Also to add - I am in central VA.

Im in DC (7b) and have several Spirea japonica and always cut them back after they flower so mid summer and again in fall (around now) to about knee height (not too low) and to a round shape so you shouldnt have a problem. next year you can wait till spring and see which results you like better.

Spiraea is one of those plants an active member here would say can be run over by a truck.....and bounce right back! And that's not far from the truth :-) It can be pruned pretty much any time of the year you want, as often as you want and as hard as you want with NO ill effects, provided the shrub is in good heath to begin with.

I had Magic Carpet spiraea in a former garden that I sheared back multiple times a year because I loved the new foliage color but hated the Pepto Bismol pink flowers. Every time the shrub budded up, I'd cut it back!! All that did was make it grow more vigorously and more densely.

Prune away!! 😁