Quick Facts

Quick facts

• Originally from south-west Asia, Giant Hogweed (Heracleum mantegazzianum) is not yet established as a weed in Australia although previously recorded in SA and TAS.
• A large perennial herb to 3-4 metres tall occasionally to 7 metres, with large leaves eventually to 3 metres long, and large umbrella-like umbels of many small white flowers, flowering once then setting seed and dying living for 3-5 years.
• Plants under stressful conditions are able to postpone flowering and can live up to 12 years.
• It has the potential to become an invasive harmful weed if it does become naturalised, as it is a weed of great significance in all countries where it has become weedy.
• Its fast growth allows rapid development of populations that form a dense cover of leaves above native vegetation.
• Giant Hogweed is highly toxic to humans, producing a poisonous sap, and toxicity may result from any activity that involves bruising, cutting or touching the foliage.
• Control is best achieved when plants are young. Older plants are generally difficult to control, requiring much follow-up over several years.

What Does It Look Like?

What is it?

Giant Hogweed (Heracleum mantegazzianum) is a short-lived perennial monocarpic (flowering and fruiting only once then dying) tall herb, flowering after 3–5 years. Normally attains a flowering height of about 3–4 metres but can be up to 7 metres tall. The large leaves are deeply divided and lobed, coarsely toothed with pointed tips, egg-shaped in outline resembling rhubarb leaves. Initially leaves forms a rosette, then are alternately arranged, increasing in size each year and can eventually reach up to 3 m long and 1.5 m wide before flowering. The upper leaf surface is glabrous but the underside and petiole are covered in pustulate bristles which produce a toxic sap toxic to humans. When it finally flowers, usually after 3–5 years, there is a single hollow stem to 100 mm in diameter. Upper leaves on the flowering stem are progressively smaller. The stem is ridged, with dark reddish-purple blotches, the hollow stems are also covered in pustulate bristles which produce a toxic sap. The base of the leaf stalk is winged. The crown below the soil and tuberous roots are spreading and give rise to new buds each year. Buds can develop into new plants.

The inflorescence (flowering head) is a white flowering compound umbel (numerous aggregates of smaller umbrella-like arrangements of flowers originating from central point on flower stalks, each umbel originating from a central point). The large compound umbel of inflorescences are arranged at the end of the branches on a peduncle (main flower stalk holding inflorescence). The small white flowers are arranged at the tops of the stems in broad, umbrella-shaped groups up to 800 mm in diameter. Each flower has five small white petals and sits on top of the ribbed ovary.

The fruits are dry and oval shaped, more than 10 mm long, with brown swollen resin canals, up to 1 mm in diameter, and on each side. Each fruit contains two seeds. The seeds are large, flattened and oval in shape, and there are approximately 1500 seeds per flower head (Nielsen et al. 2005; USDA Forest Service undated).

For further information and assistance with identification of Giant Hogweed contact the herbarium in your state or territory.

Flower colour

White

Growth form (weed type/habit)

Herb

Where it currently grows? Preferred habitat

In its native range Giant Hogweed grows on the edge of forests and glades and riparian areas alongside streams and creeks. It occurs in mountain areas with annual rainfall of 1000-2000 mm and a temperate continental climate of hot summers and cold winters (EPPO 2006). Where it is naturalised in North America, it is common along railways, roadsides, streams, rivers, uncultivated or wastelands and agricultural areas (USDA Forest Service undated).

Are there similar species?

The naturalized species Hemlock (Conium maculatum) is similar to Giant Hogweed but Hemlock does not grow as tall and the leaves are more finely divided ( eFloraSA 2021; VicFlora (2016).

Why Is It A Weed?

What are its impacts?

Giant Hogweed (Heracleum mantegazzianum) is a weed of natural areas including riparian (riverside) habitats, roadsides and amenity areas. It is highly toxic to humans, causing severe dermatitis and swelling and blistering of the skin.

Agriculture: Giant Hogweed is not normally a weed of agriculture or crops, but there are reports of its encroachment into crop fields, such as potatoes in Sweden (Lundstrom 1984), and it has also been known to invade pastures (EPPO 2006). Giant Hogweed acts as a host for some crop pests and diseases.

Native ecosystems:  In Europe, Giant Hogweed has negative impacts on biodiversity and the environment in general, Giant Hogweed is an aggressive competitor, which is able to out-compete native plant species, reducing the amount of suitable habitat available for insects, birds and mammals (EPPO 2006). When it dies back during the winter months, it leaves bare ground. This can lead to an increase in soil erosion on riverbanks and steep slopes (Davies 2008).

Urban areas: Known to grow on urban fringes and in urban riparian areas Giant Hogweed is toxic to humans, the sap can cause severe photo-sensitivity of the skin, impacting human health, native flora and tourism (EPPO 2006). It contains a substance within its sap that makes the skin sensitive to ultraviolet light. This can result in severe burns to the affected areas, producing swelling and severe, painful blistering (Davies 2008). In other areas of the world dense infestations of the weed can lower the recreational value of the landscape and seriously interfere with access to amenity areas and river banks. Along roadsides, large stands can reduce visibility and result in a safety hazard.

How does it spread?

Giant Hogweed can reproduce vegetatively from underground buds formed on the crown and tuberous roots, and is also spread by seed. Seed is spread  considerable distances by water dispersed locally by wind and water, birds. It is spread by people  and via the ornamental plant trade, dumping of garden waste and the spice trade. Seeds stick to vehicles, clothes or animal fur or are distributed in contaminated soil, and can be spread far from the place where they were produced (Nielsen et al. 2005; EPPO 2006).

What is its history in Australia?

In Australia, Giant Hogweed is known from one collection in South Australia and one collection from Tasmania, both collected in 1990 (AVH 2021). It is considered a potential weed threat from seed which has been available through the horticultural industry (Spafford Jacob et al. 2004).

How To Manage It?

Best practice management

In Europe, Giant Hogweed (Heracleum mantegazzianum) can be controlled by herbicides if applied early in the growing season. Other physical and cultural control methods are also used, and best employed on young rosetted plants before they flower. However, with these persistent and difficult to control plants, an integrated management program should be employed, with 2–4 visits required per year for several years. A coordinated approach should be employed with local government staff.

Chemical control: Spraying Giant Hogweed with an appropriate herbicide is the most effective control option, although it can take several years of repeated treatments to eradicate. Spraying can only be carried out during the growing season when there is green, leafy material present. In order to be effective, spraying must be carried out before the plant flowers and sets seed. Seeds can remain in the soil for several years, so a long-term strategy for treatment with herbicide is required (Davies 2008).

Please see the Australian Pesticides and Veterinary Medicines Authority for chemical information http://www.apvma.gov.au 

Non-chemical control: Physical control: Removal of smaller individual plants should be carried out with full Personal Protection Equipment (PPE) to avoid contact with skin and eyes, as contact with the plant can cause serve dermatitis. All roots and underground buds should be removed as these can form new plants  If plants have seeded then topsoil within a 4-m radius can be removed around Giant Hogweed plants as seeds fall within a 4-meter radius of the parent. 

Mechanical control: Mechanical cutting is frequently used to clear river banks, but provides no long-term control, as there is rapid re-growth from below ground, and it may encourage additional flowering shoots.  A quicker method of removing larger infestations of Giant Hogweed involves mechanical removal clearing of above ground leaf/stem material, and the additional removal of below ground material contaminated with seeds within a 4-m radius around Giant Hogweed plants and removal of  underground buds preventing the formation of new plants. Any regrowth should be treated with an appropriate herbicide (Davies 2008).  Follow-up will be required in many cases for several years.

Does it have a biological control agent?

NO

When does it grow? (lifecycle/growth calendar)

Giant Hogweed is a short-lived perennial herb normally flowering and dying with 3–5 years. Plants persist in a rosette stage and usually flower in the third to fifth year. Under unfavourable conditions, such as nutrient-poor, shaded or dry sites or when regularly grazed, flowering is postponed until sufficient reserves have been accumulated, and plants can live for at least 12 years. In central Europe, plants flower in summer and seeds are released in autumn. Seeds are initially dormant and do not germinate until spring. A period of two months at 2–4°C is sufficient to break dormancy under experimental conditions. In the field, dormancy is broken during cold winters (Nielsen et al. 2005; EPPO 2006).

Where Is It Found?

Which states and territories is it found?

Previously found in SA & TAS.

What areas within states and territories is it found?

Giant Hogweed is recorded as being collected in the Adelaide area of South Australia (AVH 2021), and recorded near Devonport in northern Tasmania, but appears to have been eradicated (Tasmania DPIPWE, 2019).

Where does it originate?

Giant Hogweed originates from the Caucasus mountains of south-west Asia (Nielsen et al. 2005).

National And State Weed Listings

Is it a Weed of National Significance (WONS)?

NO

Where is it a declared weed?

TAS, WA

Government weed strategies and lists – Weeds Australia

Is it on the National Alert List for Environmental Weeds?

NO

Government weed strategies and lists – Weeds Australia

Is it on the Agricultural Sleeper List?

NO

Government weed strategies and lists – Weeds Australia

Names And Taxonomy

Main scientific name

Heracleum mantegazzianum

Other scientific names (synonyms)?

Does it have other known common name(s)?

Cartwheel Flower, Wild Parsnip, Wild Rhubarb, Giant Bearclaw