just enough growth so that you have something to grip when you pull the plant from the ground. You also can attack the plant with a hand saw or some clippers, but the chainsaw probably will be much faster and save you some physical effort.

Removal

Dig a trench around the lilac plant with a very sharp spade such that most, if not all, of the roots are taken up in the root ball. Start at least 2 feet away from the base of the plant to do this, leaving more room if the plant is larger. Cut under the plant with the spade to free it and lift the plant out by grabbing onto the base of upper growth.

Raking and Finishing

Lilac roots can sprout new shoots if you do not remove them when you are trying to get rid of the plant. Thus, with a rake (a metal one is best because it is so much stronger than plastic), thoroughly rake the dirt in the hole to remove as many root remnants as you can. When you are done raking, sprinkle some salt or herbicide in the hole to prevent any missed roots from sprouting. Fill in the hole and pat the dirt down firmly. To prevent the freshly dug area from being explored or excavated by wildlife or pets, place a few rocks on top of the dirt. Remove the lilac branches, root remnants and root ball from the area once you've filled in the hole; dispose of them properly so the plant cannot regenerate.

Source:

USDA Lilac Profile

Dave's Garden: Lilacs