I'm not at all familiar with your zone but here are the two things that came to mind as fillers....Salvias or ornamental grass. Your plan sounds really lovely!

MeMo

How about holly or boxwood? I know those are evergreen here in my zone.

Is this a full sun bed?
Do you have photos to post?

In England, I saw low boxwoods as a green hedge with lavender planted inside the hedges. I think it was Kew Gardens and/or Hampton Court. It was stunning! I think someone on this forum may actually have photos of that look. Can you grow lavender?

What about roses inside the hedges? Knock-outs are low maintenance (unless you have a deer or Japanese Beetle problem).

Cameron

I'll try to get a picture to here later today ... never tried it before. The bed is in full sun, mostly amended clay, over a septic field. I was trying to acheive the "boxwood look" by using the heather. Lavender here is iffy. I have one that is hanging on ... my previous lived for a few years but got really woody & ugly the last year. Not a deer problem here but def. japanese beetles... though I do have roses. Hmmm maybe roses in the 4 quadrants with low growing herbs like thyme under them? creeping sedum? ???

Oh, I think the roses would be so beautiful! A low-growing thyme or sedum is a pretty idea.

What about a phlox subulata (ground cover phlox)? My creeping phlox has green foliage year round. There are pinks and blues and whites available.

My dianthus (cottage pinks) foliage looks good year round and goes great with roses. In the spring, it blooms for a long time and smells so good. I have 'Bath's Pink' with the clove scent, but there are others. Here is a photo with dianthus in the foreground. In the background, it lines my pathway opposite my roses.

Here is a link that might be useful: creeping phlox examples