I found it in our woods by our creek, thought water hemlock at first, but lacks purple spots and root tubers do not match pictures, also the leaves lack the jagged edges and are fairly smooth except near the tips. I have exhausted my google search, also it doesn’t have the strong weedy smell like poison hemlock. stems were hollow, smooth green, and top cluster of seeds smell like a mix of dill and cilantro. Pickens, sc area.
your plant appears to be Oxypolis rigidior, the cowbane. I have not seen these tuberous roots previously, though – but I do not usually dig it up. A similar plant is Sium suave, but that species has toothed leaves. It is generally thought to be toxic. I see a lot of these along creeks in Oconee County.
Steve Hill, Botanist, SCNPS