Technical “ History
Giant Hogweed is another plant introduced by the Victorians as a garden ornamental. Introduced in 1893 and planted in the great estates by the landscape gardeners of the time, it soon became a hit with the general public. Famed initially for its luxurious growth and huge foliage and flowers, the plant was a favourite with those looking for something exotic within their herbaceous borders.
As is typical of these Victorian introductions, the plant escaped from cultivation, spreading along linear corridors of wasteland, rivers, canals, roads and railways, along which it is easily transported.
The flowers each produce 30-50,000 viable seeds.
Identification
The object of any control strategy should include killing of the existing plants, plus eradicating the seed bank. To carry out one process without the other would be a complete waste of time
Very large perennial herb persisting for 3-5 years, height up to 5m, hollow stem often purple mottled, usually with sharp bristles, stems 5-10cm diameter. After reaching a mature stage, it flowers, sets seed then dies.
All parts have a strong resinous smell.
Leaves are very large - up to 3 metres across - and are sharply divided and serrated with bristles on the underside.
Flowers are umbrella shaped and are very large, up to 80cm across, with many small white flowers.
Dies back in winter. Growth
In a natural state, the plant is biennial, growing from seed in the first year, then taking between 2-5 years to reach maturity, then flowering and dying. However - if the plant is cut down before it produces seed - given enough time - it will flower again and set seed. If it is too late in the season, it will survive into the subsequent season, flowering and seeding in the following season.
Giant Hogweed relies solely on seeds for reproduction and spread, it does not reproduce by vegetative means (i.e. fragments of roots and stem) - thus, a single plant germinating from a single seed could start a significant invasion.
Spread
The huge amount of seeds that the plant produces (30-50,000) create a seed bank in the top 10cm of the topsoil. Within a dense stand of Giant Hogweed, there will be circa 6-7,000 live seeds/m2 seeds/m2
. Out of these, some 2,000 live
will be alive and ready to germinate, having lost dormancy over winter (typically two months at 2-4O
C). About 200 seeds/m2 remain
dormant in the soil and 8% are found to survive in the soil for more than one year - about 5% survive for two years after falling from the parent plant.
This would suggest that the seed bank is active for at least two years, and possibly as long as five years, assuming no more seeds fall from parent plants.
Seeds will also be spread by water and wind, as they disperse easily along watercourses, roads and highways.
Problems
The problem with invasive non-native species is the rapidity of spread and their domination of their new environment. Giant Hogweed produces huge amounts of seed, which leads to dense colonies establishing and supressing the growth of native species. Indigenous plants are displaced and there are usually associated negative impacts on ecologically sensitive and vulnerable habitats.
With its tall stems and huge seed heads, the plant is ideally suited for wind dispersal of its seeds. Many of these seeds will also fall around the plant and produce a huge viable seed bank within the topsoil. Seeds which fall into watercourses will all be dispersed downstream and washed up along the bank - often landing on bare earth ideal for establishment - thus allowing the plant to spread rapidly. The plant produces huge amounts of vegetation which cause problems with water flow and can impact on water movement during flood events.
PC AUGUST/SEPTEMBER 2015 I 133 Blisters appear when skin is exposed to the sun
When the plant dies back over winter, the ground beneath is exposed and bare of vegetation, leaving river banks susceptible to erosion and recolonisation by new seeds floating downstream.
As previously noted, Giant Hogweed is a serious threat to human health - any measures to control Giant Hogweed MUST take this on board!
Control
Any strategy used to control Giant Hogweed must be undertaken with extreme care.
Use of full personal protective equipment - including overalls with integral hood, boots, eye protection, face protection, full helmet, gauntlets etc. etc. - is an absolute must!
The object of any control strategy should include killing of the existing plants, plus eradicating the seed bank. To carry out one process without the other would be a complete waste of time. Methodologies can be chosen to suit the resources available - usually depending upon budget and timescales required.
The surface growth of Giant Hogweed can be effectively eradicated using an application of translocated herbicide.
The seed bank can also be dealt with by excavation and removal to licensed landfill - this is an expensive approach and one which would be considered environmentally unfriendly.
Using a longer term approach of seasonal applications of herbicide over a period of five to ten years would be a highly effective strategy.
For more detailed advice, please contact Mike Clough at Invasive Weed Solutions Ltd.
Mike Clough is Chairman of INNSA (Invasive Non- Native Specialists Association)
www.innsa.org
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