Posted: Yesterday at 20:49
It doesn't look like mare's tail to me.What did you have growing there before? It looks a bit like yarrow, achillea but there are many things with leaves like that. Is it just within that circle or is it in the existing lawn as well?
Could you did a piece up and give us a look at the roots? It would help to know if it can be dug up, suggesting multiple seedlings of something, or if it's all joined together in one mass of root. also how deep do the roots go?