Pacific Northwest Gardening: Laurie's Garden Tour 2, 1 by HoosierGreen

HoosierGreen wrote:
If one's lilac is healthy, but doesn't bloom well, first determine that it's getting as much sun as possible. Full sun is best, but half-day works. Also, how old is the shrub. Under three years, usually you won't get bloom.

Secondly, be sure you aren't "topping" the branches to keep the overall shrub smaller. You'll be cutting off the buds for next spring's blooms. I had a client once who lamented that his lilac didn't bloom, but after one time convincing him to not shear the bush that summer ("But it'll get too big!"), he was rewarded with abundant blooms the very next spring. I convinced him to let me prune it Method #2, which reduced the overall size and appearance to him, but it was more the reduced mass of foliage that worked wonders. He's had great blooms every spring since.

Third, although it usually isn't needed, apply some fertilizer or compost this spring and repeat once again in the summer to encourage bud formation for next year. Mulching the bush helps, too.

Sadly however, according to Michael Dirr, French lilacs do best in zones 3 to 6 and tend to lose vigor and not flower reliably from year to year. I guess that's a small price to pay for all of the wonderful plants you can grow in your region.

But... don't despair! You should be able to find cultivars of the Descanso hybrids that were developed in Southern California for good flower production in mild winter environments (heat tolerant, low chill cultivars). They are supposed to be great in Zones 8 and 9. Some of these cultivars are 'Angel White', 'Blue Boy', 'California Rose', and 'Dark Knight'.