Photo credit: © Henrik Ismarker, https://flic.kr/p/odrK25

Giant Hogweed

(Heracleum mantegazzianum)

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If you believe you have found this species anywhere in Pennsylvania, please report your findings to iMapInvasives by submitting an observation record.

Species at a Glance

Giant hogweed, also known as giant cow parsnip, is a perennial herb and member of the carrot and parsley family that can grow 4.5-6 m (15 to 20 ft) high. While the plant is beautiful, it is also dangerous and direct contact with its sap can cause blisters and burns on the skin. It is one of the few North American invasive plants that can cause both human health impacts and ecological damage.

Identification

Leaves: Large compound leaves are lobed, deeply incised, and 1-1.5 m (3-5 ft) wide. Hairs on the undersides are stiff, dense, stubby, and only about 0.25 mm long. Petioles have short, coarse, white hairs at the base.

 

Flowers: Numerous white flowers form a large, flat-topped, umbrella-shaped head that is up to 0.8 m (2.5 ft) across. Flowering occurs from late spring through mid-summer.

 

Fruits/Seeds: Up to 100,000, 1 cm (0.5 in) long, winged, flattened, oval seeds form in late-summer. The seeds, originally green, turn brown as they dry.

 

Stems/Roots: Thick hollow stems are generally 2.5-8 cm (1-3 in) in diameter, but can reach up to 10 cm (4 in). They are marked with dark purplish blotches and raised nodules. Leaf stalks are spotted, hollow, and covered with sturdy bristles that are prominent at the base of the stalk. Stems are also covered with less prominent hairs.

Similar Species

While it’s much larger than most native plants, giant hogweed can be mistaken for cow parsnip (Heracleum lanatum), purple-stemmed Angelica (Angelica atropurpurea), and poison hemlock (Conium maculatum). Cow parsnip only grows 1.5-2.5 m (5-8 ft) tall and the deeply ridged stems can be green or slightly purple without the dark blotches and raised nodules. Purple-stemmed Angelica has smooth, waxy green to purple stems with no bristles or nodules and a softball-sized cluster of greenishwhite or all white flowers. Poison hemlock is shorter than giant hogweed, growing only 1-3 m     (4-9 ft). While the stem has some purple blotches, it is waxy and the entire plant is smooth and hairless and the leaves are fern-like with bright, glossy, green leaves.

Habitat

Giant hogweed can colonize a wide range of habitats, but prefers rich damp soils such as those found along abandoned railroad right-of-ways, roadside ditches, stream banks, or other moist disturbed areas.

Spread

Because of its unique size and impressive flower head, giant hogweed was probably introduced to the United States and Canada as an ornamental plant for arboreta and Victorian gardens. It was also a favorite of beekeepers because the flower heads provide a large amount of food for bees. Giant hogweed survives from one growing season to the next by forming buds that can endure a period of dormancy in the winter. The numerous seeds produced by this plant can be spread by animals, surface runoff, wind, or by human activities and can remain viable in the soil for up to 10 years.

Distribution

Native to Europe and Asia, giant hogweed escaped cultivation and has now become naturalized in a number of areas throughout the United States and Canada. In the Mid-Atlantic region, it can be found in New York, Pennsylvania, and North Carolina.

 

Note: Distribution data for this species may have changed since the publication of the Mid-Atlantic Field Guide to Aquatic Invasive Species (2016), the source of information for this description.

Environmental Impacts

Colonies of giant hogweed can form dense stands that crowd out slower growing native plants. The sap from broken stems, leaves, roots, flowers, or seeds, can cause irritation in the form of severe blisters, burns, painful sores, and purplish blackened scars when it comes in contact with skin.

Video
Note

Information for this species profile comes from the Mid-Atlantic Field Guide to Aquatic Invasive Species (2016).

The Pennsylvania iMapInvasives Program is a partnership of the Western Pennsylvania Conservancy, the Pennsylvania Natural Heritage Program, and NatureServe.

Funding for Pennsylvania iMapInvasives is provided by the Great Lakes Restoration Initiative.

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Pennsylvania Natural Heritage Program, iMapInvasives partner
Great Lakes Restoration Initiative logo, iMapInvasives funding source