Hi there: Is it okay to trim last year’s lilac flowers in May/June being careful not to cut out new blooms in Eastern Ontario? I have regular lilacs and bloomerang lilacs.
Thanks for contacting the Toronto Master Gardeners.
As you are probably aware, pruning the spent flowers of any lilac (including the Boomerang series) is best done right after they have faded; they set the next year’s flower buds very soon after they’ve bloomed. If your lilac is several years old, perhaps deadheading the blooms is not as important. However, removing the old flowers allows the plant to direct its energy into producing new shoots as well as the next year’s flower buds, instead of setting seed.
You are asking if it’s OK to trim last year’s flowers now. Since most lilacs will be blooming soon, it might be best to wait to cut off last year’s flowers until after the new blooms fade; that way you will be able to prune and reshape, if necessary, the shrubs as well as make sure that the new growth will produce next year’s flowers.
If you need any more information about how to prune lilacs, please check out the TMG website. For example, see: https://www.torontomastergardeners.ca/askagardener/lilac-pruning-2/
Good luck with encouraging your lilacs to bloom.