USDA site says 2' for erosion control and farther apart for other purposes. Dave's Garden says 10' to 12'.

Neither of my two older ones have gotten much bigger than they were originally. I think one has gotten wider but not taller. Neither is in what you would call a wet area.

Three feet apart is fine, Christie. How big buttonbushes get depends on how "happy" they are. I've seen them growing roadside on a slope where they were only about ?2' X 2'. But as you go down the slope where it's swampy, they're much taller and wider. In my best tree book, it's listed as a tree, because under ideal conditions, they can get 25' tall.
Mine is about ?10' wide and tall, including the sucker plant near it. It grew rapidly from a cutting I took from one growing in a group on the side of Hwy. 49 - I'd guess it grew about 2'-3' a year, both high and wide.
If yours are in a regular, non-swampy garden/yard setting, they won't grow as fast as they would in a ditch, and may never get too big, I don't know for sure, because mine grows in the septic tank drain line field.
The 3' spacing will look good - the bushes will just merge into each other and look very pretty.
Sherry

I forgot to thank you for your replies and it's just what I wanted to hear. I really don't want to have to move them. lol It's a lot of trouble and I'm afraid I might kill them. It's not a swampy area at all. I may always have to water them in the summer when it's dry, even after they mature but that's ok. I'm looking forward to the swallowtails. : )

Thanks Sherry!

I saw the spacing requirement on Dave's Garden and started freaking out.

3' is definitely doable.

Once again, your wisdom is invaluable :) Tony

You're both so welcome!
Sherry