Question: Is August too late to prune our French lilac?
Answer: It all depends on what you want to prune and why.
If you simply want to remove the seed capsules from a common or French lilac (Syringa vulgaris), go ahead … but cut only the leafless stem at the tip of the branch that carries the capsules, not the secondary branches just below, the ones with green leaves. It’s from those branches that next year’s flowers will appear, as, by August, they already carry the buds of the blooms to come. If you cut back too far, removing foliage and not just bare stem and seed capsules, you’ll reduce next year’s blooms.
In passing, deadheading lilacs, that is, removing the faded flowers and the seed capsules that follow, is a popular but essentially useless task. Read Garden Myth: Deadheading Lilacs Improves Bloom to understand why. Do it only if those seed capsules really bother you!
If you want to prune your lilac to reshape it or to reduce its height, you could do it now, but it would be better to wait until spring. You see, if you prune in August, the lilac may produce new shoots that won’t have the time to harden off properly (prepare for winter) and they could be damaged by winter cold, obliging you to prune again in the spring.
If, however, you’re just considering removing a branch that is bothering you for any reason—it rubs against the house, hangs over a path, making foot traffic difficult, is dead, damaged or broken—, etc., by all means prune it now. You can eliminate wayward and unwanted branches at any season.
Also, if you want to eliminate the suckers that grow at the foot of the shrub—and they can be very numerous—, go for it! Again, you can remove them in any season. Cut them as close to the ground as possible, even below ground level if you can, as this helps eliminate regrowth. Or, dig them out with a shovel.