Heracleum mantegazzianum
NPPA pest plant
Poisonous plant
Common names
giant hogweed
Family
Apiaceae
Flora category
Vascular – Exotic
Structural class
Herbs - Dicotyledons other than Composites
Conservation status
Not applicable
Simplified description
WARNING: the NZ Ministry of Primary Industries (MPI) says”Giant hogweed is poisonous to humans. Touching it, or exposure to dust from weed-eating, can irritate skin and cause blisters and swelling. The toxin actually causes photosensitisation so that the skin reacts badly to sunlight.”http://www.biosecurity.govt.nz/pests/giant-hogweed
“Giant hogweed is a perennial that grows 4-6 m in height. It has stout dark-reddish-purple stems, and spotted leaf stalks with sturdy pustulate bristles which contain a toxic sap. The stems and stalks are hollow, the stems being 5-10 cm in diameter. Giant hogweed has extremely large leaves (up to 1.5 m) and tuberous root stalks. When the plant is two to three years old it produces large umbrella-like clusters of greenish-white flowers. It usually grows on the banks of rivers or creeks.” (ibid)
Detailed description
A biennial or perennial herb, up to 5 m tall; with a taproot or fascicled fibrous root; stems hollow; leaves compound with 3 leaflets, lower ones to 2.5m long; inflorescence, loose compound umbels; flowers small, perfect; fruit a schizocarp (splitting between the carpels) into two 1-seeded sections (mericarps); the mericarps are elliptical, 7-12 mm long, 6-10 mm broad, with brown oil tubes (vittae) conspicuously expanded at their lower ends (up to 1 mm wide).
Similar taxa
H. spondylium (cow parsnip). Hogweed has purple blotches on the stems and petioles.
Flower colours
White
Fruiting
Mericarps 10 mm long with conspicuous oil canals (vittae) - see Flora of NZ 4: 123, Fig 12
Year naturalised
1968
Origin
SW Asia
Reason for introduction
Possibly ornamental.
Control techniques
MPI advise “Contact your regional council to determine the status of this species and responsibility for control and/or advice on control.”
Etymology
heracleum: Named after the classical Greek hero Herakles (Hercules) who is said to have discovered the plant’s healing properties.
National Pest Plant Accord species
This plant is listed in the 2020 National Pest Plant Accord. The National Pest Plant Accord (NPPA) is an agreement to prevent the sale and/or distribution of specified pest plants where either formal or casual horticultural trade is the most significant way of spreading the plant in New Zealand. For up to date information and an electronic copy of the 2020 Pest Plant Accord manual (including plant information and images) visit the MPI website.
Poisonous plant
Sap contains photosensitiving agents which when combined with daylight cause skin to blister and become inflammed within 15 minutes after contact. After a week hyperpigmentation develops and photoxic reactions can continue for several months (Klingenstein, 2007). Affected skin may remain senstive to UV light for years and contact with moisture (e.g. sweating or dew) will exacerbate the reaction (ibid.). Utmost care should be taken when controlling this plant as due to the delayed reaction, especially avoid use of machinery such as mowers/line trimmers which might fling pulped material about (ibid.). Wash eyes with water if sao goes into eyes and seek medical help if concerned about any reactions (ibid.).
References and further reading
Klingenstein, F. (2007). “NOBANIS – Invasive Alien Species Fact Sheet – Heracleum mantegazzianum” (PDF). Online Database of the North European and Baltic Network on Invasive Alien Species. NOBANIS.