This is a weed and we pull it up but it’s actually fairly attractive. It’s called white snakeroot (Ageratina altissima) and it’s a fairly common native plant in our area. It’s similar to the blue mistflower (Conoclinium coelestinum) that we have in some of our borders but quite a bit taller (it’s three or four feet tall, compared to about about a foot and a half). This one is behind some shrubs so managed to get pretty much full grown before I noticed it. It will be gone shortly but I thought I’d take some pictures, anyway.