I have pruned mine this time of year and still have flowers. Your growing season is much longer. You should be okay. I probably wouldn’t prune much later.I planted my winter hazel in an oversized pot to let it grow out for a couple of years and pretty much ignored it this summer. It has exploded with new growth. A lot of the new branches are crossing and rubbing together. In zone 7 would it be ok to prune it back in mid August or should I just leave it until spring? I don’t care about the flowers.
If you’re letting it grow out, why are you pruning it?