what can do with an overgrown spirea japonica?

Cut it as hard as you like it will come back. Be brave, most plants are very resilient and can survive most things other than poison ,drought and actually removing them from the ground. I would prune it after the flowers have finished.

It depends if you want to wait till it flowers or not. I suppose most would wait till after it flowers but if its stopping you doing other things in that part of the garden you could do it now or do part of it now.

I would trim it down to the shape you want and as your first reply said, it will almost certainly come back again. And it will probably be less twiggy and leggy and be more bushy and attractive too next year.

Thanks Wyeboy!
Where else would you get an instant answer like that! I really love this site
I always struggle with tryng to keep a good shape when I'm pruning. Hacked at a Mexican Orange blossom earlier this year and it looks really stupid now - all branch and not much leaf!
Oh well - I suppose practice makes perfect

Another instant reply! - thanks Inverglen - I may be brave and have a go with just part of it - I'm such a coward when it comes to making decisions about pruning!

Agree with Wyeboy, cut it back hard, Spirea tend to bounce back prety fast in my experience.

Don't worry about the Mexican Orange just try again!!! Be bold it will look a bit odd if you do part of it.