- Scientific name:
- Heracleum mantegazzianum
- Cartwheel-flower, Efwr enfawr (Welsh), Giant cow parsnip, hogsbane, giant cow parsley
- Native to:
- Caucasus mountains in south west Russia and Georgia
- Widespread, most common on river banks, but can also be found along railway banks and on waste ground
What is Giant Hogweed?
Giant Hogweed is a non-native highly invasive plant in the family Apiaceae introduced to Britain in the 19th century, mainly for ornamental reasons and is recognised as one of the most invasive species of plant in Britain and Ireland today. Due to our climate being warmer and due to the higher nutrient content of our soils giant hogweed seeds germinate more readily here and subsequently it has became very invasive. There is no significant insect or pathogen control of this plant.
Because of its heavy seeds, which are its sole mechanism for dispersal, this invasive weed favours river and stream bank habitats. The large seeds are shed into the flowing water and carried downstream where they can germinate and create new populations. The weed also grows luxuriantly along railway lines, on disused waste areas and on undisturbed damp ground.
In Ireland Giant hogweed is continuing to spread, particularly along river corridors. This continues to be a cause for concern because of the human health hazard that the plant represents and because of its dramatic impact on biodiversity and bankside integrity.
Identification of Giant Hogweed
Giant hogweed is a tall (usually 3 – 5m), perennial herbaceous plant with hollow, red-spotted hairy stems and several hundred small white flowers in large umbrella-like flower heads up to 500mm across. Stems are green with dark-red or purple blotches and are hollow; they can be up to 100mm in diameter. The leaves are large (to 1.5 metres across), deeply divided and often jagged in form. They have hairy spines or bristles underneath. The root is tuberous and fleshy. Individual plants set seed after three to four years growth and then die. The large umbrella-shaped flower heads are borne on tall (up to 4 metres) flowering stems. The flowers are relatively small and are normally white, although they can be pink. Flowers are produced in June / July with seeds appearing in late July and August. Each plant is capable of producing 50,000 viable seeds. Seeds can remain viable in the soil for up to 15 years, although the majority of seeds germinate in the first few years.
The following species are sometimes confused with giant hogweed due to their flowering head: wild carrot, fool’s watercress, cow parsley, hedge parsley, pignut, sweet cicely, hogweed, wild angelica, alexanders, sweet cicely, water hemlock, wild parsnip, fennel and hemlock. https://youtu.be/z7rWZ5H63Gc
Problems caused by Giant Hogweed
Giant Hogweed poses a very real threat to humans as well as to our native fauna and flora.
- The plant’s sap contains a chemical that sensitises the skin, which leads to severe blistering when exposed to sunlight. This may recur for several years after the initial exposure. The intensity of the reaction varies with individual sensitivity.
- Giant Hogweed forms dense stands that severely depletes local biodiversity by light exclusion.
- It restricts access to rivers for leisure or inspection purposes.
- In winter, following dieback, its leaves can cause blockages in water courses, as well as exposing river banks to severe erosion.
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Solution for Giant Hogweed
This is a job for the professionally trained operator. There have been a number of litigation cases taken against Public Authorities by both workers and the public who have received severe burns and scarring from this plant.
A control program will typically be planned over four years.
- Strict adherence to health and safety and appropriate PPE must be worn.
- An approved herbicide will be applied to all plants in Spring and Autumn.
- The site must then be monitored and any live plants sprayed.
- If and when any further live plants flower, the flower heads should be removed and burned.
Giant Hogweed a significant health hazard!
How to treat Giant Hogweed
This is obviously a complicated and time consuming task that INVAS Biosecurity is happy to undertake for its clients.
This is a job for the professionally trained operator. A full knowledge of the methodology and of chemical application is required, and strict adherence to health and safety regulations and the wearing of approved PPE must be observed. There has been litigation taken against Public Authorities by workers who have received burns from this plant.
It is important that a detailed control program be drawn up after the completion of a survey of the site that covers the catchment area. Included in this plan, provisions must be made to prevent the further spread of the plant.
The seeds can remain viable up to 15 years but this is more the exception than the rule with the vast majority being past their sell by date after three years. A control program will typically be planned over four years.
Consideration must be given to the location of the infestation and a suitably approved herbicide chosen should the application be made near water courses. Herbicide treatment has proven to be the most effective method of control and must be followed by collection, bagging and approved disposal of any seed heads that have been missed. The overall aim of the program is to eliminate the seed bank. This procedure must be followed over the four years of the program.
This should be avoided as the risks of getting sap on the skin and the consequent burning is extremely high. If it must be undertaken it is essential that the operator wears fully approved PPE and it is important to remember that cut material can remain active for many hours. After cutting the large perennial root can send up new shoots to reinfest the area so cutting is not an option if long term eradication is envisaged.