Sorry if this is a dumb question, but I am completely new to peonies and have inherited a great deal of them with my new home. Do they spread on their own like some other perennials do? I have so many of what appears to be the same variety in my backyard that I cannot imagine they were all separately planted. Some of them look quite young and only have a couple small buds, much smaller than the bigger, more mature-looking plants. They stretch over an area that's about 10-15 feet long.

Thanks!