What is giant hogweed?
Giant hogweed (Heracleum mantegazzianum) is a non-native, invasive weed. Native to southern Russia, it’s a member of the Apiaceae family and is closely related to cow parsley, carrots and parsnips. Originally introduced to the UK as an ornamental plant in the 19th Century, it has large, umbrella-shaped flower heads and can grow to over 3m (10ft) in height. However, its sap can cause severe burns if it comes into contact with human skin.
Giant hogweed can now be found throughout much of the UK, especially waste ground, roadside verges and river banks, where its seeds are transported by water. It grows in dense clumps and can out-compete native plants.
It’s listed on Schedule 9 of the Wildlife and Countryside Act in England and Wales. This means that, while it’s not an offence to grow it in your garden, it is an offence to plant it in the wild or let it escape from your garden into the wild.
What does giant hogweed look like?
Giant hogweed looks like a much larger version of cow parsley. Its stems are hollow and ridged, green with purple blotches and stiff, white hairs. Biennial, it forms a rosette of lobed leaves in its first year before sending up a flower spike bearing a large umbel of white flowers in its second year, and then setting seed and dying.
Giant hogweed looks similar to, and therefore may be confused with, the following plants:
Common hogweed (Heracleum sphondylium) – growing to just 2m, this British native hogweed is much smaller than giant hogweed, and has more rounded, and less jagged leaves. It’s not toxic and is a fantastic wildlife plant.
Cow parsley (Anthriscus sylvestris) – cow parsley has a more delicate appearance than giant hogweed, with feathery, almost lacy leaves, and smaller flower heads. It grows to a maximum height of 170cm.
Is giant hogweed dangerous?
Giant hogweed has phototoxic sap, which makes skin extremely sensitive to sunlight. If giant hogweed sap gets onto your skin and the skin is then exposed to sunlight, it can burn and blister. These blisters may last for several weeks and may even recur over months or even years.
If you touch giant hogweed and do get its sap on your skin, wash your skin immediately, taking care to ensure all sap has been rinsed off. Then keep the area covered for the next few days, to avoid it being exposed to sunlight. You may want to seek medical advice.
It’s important to also keep dogs away from giant hogweed, as they are also affected by its sap.
How to get rid of giant hogweed
Giant hogweed is biennial, so forms a basal clump of leaves in its first year and then flowers and sets seed in its second year. Removing it by hand requires full protective clothing, to stop any sap from being in contact with the skin. To remove it successfully, dig out the whole plant, ideally in its first year, before it flowers and sets seed. Bear in mind that seeds can survive for several years, so you may need to remove seedlings in future years.
Weedkillers may also be used to get rid of giant hogweed but bear in mind that they could harm other plants growing nearby and could leach into the soil and waterways. What’s more, you will still need to wear full protective clothing, dispose of the plants and then continue making applications as any seeds germinate.
How to dispose of giant hogweed
Giant hogweed can be disposed of in landfill sites but only those with licences to deal with ‘controlled waste’. Instead, burn or compost giant hogweed in your garden. If composting, keep the seeds to one side and burn them anyway, as they may survive the composting process.