- Heracleum mantegazzianum
What’s the problem?
- Legal implications.
- Invasive non-native species.
- Rapidity of spread.
- Difficult to eradicate seed bank – may take years.
- Causes blistering of skin on contact.
What are the legal implications?
- You must not allow Giant hogweed to spread onto adjacent land – the owner of that land could take legal action against you.
- You must not allow or encourage the spread of Giant hogweed – this includes moving contaminated soil from one place to another or incorrectly handling and transporting contaminated material and cuttings.
- You do not need to notify anyone.
- You are not obliged to remove or treat on your own land.
Giant hogweed originates from the Caucasus Mountains between Russia and Turkey and was introduced as a popular wetland ornamental plant in the 19th Century. The earliest records originate from the seed list at Kew Botanic Gardens in London which show that Heracleum persicum was received in 1819. It quickly spread as its large flower heads produced thousands of small light seeds that are easily spread by wind and water as well as by human activity including in some areas deliberate planting.
Giant hogweed is a perennial growing up from a single tap root to a height of 5m and with leaves of up to 1m. The plant can take several years to mature where it then flowers producing some 20,000 seeds on average before it dies.
Giant hogweed thrives on disturbance and has been spread by both natural means and by human activity. In riparian areas, high water flows disperse seeds of the plant downstream where new colonies form.
Health hazards if not removed
The large size of the plant and the hollow stems make Giant hogweed very attractive to children who often use them as pea shooters and telescopes – however, the stems leaves and sap contains several photosensitizing furanocoumarins. In contact with the human skin and in combination with ultraviolet radiation, these compounds cause burning of the skin. After a period of about 24 hours flushing or reddening of the skin (erythema) and excessive accumulation of fluid in the skin (edema) appear, followed by an inflammatory reaction after three days. The reaction of the skin will also depend on individual’s sensitivity. Approximately one week later a hyperpigmentation (unusual darkening of the skin) occurs in the affected areas, which can last for months. The affected skin may remain sensitive to ultraviolet light for years.
All blistering should be treated as a burn and in severe cases, medical advice should be sought as untreated blistering can lead to permanent skin damage and recurrent dermatitis.
Biology and Ecology
Giant hogweed plants can live for several years but die after bearing seeds once. They persist in a rosette stage and usually flower in the third or fifth year. In unfavourable conditions flowering is postponed until sufficient reserves have been accumulated. Given such conditions plants can live for 12 years. On average, it flowers in the UK earlier than its native Caucasian range. They do not reproduce vegetatively and rely exclusively on reproduction by seed.
Flowers are arranged in compound umbels and are composed of insect-pollinated and hermaphrodite flowers (having both male and female parts in the same flower), the pollen grains mature before the female reproductive structure becomes receptive. Seeds normally result from fertilisation between two plants (outcrossing), but there is an overlap in male and female phases, which makes self-fertilization possible. Seeds produced by self-pollination are viable; more than half of them germinate and give rise to healthy seedlings. This means that even a single isolated plant, resulting from a long dispersal event, is capable of founding a new population.
The plants flower from mid-June to late July and seeds are released from late August to October.
- Spring – Dark green spiky leaves start growing very early in March/April reaching a height of 30 – 50cm.
- Summer – The large hollow stems are ribbed and green in colour with purple blotches – large flower heads up to 50 cm across.
- Autumn – Some immature plants may die back but many will keep some colour in their leaves through winter – mature flowering plants will die leaving seed heads with up to 20,000 seeds.
Giant hogweed is a particularly virile and aggressive weed, which propagates through seed, root and rhizome systems. It has the ability to establish growth from very small quantities of this material and therefore all processes of control and eradication must take account of this ability. An understanding of the plants flowering biology and mechanisms of reproduction is crucial for successful eradication.
- Plants do not flower until the second, third or even fourth year. Some plants are monocarpic and die back after flowering, others are perennial and flower for several years.
- By mid-April, mature plants are 3 – 4 feet tall and up to three feet wide. Seedlings are 300 – 450mm tall with leaves that are much more palmate than the mature plant.
- By May, the mature plant starts to bolt sending a thick hollow stem up to a height of 2 to 3 metres.
- Flowering starts from mid-May to mid-June and lasts from several weeks to more than a month.
- Green fruits form by July and then turn brown as they ripen.
- From late August through September the plants become senescent, dying back to the roots. The dried stalk and bare flower stems will persist through autumn and winter.
- Winged seeds are dispersed by water and soil movement.