Which spirea?

Shrubs are pruned right after they flower. Cutting it back now will reduce the bloom next season. If it's very thick, you need to do rejuvenation pruning rather than cutting down the whole plant.

https://duckduckgo.com/?q=rejuvenation+pruning+of+flowering+shrubs&t=ffcm&ia=web

It depends on the type of Spiraea whether it will effect the flowers. Unless it is one of the early spring flowering Spiraeas, it will bloom just fine after a hard prune. Like Wayne suggested, I would wait until very early spring, right before the buds start pushing. It will most likely be well covered in snow relatively shortly if your area is anything like here, so I would either pull out the leaves by hand or with a yard vacuum and ignore it until spring.

I have hard pruned my summer-blooming Spiraea a couple of times over their lifetime to remove old dead twigs and tidy them up, and they have regrown to full size, ~4,' in a couple of months and bloomed just fine. I take them down to about 6"-8" and to just above a bud on each branch. Pruning in spring may help you see where the buds are.

Right after pruning

Two weeks later

My Spiriva (possibly 'gold flame') did quite well this year after being pruned down to 6" last spring.

The only reason I didn't do it in the Fall is bc I'm having too many other things to do this time of year.

I don't think fall pruning would hurt it once it's completely dormant. Plants that I didn't prune at all, didn't do as good as the ones I did.

The entire tops of established Spiraea japonica selections of normal vigor should be pruned down low every year as a normal maintenance procedure. Other kinds however should not be. Particularly those that flower in spring only.

Wait till spring in your climate. I cut back s. goldmound (or similar name) every year in spring. It keeps its shape and still flowers very well. I used to hold back on s. shirobani But it got tangled and ratty so quickly that I started cutting it back hard in spring as well. Again, it grew back compact and floriferous. .

On the other hand, if your neighbor's survives a Michagan winter cut back, I suppose you could risk it. Maybe his are well established with a good root system. Are yours?

Here in Michigan we cut our spirea in April -- it always comes roaring back. We cut it as low as we can go.

Can I cut back very hard an 8' bridal wreath spirea? I live in south central PA.

Because bridal veil Spiraea bloom in spring, you would lose the flowers for the coming season since the flower buds have been set for next year.

I did some major pruning of my large spireas ( van houttei) after blooming, shortening some canes and removing others at the base. This fall I noticed that they were the first shrubs to go into dormancy. From afar they even look dead! It is disconcerting because I assumed spireas can take anything, but maybe they are more sensitive than we think?

Kitasei, I would be shocked if they don't come back vigorously next spring. As long as they were in a suitable spot to begin with, IME Spiraeas are the type of plant you could run over with a mower and they will return vigorously.

I have no idea the variety, but I cut back to the ground an overgrown spirea at my Mom's house last October. When I say to the ground, I mean flush with the ground. It made it a lot easier to clean out leaves.

Well, lo and behold, this is what it looked like a month ago when I was up there. I'm sure it's much bigger now. So, for those of you wondering, these shrubs seem pretty bulletproof, even in a cold climate.

BTW, this is in southern MN.

I cut my two down to 3" earlier this spring, they both came back great!

Added pictures: 6-21-18