I have lilac bushes that do not bloom. This year, one bush has three or four purple clusters, another next to it has no clusters, and a third bush many feet away has about three white clusters. Several years ago, I pruned back some branches for more blossoms, but nothing happened the following year. The shrubs grow many leaves and are about 9 or 10 feet tall. At the end of summer I get powdery mildew on the leaves. The lilacs get about seven hours of sunlight. Should I amend the soil with something? What’s wrong?
The Washington region is categorized by the Agriculture Department as Zone 7. Lilacs will not flower well in this or warmer hardiness zones except in rare microclimates such as in frost pockets in low-lying areas. The way to have some success with lilacs is to plant them where they receive full sun, cold weather in winter, and mild weather with good air circulation in spring and summer. The oppressive heat of July and August in this region causes perfect conditions for powdery mildew; they will recover when leafing out next spring but produce fewer flowers. Common lilac prefers a neutral soil that has a pH of 6.5 to 7. A pH test from agricultural extension or a local garden center will indicate if you should add some pulverized agricultural limestone around the roots of the plant. Lilac blooms occur on stems that formed last year. Branches should only be pruned as flowers fade from these stems. Branches that have gotten large enough and have furrowed bark should be cut to the ground after blooming. New plants will grow from the roots.