Is this a weed?
What is this plant? Assuming it's a weed what is best way to get rid of it? It is taking over our rain garden! Third photo is before This weed? Took over this Year.
This looks like our native obedient plant, Physostegia virginiana: http://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?kempercode=g620
There are many flowers with very similar foliage. To be certain of the id, let it flower. If it is not obedient plant, send us a photo and should be about to identify with certainty then.
Many good plants spread by roots or rhizomes, and we assume you want to area to fill in with vegetation. If it is a good native, it's just a matter of how much you want to devote to it.