Question
how late in the season can one still prune lilacs without taking off the new buds?  Burlington, Vermont

Answer
I'd like to give you a perfect answer, Charlie, but I can't tell you precisely WHEN this plant produces Florigen.

Lilac leaves make Florigen.  This is the flowering hormone that transforms Shoot Apical Meristem cells into Floral Apical Meristem tissue -- those fragrant Lilac flowers.  Florigen is made in the leaves.

Syringa vulgaris is a Woody Angiosperm that forms flower buds ONLY after a period of cold weather -- between freezing and apx 50 degrees F. Lilacs not exposed to cold temps are never able to signal flower development.  Scientists have tested bud dormancy triggers and found that when they chill some Lilac buds, and not others, only those that were chilled grow into flowering tissue.

There is a very small window during which stems can be removed from a Lilac without removing the Florigen-producing leaves in the process.  That window opens and closes in Spring.

Bottom line: If you prune NOW, don't expect Spring flowers in '09 on this shrub.  No Florigen, no flowers.

Sorry, but that's the deal.

THE LONG ISLAND GARDENER