The lilac (Syringa spp.) is renowned for its glossy green leaves and blossom clusters that come in a wide range of colors, including purple, yellow, pink and white. If a lilac bush overwhelms your landscape or you wish to plant a different type of shrub in its place, you can kill it using one of several different manual and chemical control methods.
Try flooding the lilac plant twice a day with water, essentially overwatering the plant. Lilacs are very susceptible to excess moisture, and North Dakota State University says applying too much irrigation will often kill the plant faster than spraying it with herbicide.
As an alternative, cut the lilac bush down with pruning shears. Trim off all its side branches, leaving just its main central stem. Use the pruning shears or a saw to cut the central stem to the ground. This will kill most lilacs, but well-established plants may send up one or two new shoots. Keep trimming the plant to the ground to effectively starve its roots and kill it.
As a last resort, spray the lilac with a broad spectrum, systemic herbicide like glyphosate. Apply the glyphosate according to its labeled guidelines, as toxicity varies by product. The lilac bush will absorb the glyphosate through its foliage and stems, and will typically die within seven to 14 days.