When do I prune lilacs in Toronto?
Lilacs (Syringa), in any climate, are pruned immediately after flowering. Pruning them any later will remove the flower buds being formed for next year. New reblooming lilacs, however, have recently been introduced. For these, do light pruning after the first flush of bloom.
Some lilacs become crowded over time, and flowering on these can be improved by radical pruning. This is also done right after flowering, cutting out 25% of the oldest trunks, right to the ground, annually over four years.
Wishing you success with your lilacs.