Heracleum mantegazzianum is a biennial or perennial herbaceous plant that grows up to 20 feet. It reproduces by seeds and thrives in temperate climates.
It is easily identified by its stiff-haired green stems with purple blotches, huge leaves (up to 3m long), and white-topped umbel inflorescence.
Giant Hogweed Classification
- Domain: Eukaryota
- Kingdom: Plantae
- Phylum: Spermatophyta
- Subphylum: Angiospermae
- Class: Dicotyledonae
- Order: Apiales
- Family: Apiaceae
- Genus: Heracleum
- Species: Heracleum mantegazzianum
- Common Names: Cartwheel Flower, Giant Cow Parsley, and Giant Cow Parsnip
Nativity and Distribution
Giant hogweed is native to the Caucasus regions in Europe and Asia, in countries such as Georgia and Southern Russia.
It is now naturalized throughout Europe, Canada, the United States, New Zealand, Australia, and West Asia.
Physical Characteristics
- Leaves: Large, deeply incised and lobed, 3m long
- Fruits: Elliptical, narrowly winged, 6-18 x 4-10mm wide
- Stems: Hollow, ridged, green stems 3-8 cm wide
- Flowers: White or greenish-white umbrella-topped umbel 80-100 cm wide
- Roots: Tap root system
Giant hogweed is a large biennial or perennial herbaceous plant that grows up to 20 feet. It belongs to the Apiaceae (carrot) family.
It is easily identified by its stiff-haired green stems with purple blotches, huge leaves (up to 3m long), and white-topped umbel inflorescence.
The leaves are large, green, compound, deeply incised and lobed, and coarsely toothed, about 3m long and 1.7 m wide in maturity. The upper surface is glabrous while the underside is covered in bristles.
Stems are hollow, ridged, and green, with dark reddish-purple spots surrounding pustulate bristles. They are typically 3-10 cm wide and can grow to more than 4m high.
The plant has an umbrella-topped compound umbel composed of white or greenish-white flowers about 80-100 cm wide. Flowering occurs between June and July.
Fruits are a pair of mericarps, elliptical, narrowly winged, glabrous to villous, 6-18 mm long and 4-10 mm wide. The seeds are dry, flattened, tan-colored, and 1 cm long, with brown lines running through most of the length.
Reproduction, Dispersal, and Life Cycle
- Life Cycle: Biennial
- Seeds: A single flowering plant can produce an average of 20,000 seeds
- Climate: Temperate climate with dry summers and cold winters
- Dispersal: Wind, water, contaminated implements and vehicles, and as a produce contaminant
Heracleum mantegazzianum behaves as a perennial or biennial and exclusively propagates by seed.
A single plant can produce between an average of 20,000 and up to 100,000 seeds. Once set, pre-flowering plants appear as rosette leaves on the ground level without an upright stem. This stage may last for one or several years, 1-5 years, depending on the area’s climatic conditions.
In midsummer, in the next year or after several years, the weed bolts an erect stem from the rootstock and flowers, (for about 10 days), producing thousands of seeds.
After producing seeds, the plant dies off, leaving dead stems and hanging seed heads with the seeds slowly falling for dispersal to other areas. This is why the giant hogweed is considered a monocarpic perennial.
The seeds are dispersed shorter distances by wind while long-distance dispersal is mainly achieved through water.
Movement can also occur when the flowerheads (initially picked as ornamentals) are disposed of as rubbish, through movement of soil during construction, attachment to vehicles or rail trains, or as a seed contaminant.
The dispersed seeds don’t grow immediately but remain dormant until the next spring. The dormancy is broken by cold and wet conditions in autumn and winter. They have a 90% germination after the first winter with the rate declining in successive years. Buried seeds remain viable for up to seven years.
Similar Plants
- Heracleum maximum
- Heracleum persicum
- Heracleum sosnowskyi
- Heracleum sphondylium
- Daucus carota
Uses
- It is grown for its imposing and attractive foliage in Europe and North America.
- Young shoots are collected and consumed for their high Vitamin C.
- Its allelopathic acetone extracts can be used to suppress other weeds and as insect repellent.
- It was used to make traditional medicine for sores and rashes.
- The plant’s antiviral, antifungal, and antibacterial compounds can be used for medicinal purposes.
Impact on Farms and Environment
Heracleum mantegazzianum is a noxious weed in several US states and Tasmania in Australia and its sale and distribution is subject to regulations in the UK, EU, and New Zealand.
It is a common weed of stream banks where its gait suppresses grass growth, leading to increased soil erosion. Apart from promoting winter erosion, its presence along the banks can also disturb salmon spawning grounds.
In natural habitats, giant hogweed can outcompete all native vegetation except trees, causing catastrophic biodiversity changes of up to 50-60%. Its early germination allows it to overtake native plant species and its numerous seeds help invade vast open areas.
As a weed of river banks, open areas, and roadsides, an infestation can lead to the reduction of the value of landscapes, access to amenities, and decreased road visibility.
Of course, giant hogweed is most known for its severe phototoxic characteristics. The plant’s sap, which contains furanocoumarins, causes photodermatitis in humans, resulting in severe skin burns or painful water blisters. Large doses of the sap have been reported to cause fetal malformation or cancer.
While washing the affected area with soap and cold water and avoiding sunlight exposure can help alleviate symptoms, some severe cases may require hospitalization.
Control
- Cultural control: Years of grazing by sheep, goats, and pigs to eradicate the weed.
- Mechanical control: Hand-pulling, digging, and ploughing to remove the weed from the roots.
- Chemical control: Herbicides like glyphosate, imazapyr, and triclopyr applied from March to May.
H. mantegazzianum can be controlled through intensive grazing of sheep and pigs for 2-5 years. It may be less palatable to cattle, but they are also included in some countries like Ireland. Grazing is recommended to begin in spring and the animals should be familiar with the weed to prevent overeating.
Physical strategies to control giant hogweed are limited as the majority don’t completely eliminate the weed. However, hand-pulling (while wearing gloves) the young seedlings can be effective.
For older plants, digging and ploughing to remove the crown (10 cm deep in the soil) or the tap root can kill the plants.
The recommended herbicides for cartwheel flower include glyphosate, imazapyr, and triclopyr. Since the weed occurs near water bodies, there are application precautions prescribed for each product.
A combination of mulch and pyrolysis liquid from birch wood is a biochemical alternative for giant hogweed control.
Lead Editor for Insight Weeds.