As far as poisonous plants go, giant hogweed (Heracleum mantegazzianum) is up there as one of the worst. Living in hedgerows and masquerading as one of the other harmless umbellifers you might find in the warmer months, it is easy to mistake this invasive non-native plant for something else. But do so at your peril.
Native to Russia and Georgia as well as southwest Asia, it was originally introduced to Britain in the 19th century as an ornamental plant. Since that point it escaped and naturalised in the wild and has spread throughout the world and arrived in the United States and Canada in the early 1900s.
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Just touching the sap of this plant can cause terrible and long-standing damage. Its sap contains furocoumarin, which, if you get it on your skin, will make your skin very sensitive to sunlight (phytotoxicity). If it does land on your skin, and then your skin is exposed to sunlight, it will form very bad blisters. This phytotoxicity can then recur over months and years.
What is giant hogweed?
Giant hogweed is a large, usually biennial plant that is often found on river banks, hedgerows and fields. It looks like cow parsley, or cow parsnip, but is significantly larger, and is part of the same family as these two as well as cumin and coriander - the Apiaceae family, which was until recently known as the Umbelliferae family. These plants are still known as umbellifers, as they are characterised by their flowers in umbels - a little like umbrellas.
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How to identify giant hogweed
Giant hogweed can look a lot like other hedgerow or river bed plants you might find such as cow parsley, but very large. Full grown giant hogweed can reach between 1.5-5m and can spread to 1-2 meters. Its stems are green, often with purple blotches and white bristly hairs. Their leaves are large and look a bit like rhubarb leaves or small gunnera leaves. Flowers appear in June and July and can reach up to 60cm in diameter.
When the plant is at its full height, it is not difficult to identify, but in its early growth it is easily to confuse it with other common plants.
If you are in any doubt about whether it is giant hogweed, do not touch it and call an expert in.
Is giant hogweed poisonous?
Yes. The sap of giant hogweed causes phytophotodermatitis, extreme sensitivity to sunlight, and if you get its sap on you, you will get severe burns if the area is exposed to the sun.
This will then result in painful blisters and sometimes blackened scars, a reaction that can happen immediately or up to 48 hours after exposure. This sensitivity can recur over months and even years and you will need to cover up the area to avoid future flares.
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Brushing through patches of giant hogweed can expose yourself to plants that have already broken and therefore its sap.
Is giant hogweed poisonous to pets
Yes - pets are as susceptible to the sap as humans are. Dogs and cats are protected somewhat by their fur, but this will not stop it entirely and hairless areas are at risk too. Getting the sap in their eyes can also cause blindness.
How do you control giant hogweed
If you find giant hogweed growing in your garden and want to remove it, you should do it it in full protective clothing. However the best way of dealing with it would be to get an expert to remove it for you.
One of the best ways of stopping it spreading is by pulling up whole plants before they start flowering in spring.
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Interestingly, not all livestock have the same response to the plant as pets and humans and there has been some trialling of intensive grazing of it by sheep which seems to be effective.
What to do if you get giant hogweed sap on your skin?
Wash the area thoroughly and seek medical advice immediately. Do not expose the area to sunlight for the next few days.
Is it illegal to have giant hogweed in your garden?
Giant hogweed is on the UK's Wildlife and Countryside Act 1981, Schedule 9 list of non-native invasive species and it is illegal to plant giant hogweed or cause it to spread. In the US it is illegal to transport or import the plant or its seeds across state lines without a permit.
In the UK, if it is already growing in your garden it is not an offence but you are obligated to remove it if ordered to do so. Anything that you remove will need to be disposed of at a licensed landfill site as it is classed as 'controlled waste'. If you come across it on council-owned land you should report it, and if you find it on private land you should tell the landowner.
