Based on my experience it is always better to keep the plants that stay at one place - irises are nice with peonies, catmint Walker's Low, liatrises are not too bad for me. Ground cover is fine, as long as does not multiply too fast. Different plants for different regions.

Ok thank you ;)

Do however make certain that your ground cover does not form a mat either of stems or roots. Too thick a mat could cause rotting of the stems if it grows so close to the stems that it blocks out air flow. Air flow is important to keep molds and other moisture caused problems at bay. Rather than a permanent ground cover I prefer low growing annuals so that at the end of the season they can be ripped out. Helps with cleanup if you remove your peony stems and each year you can change what you plant. Changing plantings can help the soil conditions improve and also lessens the chance of problems that may accumulate in the soil. "Think" similar to various tomato virus's that have made some tomatoes impossible to grow except in containers.

Great tips! Thank you