My lilac bush has blossoms only on the very top (12 ft) branches. I uderstand that I can prune it back to develop blossums on the lower branches. When I should I do this?
Right after it blooms. Google 'rejuventaion pruning' - lilacs are considered caning shrubs and you can remove 1/3 of the oldest growth at the base. This will encourage the development of vigorous new growth. You can do this over a period of 3 years to achieve a smaller and fully rejuvenated shrub.
just wondering if any of the "cultivar" lilacs are grafted.
So would any of the new "canes" be considered suckers and not true to the cultivar?
If so, how far back would the original canes be safe to cut back?
Cut them back to the ground. They are not grafted.
"topping" or shearing like a hedge will not keep a lilac smaller than it's natural size.
If you want a smaller one, plant 'Miss Kim' she is 6' after ten years, much smaller than 12'.
I can attest to the miss kim(syringa pubescens), although I honestly find it a little less satisfying than my syringa vulgaris(es?).
I do like the small size, but the blooms are also not as colorful, think an icy pinky purple, and much smaller than the vulgaris blooms. Additionally, it blooms later than the standards. All of my vulgaris are in full bloom now and have been for a couple weeks. The pubescens has visible flower buds, but it appears as though it'll be a few weeks before anything happens with them. And IIRC, they're not quite as fragrant.
But they stay small, in fact basically look like a shrunk down vulgaris... smaller leaves, smaller flowers, smaller stature.