I have a 2-3 year old reblooming Lilac grafted on a standard. It bloomed great in spring, i pruned it after it finished blooming, mostly dead heading along with some long shoots. Since, it’s been growing crazy long stems all over itself. It’s looks very messy/wild but if I prune these, won’t I miss the late summer re-blooms? But this can’t be right, some of the new stems are over a foot long with no branching!
I’m in Toronto by the lake, clay soil modified with compost and mulch. The standard gets sun 10am – 6pm and we water 1-2 times a week when no rain.
Thank you for contacting Toronto Master Gardeners with your question.
Reblooming lilac (Syringa penda), such as “Bloomerang,” has a nice, rounded shape, The tree is about 4 to 5 feet in height above the graft with a spread of 4 to 5 feet.
Accordingly to the breeder “The rebloom of Bloomerang lilac occurs on the new growth the plant creates after its spring bloom. For the best rebloom, it’s vital that the plant grows vigorously during late spring and early summer. If you want to prune Bloomerang lilac, do so immediately after its spring bloom. Never cut it back in fall, winter, or early spring – doing so will remove the spring flower buds. It is not necessary to prune Bloomerang lilac in order for it to rebloom. However, giving it a light trim after blooming does remove the developing seed heads, providing a neater look, and encourages more new growth for reblooming. Trimming after blooming will delay the rebloom by a few weeks compared to an untrimmed Bloomerang lilac.” (https://www.provenwinners.com/plants/syringa/bloomerang-purple-reblooming-lilac-syringa-x)
Please refer to this article which gives clear instructions on pruning your reblooming lilac: https://homeguides.sfgate.com/deadhead-reblooming-lilacs-72129.html