There are a number of wonderful, fragrant lilacs on the market but if you don't trim them at the right time, you will loose next season's blooms. In Missouri, USDA Zone 5b or 6, old-fashioned lilacs bloom in May. The best time to trim lilacs is immediately after they've finished blooming so you're not cutting off branches that will set blooms for next year. You'll know lilacs have finished their bloom when all their florets have unfolded and the flowers are going brown. If you want to keep your old-fashioned lilacs contained, cut out all new growth sprouting around the base; remove dead branches and cut top branches about 6 inches from where you want the plants to be next year. I have old-fashioned lilacs under my bay windows. I know it's spring when I can open the windows and welcome their wonderful scent!