Poohtown, are your lilacs getting enough sun? Are you over fertilizing them? Are they planted in an area that drains well?
When we first planted our lilacs, many years ago, we were so anxious to see them bloom, that we fed them, a lot. They got bigger, with luxuriant foliage, but not a flower to be seen! We learned that was the wrong thing to do, so we stopped. At the same time, we were told that the heavy clay soil we had them planted in might need to be further amended, which we did by hammering an 18" long 2"x2" wood stake into the ground, and filling up the holes we'd made with clear sand. The combination of those two changes resulted in lots of blooming.
It's also important not to prune your lilacs too late, or you could remove the flower buds that have set for next year's blooming. I also clip off the spent flower heads to keep it from growing seeds.
Here are some links with more info on Lilacs, that might be of help to you.
~Questions on Lilacs~
~Pruning Lilacs~
~Lilacs, The Lovely Ladies~
Anyone else with some suggestions for poohtown?
Let us know if you think you've found some answers, okay? Good luck!!!
