Lilacs are a gorgeous, fragrant addition to an old-fashioned garden--for those couple of weeks each spring when they're in bloom. The rest of the year, though, they can look scraggly and unattractive. If you've got a lilac that needs to be tidied up, use a three-year plan for best results.
Examine the bush carefully before you start cutting. You'll notice that the bush grows from many individual stems, not from a single trunk. The way to get a lusher and more compact plant is to remove entire stems, rather than trimming off the ends of branches.
Remove dead and diseased branches and stems. The main two main pests are scale and borers. Scale looks like warty growths on the branches; borers leave holes up to 1/8 inch near ground level. If you see evidence of either of these problems, remove the diseased stems all the way down to ground level. Discard diseased stems and branches by burning them or putting them out in the trash to prevent spreading disease to the rest of your garden.
Examine the remaining stems to find the oldest ones. These will be thicker than the others, and usually have long bare stems with a few leaves at the end. Remove these stems, cutting them all the way down to ground level. Don't remove more than a third of the entire bush.
Remove another third of the bush in year two. Again, start by removing any dead or diseased wood, then choose the oldest stems, the ones with the barest trunks and the fewest leaves.
Remove the last third of the bush in year three. At this point, you'll have a nicely renewed lilac, bushier and more compact.
Keep your lilac looking lovely by cutting back dead wood and overlong branches each year, never removing more than a third of the entire bush. Also remove most of the suckers (brand new stems) each spring, since they can rob the energy the bush needs for blooming.
Encourage bountiful flowering by deadheading. Within two weeks after your lilac is finished blooming, remove all the dead flowers, cutting back to right above a set of leaves.