Giant hogweed (Heracleum mantegazzianum) is a native of Europe's Caucasus Mountains. Imported to the United States, it naturalised in many areas. The plant is a member of the carrot or Apiaceae, formerly Umbelliferae, family and is distinguished by its height between 9 and 14 feet tall and giant leaves, which can be 3 to 5 feet in length. Giant hogweed is often confused with other members of the carrot family with similar tall, hollow stalks and flattened clusters or umbels of tiny individual flowers.
Start at the top to identify true giant hogweed. The flowers look almost exactly like those of common Queen Anne's lace (Daucus carota), only much larger with clusters of rounded umbels up to 4 feet across. Next, examine enormous leaves, which are palmate or deeply lobed and incised. The thick stalks are marked with purple blotches and rings of coarse, white hairs. They are also ridged and hollow. Be careful when examining a suspected giant hogweed. The sap in the stems and leaves can cause skin reactions and even blindness if rubbed in the eyes.
Another member of the Heracleum family, cow parsnip (H. lanatum) has a similar appearance to giant hogweed. The differences include flower shape. Cow parsnip flower heads or umbels are flattened rather than rounded on top and somewhat smaller than those of giant hogweed. The leaves are also half-size and not as deeply incised. Cow parsnip's hairy flower stalks are about half the width of hogweed, 1 to 2 inches in diameter rather than 2 to 4 inches, and do not have the distinctive purple blotches. Cow parsnip's seeds are heart shaped and smaller than hogweed's ovoid seeds.
Poison hemlock (Conium maculatum) another member of the Apiaceae family, is a dangerous plant in its own right. Ingestion can be fatal to humans and animals. Unlike hogweed, hemlock's purple-blotched stems are hairless. The small, white flowers are gathered into many flat-topped clusters, rather than relatively few, very large umbels. Hemlock's foliage is smaller and sometimes glossy, bearing a resemblance to fern fronds. At 4 to 9 feet, poison hemlock is smaller and somewhat more delicate in appearance than its giant relative.
Angelica archangelica is one hogweed imposter with leaves and stalks that are not only nonpoisonous but also edible. The stems are often candied and used in cakes and confections. It is significantly smaller than hogweed, at about 4 to 9 feet tall, with yellowish green flower umbels instead of white ones. Purple hairless stems support the ball-shaped flowerheads and the pinnate leaves are made up of many distinct leaflets.