Giant Hogweed (Heracleum mantegazzianum)

Gallery:

Giant hogweed flower head

Giant hogweed leaf with hand for perspective

Giant hogweed stem

Giant hogweed seed head

Giant hogweed

Giant hogweed infestation

Common names:

Giant hogweed, giant cow parsley, giant cow parsnip, wild parsnip, cartwheel flower

Scientific Name:

Heracleum mantegazzianum

Description:

Giant hogweed is a distinctively large, escaped ornamental in the Apiaceae (carrot) family. The hollow stems have reddish-purple spots, are covered with bristles, and are 2 – 4 inches wide. The leaves are sharply and deeply cut and can be 5 feet wide, while the white flowers grow in flat-topped clusters that can be 3 feet wide.The plant spreads by seed, producing anywhere from 5000 to 100,000 seeds per plant. It takes 3-5 years to produce flowers, and is monocarpic, which means the plant dies after it flowers.

Life cycle:

Perennial

Height of mature plants

15-20 feet tall

Flower color:

white

Bloom time:

Mid-May to July

Look-a-likes:

Giant hogweed looks extremely similar to the native cow parsnip. When mature, giant hogweed is larger than the native cow parsnip that only grows 6 feet tall with flower heads that are 8-12 inches. However, before maturity, the distingishng features are subtle and relative to each other: the purple spots on giant hogweed are more raised, the hairs on the leaf underside are shorter, and the leaves are more deeply cut.

Habitat:

Giant hogweed is somewhat shade tolerant and prefers moist soils. However, it is also grows well in full sun. Escaping cultivation by seed, this plant can be found in backyards, grasslands, ravines, parks, vacant lots, streamsides, woodlands, and roadsides.

Impacts:

Giant hogweed has a clear sap that is photo-toxic, which means skin contact followed by sun exposure can cause severe, painful blisters that can leave dark scars (contact with the smoke from burning the plant has similar effects). It is a prolific seeder, and the seeds can float in water up to 8 hours. Furthermore, it can out-compete native plants and invade stream areas.