Difficult to tell from this photo. It could be Ageratina altissima or a Eupatorium. Both would be native.

It looks like Ageratina altissima.

Yes to Ageratina altissima. The Eupatorium species have smaller heads with shorter flowers.

As far the weed part. It's what I would consider a weedy plant. I grew some in my small garden and I was beginning to spend a lot of time pulling up white snakeroot vollunteers, and I also had the chocolate variation which also spread everywhere like wildfire. This is most common plant in my area, even more numerous than Canada goldenrod, so no need to grow it anyways, not like you never get a look at it. I removed all the plants from that small garden this year, and saved some Ageratinas to be part of a planting at the new, more wooded place because they might hopefully throw the deer off the scent of my other far more special plants. The deer won't go near this plant, they somehow know it's deadly poisonous. I hope they eat those dreadful hostas tho, rootballs and all.

Here comes the deadheading lecture I'm sure!