Heracleum mantegazzianum Sommier & Levier
Giant hogweed, cartwheel-flower, wild parsnip, white rhubarb, giant cow parsnip, giant cow parsley
The Return of the Giant Hogweed. Sounds like a good name for a horror movie, but it is actually a song by Genesis that satirizes the spread of this plant. It really is a scary plant, reaching as high as 23' (7 m), and armed with poisons that can cause blindness or permanent scarring. This species is listed in the 2003 Guinnesss Book of World Records as the world’s largest weed. Ironically, it is not a native of western Europe or North America; it was introduced as an ornamental. Introduced? That’s like bringing, say, Burmese pythons to south Florida and having them take over the Everglades, eating 90% of the local mammals. Oh, wait, that actually happened...
Here are some similar-appearing species:
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|Plant||Extremely large: 6½-16' (2-5 m) tall, sometimes reaching 23' (7 m). Poisonous : causes phytophotodermatitis.||Up to 6½' (2 m) in height. Poisonous : causes phytophotodermatitis.||Grows to 3’ in height. Plant smells like carrots.|
|Flowers||White flowers form flat-topped compound umbels (flowerheads) up to 30" (76 cm) in diameter.||Large white compound umbels about 8" (20 cm) in size. Individual flowers have five petals of inconsistent size.||Pale pink before opening. When open, forms an umbrella-shaped flower cluster up to 3” across, composed of tiny 5-petaled flowers.|
|Leaves||Leaves are up to 5' (1.5 m) across, extensively divided into sharp-tipped sections.||Up to 16" (40 cm) across, split into sharp lobes that vaguely resemble maple leaves, with a disagreeable odor.||Compound leaves are deeply divided and subdivided.|
|Stem||Hollow stems are 1-4" (3-10 cm) in diameter, with deep purple raised blotches containing white hairs.||Thick, grooved, hollow, hairy, and often reddish.||Fine hairs on stems and leaves.|
|Seeds||Each seed is up to ¼" (8.3 mm) long.|
|Fruit||Flat, green, egg- or heart-shaped, ⅜-½" (9.5-12 mm) × ¼-⅜" (6.3-9.5 mm).|
|Habitats||Rich, moist soils in ditches, stream banks, vacant farmland, and fence and tree lines||Moist, shady mountain woodlands, streambeds|
Plants: These plants are annuals, making it all the more amazing that they routinely grow to a height of 6½-16' (2-5 m) and sometimes reach 23' (7 m) in a single growing season. Hollow stems are 1-4" (3-10 cm) in diameter, with deep purple raised blotches containing white hairs. Giant hogweeds look most similar to extra large versions of common hogweed (Heracleum sphondylium), or Sosnowsky’s hogweed (Heracleum sosnowskyi), or garden angelica (Angelica archangelica). If you don’t know what any of those look like, they also look a bit like ”Queen Anne's lace on steroids,” as one botanist quipped. Here is a good comparison of several similar species.
Leaves: Leaves are up to 5' (1.5 m) across, extensively divided into sharp-tipped sections.
Flowers: White flowers form flat-topped compound umbels (flowerheads) up to 30" (76 cm) in diameter.
Fruits: A single plant produces 20,000 oval seeds, each up to ¼" (8.3 mm) long.
Edibility: Poisonous These plants contain furocoumarins in their sap, a defense against fungal attack. It happens that these chemicals cause phytophotodermatitis in people—that is, severe skin blistering from exposure to light. If you come in contact with the plant, then are exposed to long wave ultraviolet light (sunlight), the dermatitis develops. The exposed skin becomes bright or dark red, developing large blisters. The effects resemble chemical burns, and can leave permanent discoloration or scarring. Temporary or permanent blindness can result from eye exposure.
Removal: Don’t try to remove this plant yourself. Call the local Department of Agriculture and have it removed by professionals (or perhaps the Men in Black). Giant hogweed is federally listed as a noxious weed in the United States, so it is illegal to import, export, transport, or (presumably) grow this plant without a permit.
Heracleum mantegazzianum on massnrc.org
Heracleum mantegazzianum on luirig.altervista.org
Heracleum mantegazzianum on www.oregon.gov
Heracleum mantegazzianum on Wikimedia Commons (Many photos)
Heracleum mantegazzianum on ohioline.osu.edu
Heracleum mantegazzianum on Wikipedia
Heracleum mantegazzianum at the USDA National Invasive Species Information Center
Heracleum mantegazzianum on the USDA Forest Service's Fire Effects Information Database
Heracleum mantegazzianum on www.nrs.fs.fed.us (PDF)
Heracleum mantegazzianum on www.seagrant.sunysb.edu (PDF)
Heracleum mantegazzianum description by Thomas H. Kent, last updated 16 Sep 2020.