Q: Please identify this 3 – 4’ tall stuff. How do you get rid of a lot of it?
A: It’s dogfennel, Eupatorium capillifolium. Don’t confuse it with Anthemis cotula, which is also known as dog fennel or Mayweed.
I used to have to pull it out of our family’s cow pasture because it made milk stink if the cows ate it. They didn’t often eat it, but they would avoid the grass around a clump too.
It is a perennial, and so will come back from the roots if they aren’t pulled out. You can control it with sprays of glyphosate (Roundup, etc) or triclopyr (Brush B Gon, etc).
I have often thought this plant was quite ornamental….but you can make your own decision in your particular situation.