Sure looks like it to me, but it is early in the season for it to be that big. The stuff on my land looks identical to that but it won't be that big until a month or two later in the summer, and I'm in Maryland - zone 7.

Could it be milkweed, you could do a pic search and compare.

For Pweed the foliage- and stemdetails don't look right

Thanks, everyone -- I pulled up one of them, and the root looked more like pokeweed (thick tuber) than milkweed. Maybe just the weirdly long winter followed by the past couple weeks' heat wave made it grow so quickly.