About Giant Hogweed
For the phrase ‘Schedule 9 Invasive Weed’, read ‘troublemaker’. If a plant is included in this list, it’s there because it will cause major problems to our natural environment. Giant hogweed will form dense colonies that suppress the growth of our native plants, disrupting our native biodiversity and leaving river banks bare of vegetation in winter, increasing the risk of erosion and recolonisation from seeds washed downstream. Infestation is precisely the right way to describe areas impacted by giant hogweed. Growing up to five metres in height with huge, incised leaves, mature plants can span over two metres, so it doesn’t take many individual plants to overrun an area. When you consider that each plant will bear up to 100,000 seeds it’s easy to see why an infestation can quickly get out of control.
Why worry about giant hogweed?
With giant hogweed, there is a second reason to be cautious; it has developed a fearsome internal chemical weapon that’s designed to protect against micro-organisms, but which can have devastating effects on humans. Sap from the plant contains chemicals, known as furanocoumarins, which can cause serious skin damage which start as a violent sunburn-like response on the skin. This is often accompanied by blisters and pigmentation which, along with the remaining scars, can remain for many years and are often further aggravated by exposure to sunlight. As if this wasn’t enough, a tiny amount of sap in the eyes is enough to produce temporary and sometimes permanent blindness. There is then both a social and legal responsibility to deal with giant hogweed infestations.
Giant hogweed facts
- Giant hogweed is usually biennial, forming a rosette of jagged, lobed leaves in the first year before sending up a flower spike in the second year and then setting seed. True biennials only live for two years, dying after flowering, but giant hogweed does not always behave as a true biennial and in fact some are perennial, coming up year after year.
- Highly poisonous sap causes severe skin blisters and resulting scars which are aggravated by exposure to sunlight. Anyone thought to have come in to contact with this weed should seek immediate medical advice.
- Reproduces by seed only so its spread is managed by controlling the seed bank
- 95% of released seeds will reside in the first five centimetres of soil depth.
- Seed persistence is limited as the vast majority will die or fail to germinate after 20 years
- You need to be aware that pets can transfer the sap from plants on their fur.
- Typical locations: along rivers, on derelict land. Often found along verges and in parks.
- Giant hogweed (aka Heracleum mantegazzianum), is a tall, cow parsley-like plant with thick, purple-speckled (once mature) stems that are covered in thick white bristles.
- The flowers are white and held in umbels, (flat-topped clusters), with all the flowers facing upwards. The flower heads can be as large as 60cm across.
- It can reach a height of 5 metres and has a maximum spread of about 2 metres.
- Giant hogweed will usually form a rosette of jagged, lobed leaves in the first year before sending up a flower spike in the second year, then setting seed before dying
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