One of the most deliciously fragrant moments in spring is when a lilac bursts into bloom. The flowers, which can be white or any shade of purple, are almost an afterthought. The rest of the year, lilacs aren't much to look at. Ranging from 5 to 20 feet or more, lilacs are often scraggly and are prone to mildew in the late summer. With a little TLC, though, they're a welcome addition to an old-fashioned garden.
Maintain the soil at the proper pH level. Lilacs do best in "sweet" soil that's neither highly acidic nor highly alkaline. A pH of ranging from 6.0 to 7.0 is best. If your soil is acidic (pH below 6.0), add lime (ground limestone), wood ashes or bone meal once a year. If your soil is alkaline (pH above 7.0), add sawdust, composted leaves or peat moss.
Fertilize twice a year, in early spring and right after your lilac has finished blooming. Use a balanced 10-10-10 fertilizer mixed to standard strength and water in well. If you prefer not to use chemical fertilizers, a thick layer of compost once a year, in the spring, will help keep your lilac well-fed.
Avoid overwatering. Lilacs do best in well-drained soil, and you should only water yours during periods of severe drought. If you do need to water, water deeply once a week.
Mulch for weed control. Use 2 to 4 inches of mulch, and leave some breathing space around the actual trunks of the lilac.
Deadhead after blooming. Cut off the flower heads to the next leaf junction within two weeks of completion of the bloom. This will prevent the lilac from setting seed and encourage great bloom year after year.
Prune to maintain the overall shape of the tree. If you look carefully at the middle of the lilac, you'll notice that rather than one single trunk, the lilac has a group of trunks. Because of these multiple shoots, lilacs can get leggy and out of control. Use a three-year plan to get an overgrown lilac back into shape. Cut back a third of the lilac each year, pruning the oldest stems all the way around. You'll rejuvenate the whole plant over the course of three years.
Treat pests and diseases ruthlessly. The 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. Treat both the same way: Prune off all affected branches and discard of them completely, either by burning or by putting them out in the trash. Do not compost damaged branches.
Ignore powdery mildew unless it really bothers you. This will appear in late summer as a powdery coating on the leaves. Though it looks unsightly, it won't damage the plant. If it bothers you, treat with a milk spray. To do this, dilute a quarter-cup of milk in a gallon of water and spray the affected leaves.