Hogweed Hazards

Hogweed has recently been getting a lot of attention in the U.K. after several people were seriously burned as a result of coming into contact with its sap. As is the way of the mainstream media, there has been quite a bit of scaremongering, along with some useful information about how to avoid a similar fate. In this adventure, I hope I can help to clear up some misconceptions about hogweed, while helping to ensure that everybody stays safe.

The first key thing to note is that there are two distinct species of hogweed in the U.K., our native hogweed (Heracleum sphondylium) (below left) and giant hogweed (Heracleum mantegazzianum) (below right), introduced from Asia as an ornamental garden plant.


Please note that it is giant hogweed that has been responsible for the recent, tragic incidents. The reason that giant hogweed is so hazardous is that all parts of the plant, but particularly the sap found in the stem and leaves, contains furanocoumarins – a chemical defence measure that only becomes active in the presence of ultraviolet radiation (specifically UVA), which is abundant in daylight. This leaves the unfortunate person who gets giant hogweed sap splashed on their skin in the strange position that, if it is night-time no harmful effects will occur, but in daylight a serious reaction ensues.

For those interested in the science, UVA rays excite the furanocoumarin molecules, putting them into a high energy state where they are able to react with the DNA contained in the cells of the epidermis (the outer layer of the skin) causing cell damage and cell death. Further cell damage and destruction is induced by the high energy furanocoumarins also reacting with oxygen to produce free radicals – highly reactive ions that damage cell components. Together these processes destroy areas of the exposed epidermis, and around 24 hours after coming into contact with the sap swollen red patches and severe blisters develop. A secondary response is the stimulation of the epidermal cells that produce melanin – the skin pigment that protects the skin from sun exposure. This means that after the blistering has healed, the affected area can remain pigmented for several months and may still react with redness and blistering on subsequent sun exposure. Perhaps the most serious hazard involved with exposure to giant hogweed is the potential for the sap to get in the eyes where it can cause blindness.

With this in mind, if you do become exposed to giant hogweed sap, if possible wash it off straight away with soap and water, and keep the area covered – i.e. totally protected from any UV exposure, which will activate the furanocoumarins. Keep in mind that windows do filter some UV radiation, but they still allow UVA to pass through, so just being inside or in a car won’t be enough protection. If all this sounds worrying, bear in mind that furanocoumarins are pretty common in the plant kingdom; lemons and limes both contain them, and some people have developed blistering from squeezing limes in bright sunlight.

This brings us back to our native hogweed; by far the more commonly encountered of the two species, hogweed, can be found throughout the UK on meadows, road side verges, woodland edges, waste ground, river banks and hedgebanks. Unlike giant hogweed, hogweed is a well-known wild edible plant; its young shoots (see below) can be fried, steamed or boiled, and John Wright includes a recipe for hogweed tempura.


His description of its taste is apt, as while it is frequently likened to asparagus, the taste is nearer to parsley, and becomes sweeter and less strong the longer it is cooked for. With this in mind, Miles Irving recommends frying shoots in plenty of butter until the sugars in the leaves have caramelised and they become crispy. He is also a little more adventurous, going beyond John Wright’s cautious recommendation to only use the barely developed leaf shoots, and uses the older leaves for stock flavouring (discarding afterwards). He also uses the dry, brown, ripe seeds as a spice, ascribing them an orangey almost cardamom flavour. It is worth noting that hogweed also contains furanocoumarins, but at a much lower concentration than giant hogweed; nevertheless, gloves are recommended if collecting large quantities in bright sunlight. However, any symptoms of skin exposure would be very mild compared with giant hogweed, at most you would probably experience some reddening and increased tanning on heavily exposed areas.

So, if hogweed is edible and giant hogweed is hazardous, how do we tell them apart? One obvious clue is the name; when mature, giant hogweed (see below) can reach an enormous 5 metres in height, with leaves up to a metre long.


Native hogweed (see below) is still impressive, growing between 50cm and 2 metres tall, with exceptional specimens reaching 3 metres, while its leaves can also reach anything from 15 to 60 cm in length. It is perhaps the reason why, in the current media attention centred around the hazards of giant hogweed, I have seen several photos of our native hogweed being wrongfully labelled as the giant species; it doesn’t reach the immense sizes of giant hogweed, but nevertheless can be a very large plant.


For identification purposes, a fully grown 4 or 5 metre tall giant hogweed is unmistakable; but what about younger, smaller specimens, how can we tell them apart from native hogweed? One tip is to look for last year’s dead stems – any monster sized ones are a sign that this is a giant hogweed patch. However, stems get trampled, cut down or blown over, so again this is not a consistently reliable ID feature. Instead, I find the leaves and the stems are the giveaway. Both species have leaves comprising smaller, lobed leaflets branching from the main leaf stem. In native hogweed these leaflets are more bluntly lobed, a duller, darker green, with hairs on both sides and a rougher texture:


Though it is worth bearing in mind that native hogweed’s leaf shape can be quite variable (as shown below) and there are several subspecies:


By contrast, giant hogweed has much more sharply pointed leaflets, the leaves are lighter green and usually hairless on top and softly hairy below.


Before the leaves have fully developed they can be hard to tell apart from native hogweed, but once they have properly unfurled even young, small leaves show the characteristic sharply pointed lobes:


The stems of both species are also very different – native hogweed has hairy green or dull red stems,


whereas the stems of giant hogweed are thicker, more bristly and green with characteristic red or purple spots.


Both plants belong to the Carrot family – the Umbellifers – named after the characteristic upside-down umbrella spray of flowers that they produce (umbels). In native hogweed the umbels are 5-15cm across, comprising many dirty-white petalled flowers, while in giant hogweed the umbels can span 50cm across. This feature can make the plants look similar to other common wild umbellifers such as cow parsley and the deadly poisonous hemlock, however, these plants have very different, feathery leaf shapes. Both native and giant hogweed can be found from February to October.


So hopefully that has cleared up some confusion. It would be sad to think of people being needlessly concerned while enjoying the countryside due to misidentifying our native, edible hogweed as its hazardous counterpart. I encounter native hogweed far more commonly than giant hogweed, though the latter is spreading throughout the UK and I know several river banks where it grows. As ever, knowledge is power, and learning to tell the plants apart is a useful skill and should be especially taught to children and those working outdoors – for example, anyone using a strimmer around overgrown river banks would be particularly at risk. I recommend getting out into nature and seeing if you can find both species of hogweed, once you have seen them both a few times, telling them apart becomes pretty straightforward. If you do happen to find giant hogweed on your travels, take a photo and upload it and the location of your sighting to http://planttracker.naturelocator.org/ a website set up by the Environment Agency to help combat the spread of invasive plants.

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