You may think this is boring but I love the look of silvery, big leaved plants like Helen von Stein lambs ear and big iris with your combination. Or even the herb Bergarten Sage. When my garden filled out I found that everything I had planted had tiny leaves or strappy leaves like daylilies. So I made a point of adding big leaves and it really made a difference and defined the edges a bit.

Thank you, Bouquet!! I was thinking silvery, too. Maybe artemesia silver mound...

I'd also vote for the artemesia...I've got an upright backing my spiderwort collection, and use the mounding forms to edge the iris beds that I babysit for my MIL.

you might also look into a variety of succulents- I've got both tall and creeping sedums, and wondered how I lived without them! the leaves run from soft greens into reds and purples, and they flower in many warm shades...

if the bed's big enough, I'd think about air layering the butterfly bush, and making a few more for yourself...

and maybe adding a white one?

http://www.leevalley.com/garden/page.asp?page=46938&category=2%2C47236&SID=&ccurrency=2

the rooter pot's made things a lot easier on me, trying to fill in a bed with nothing in it but tulips and phlox!

I've got a Belinda's Dream rose, a Black Knight butterfly bush (deep purple) and annual salvia farinacea (blue).

To these, I added: pink Stargazer lilies, white four-o-clocks, pink and white cleome, blue Stoke's Aster, and Bath's Pinks (silvery-green foliage all summer). This gives me blooms all spring and summer. They should all do fine in the heat, and the lilies would appreciate a little shade, if you have a spot, or you could put them in the back.