The hydrangeas will be taller than the clethra, in my experience.

Thanks ademink, that's what I needed to know!

Clethra can get tall in wild situations (I've seen 7 or 8 feet) usually when they are in a wet shady area. In a landscape situation they usually don't get that tall. Also if you're in zone 5 you may get tip dieback every year which could limit their height.

thanks jimmy~I'm going to keep the spirea there for now, they get just enough sun to flower once. I may put Ruby Spice off in the corner. It does get tip dieback, and chlorosis, and is not my favorite shrub right now. Eventually, I'll replace the spirea with hydrangeas, or something taller. Possibly layering a couple of shrubs, any suggestions for mostly shade with alkaline, clay soils?

I wonder if I prune the clethra down hard if it will get fuller at the bottom? I think if I could get it to grow low and spread out, that would work for me too. I just want every inch of ground covered.

I had another brainstorm. If I leave the spirea for now, would viburnum Summer Snowflake look good as a background shrub? Several sources I have consulted say this shrub will be 6-8 feet H x W, all the way up to 10 x 5 feet. They're about the same height as the spirea now.