Should I prune after first bloom or will that affect/ encourage second bloom
Thank-you for contacting Toronto Master Gardeners with your question about lilac pruning. This question has come up before, related to a standard-trained plant. I am including the previous answer below:
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
So, basically, it is your choice whether to prune immediately after the current (spring) flowering or not. Clearing out the previous flower/seed heads will certainly give your plant a tidier appearance, and will allow the 2nd flush of flowers to look their best. Good luck!