This blooms like double-flowered baby's breath in spring along a fence in mostly shade except in spring when the trees haven't filled out. I had/have quite the job uprooting it's extensive underground network when I decided I wanted a birdbath and other plants there two years ago. It still comes up in the bed. I don't know how long its been here--over ten years is my guess. It's so pretty I can't pull it all up. What is it?