Lilacs have been cultivated for centuries and are a classic old-style garden shrub with sweet-smelling blooms. The plants grow as shrubs, but some varieties can become small trees. Lilacs send up suckers from the roots to spread the plant and provide fresh new vegetation for following seasons. This habit tends to make the plant invasive and hard to control. Old neglected lilacs can lose foliage on the shaded lower portions, making them leggy and unattractive. Hard pruning can bring them back in a few years, or the gardener may decide to simply remove the old bush and start again.
Prune away the top part of the bush. Since you are killing the plant, it doesn't need to be a professional job. This will make it easier to dig and get at the roots. Cut down to the trunk or as far as you need to so the access is easy. Saw the bush off at the trunk leaving about 5 cm (2 inches) of stump.
Use rubber gloves when mixing and applying the chemical. Apply Trimec or glyphosate to the newly sawn trunk. Do this within an hour while the plant cells are still open and exposed. Use a 50 per cent solution as the manufacturer instructs and spray or paint it on. The treatment needs to penetrate for at least 24 hours. If rain is expected, cover the stump with a tarpaulin.
Dig a trench 30 cm (1 foot) around where the perimeter of the foliage used to be. The roots will have spread, so you need to get farther from the trunk to find them. The chemical should have penetrated to the roots by now, killing them. However, in the interest of planting there again, you must remove the vegetation.
Use the shovel and a saw to cut out the roots. Dig deeply close to the trunk after you have the roots out and under the lilac. This will take a lot of elbow work and time, especially if it is an old, well-established tree. When you have undermined the stump, use the shovel to pop it out.
Plant in the excavated and treated area no sooner than two weeks. The chemical needs time to break down, or the soil will still be contaminated and can kill the new plants. Watch for lilac suckers breaking through and treat them as needed with the chemical. You may need to be vigilant for a couple of seasons.