I hope you get an answer because I was about to post (or try to) pics of the same plant. Or I think it is the same one. With the same question. It grows all over here including the hillsides and one of the few plants the cattle don't eat. (Along with Ironweed and cockleburs).
Does yours smell very sweet? I could see that the pollen collected was white or offwhite. I was having difficulties taking pics as I had "help" of a half dozen year old half lab pups that were on their morning run. When I stopped to look at anything, they had to see what it was too. They were knocking into the plants and bushes.....and me.