I don't know much about its blooming habits, but now (mid/late summer) is a good time to cut it back. New growth in October will have time to harden off before the winter frosts. Your other option is late winter, when the likelihood of a late frost minimal.

Pruning on sambucus is recommended for late winter/early spring, when the plant is still fully dormant. They bloom on new growth.

Pruning now before the plant enters dormancy can encourage new growth that may not sufficiently ripen off before cold weather sets in, potentially resulting in winter dieback.

This post was edited by gardengal48 on Sun, Aug 18, 13 at 13:45

they are super tough and tolerate coppicing.

if you really hate the current height, i would guess that you could violate best practices, and just hack it without much damage. just suboptimal growth.

Here is a link that might be useful: hacking back elderberry

I wouldn't expect either a big push in response to pruning this late or a freezing off of whatever might be produced, as this is a very hardy shrub. And being a vigorous cane grower it really wouldn't matter if there was some damage to secondary sprouts. As mentioned you could cut the whole thing off some time and that wouldn't be likely to faze it.

Did you get any berries ?