I've been underplanting with creeping phlox and I've been really happy with it. It covers a lot of ground, but the roots are only at one central spot (for each plant). It suppresses most weeds, but when a few make it up through it, it's easy to lift it up and pull them out. You can keep its roots a good distance from the peonies. If it does grow too close to them it is easily trimmed away. And it blooms before pretty much any of my peonies, looks fantastic, and adds another season of flowers to the bed. Basically, I now want to take everything else out from at least a few of the beds and just have the phlox and the peonies, and maybe a few sturdy bulbs (lilies?) to come up later.
I believe there are a number of geraniums that do pretty much the same thing, except they likely bloom at the same time as the peonies, and may not suppress the weeds quite so well.
