Hog weed leaves have broader sections , where as hemlock has more sections to the leaves . i think i'm right in saying that the hogweed flowers a bit later too but that may depend on where you are. Unfortunately hogweed , hemlock and cow parsley are all very similar , i think they maybe part of the same family , with similar white flowers , coming off one stem. We,ve got them all growing here , although the hemlock tends to be along the river bank , the hogweed and cow parsley tends to be in the hedgerow. I will get a pic of all three and post it tomorrow for anyone whos interested.