Answer: It might be a flower, there are some aggressively spreading and very hardy chrysanthemums out there such as Mary Stoker and Clara Curtis as well as Chrysanthemum pacificum which is sometimes planted as a groundcover (or escapes); it might be an artemisia or even an anthemis of some kind, or maybe bishop's weed -- now that I think about there are quite a few other possibilities -- including some weeds. But to be sure, I would suggest you take a sample to your local nursery and see if they can identify it for you or if you can visually match it to anything in their perennials section. If it is spreading with underground roots like that then cutting it down will not get rid of it, you will probably need to dig it out taking every shred of root. Good luck!
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