Giant hogweed Heracleum mantegazzianum
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A non-native invasive plant.
As its name implies, giant hogweed grows tall, up to 6m high. This, and its larger, more pointed leaves, distinguishes it from the native hogweed, which is much smaller (up to 2m).
Marginal habitats and along river banks.
What's the problem?
Giant hogweed often becomes established on waste ground, neglected urban areas, around rubbish tips, roadside verges and river banks, where it forms large, dense colonies and crowds out native plants. It reproduces by seed which is produced prolifically.
Sap from the plant is "phototoxic" - this means that it can burn and blister the skin in the presence of sunlight so precautions must be taken when touching it and when undertaking control work.
This species is listed on Schedule 9 of the Wildlife and Countryside Act in England and Wales therefore, it is also an offence to plant or otherwise cause to grow these species in the wild
Removing giant hogweed
It colonises waste land and river banks but it produce a dense colony and grows up to 5m high as well as producing 30 to 50,000 viable seeds per year. Water can then disperse these seeds downstream where they wash up along the bank so it can spread quickly along watercourses. If the plant is cut down before it produces seed, it will survive into a third or subsequent season, attempting to flower each year.
The sap of giant hogweed contains a toxic chemical which sensitises the skin and leads to severe blistering when exposed to sunlight. Hand cutting should never be undertaken unless the operator is wearing full protective clothing to prevent skin contamination by the sap. Infestations need to be controlled by digging out the whole plant as cutting through the stem must be done below ground level to ensure damage to the rootstock and to prevent regrowth from the base. However, using chemical control may be the most practical option. Individual plants or whole colonies should be sprayed with glyphosate in April or May. The area should be reseeded once the plants have died off and spot treat should be used on new plants as they appear. See here for more information.