Hi Tim: I don't have any real advice since my Bridal Wreath Spirea is a single plant, which looks like a beautiful waterfall each spring. But you may want to have a look at this article and see her take on spirea hedges:

http://gardenrant.com/2009/05/spirea-me-baby.html

Good luck.

I did not read the gardenrant article but this is what I learned in a landscaping class years ago.
A woman in the class had the same question. The instructor advised she cut her whole hedge along side of her driveway to almost ground level. A drastic solution! She did and the hedge grew back with new healthy branches. That was 20 years ago and she's has the nicest flowering spirea hedge ever since.

No,no, no! Do not cut it all back at once if you want to maintain a nice flowering hedge! The 1/3 a year is the right thing to do. A property in the neighbourhood has/had a spectacular Bridalwreath spirea hedge. When the property was sold a few years ago, the new owners tried to trim it up as a more formal hedge - which looked awful and, since they trimmed it in late summer, eliminated 90% of the flowers for the following year! We got to know the new owners at a neighbourhood Christmas party and got talking about the hedge and how to prune it properly. It is now back to being a spectacular cascade of flowers in June and they love it.

By pruning 1/3 of the stems (choose the oldest/thickest ones to remove each year - prune immediately after flowering is finished) you control the height and width of the hedge while ensuring that the hedge consists of the 'old wood' (at least one year old growth) necessary for maximum flowering.

By taking 1/3 of the stems out each year the hedge will consist of 1/3 relatively short one-year-old wood, 1/3 mid-height 2-year-old wood, and 1/3 taller 3-year-old wood. The hedge should be about 5-6' tall by the third year of pruning this way and it would remain that height if the 3-year-old wood is the stuff that is removed after flowering each year. Having the equal mix of 1,2, 3 year old wood ensures a lovely cascade of flowers. If you cut it all down at once, the cascade effect is lessened because most of the stems will be roughly the same height.

Yes, that method is what is mostly recommended to maintain a flowering shrub. I was passing along a suggestion made for an old row of out of control shrubs.:)

sharon - even an old row of Bridalwreath can be brought back into shape/control using the 1/3 method. That way, if the shrubs are a hedge, you don't lose privacy for a year or two while the cut-down hedge regrows.

My bridal wreath spirea is taking up too much space in my new front garden. I love it only when it blooms in May. I trimmed a little in early June but not enough; I desperately want to take my shears to it right now. How much damage will I do if I took my shears to it right now, in August?!

You would lose many of next year's flowers if you cut it back now. It is better to hold off until next year and whack it down just after the flowers start turning brown. If you want to keep it short always, you could cut it back hard every year after it flowers, rather than doing the 1/3 each year. But that will eliminate the tiered cascade effect as all the growth will be one height, although it will still be a nice fountain-like effect - but too short to be maximum impressive :-) And, if the height serves any screening purpose, you'd lose that...

It sounds like perhaps the shrub is just too big for the space and maybe you should consider replacing it with something more approprite in size...?