Are you talking about the serrated leaves with the pink flower? That looks like astilbe to me. Where I live astilbe does not multiply so I am guessing you are referring to something else growing there? Where are you located? Need a better picture if that is not it.

Yes, the serrated leaf and the pink flower. It looks a lot like an astilbe, which I have a lot of, but this has a much larger leaf and it definitely is freely multiplying. And it is more gangly. Perhaps it is a variety of astilbe. I am in Michigan.

When you have time, google pink flowered astilbes and see if you can find a variety that matches what is growing in your yard. I can't imagine that it is a weed. It may be gangly if it is in too much shade.

Callirhoe...that sounds right. I was trying to think of another perennial that has astilbe-like flowers and QotP certainly does and the leaves also match. I guess I should be glad I never got around to planting it in my yard. :)

It is definitely Filipendula. I planted some last year, and it put on quite a show this summer, but I am now being told that it can become quite the spreader. So... it may have to come out. A pity because it is quite tall and stately at the back of a border