Common Milkweed (Asclepias syriaca) has a stem that is squarish in cross section, leaf size decreases going up the stalk and overlaps significantly (if folded upward), leaf bottom and stem is hairy, shoot is shorter and thicker. Common Dogbane has a stem that is round in cross section, leaf size increases going up the stalk and overlaps only slightly if any, leaf bottom and stem is smooth and hairless, shoot is taller and thinner. Young Milkweed stems tend to be green, while young Dogbane stems have a faint redish bloom or spots.
