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Pontifications on Poison

Being some ramblings on events associated with poisonous plants.

Friday 26th August 2011

The journalist I wrote about on 16th August has been in touch again but not so much about the piece she hopes the Scottish Daily Mail will run shortly as about her dealings with Edinburgh City Council who she has been in contact with about a large patch of Heracleum mantegazzianum, giant hogweed, she found near Heriot Watt University.

By the sound of it, the official she’s been dealing with tried to fob her off by saying the plant she had seen was common hogweed rather than giant hogweed. So I thought I’d look at the similarities and differences between the two plants and a surprising third.

Heracleum mantegazzianum, giant hogweed

Heracleum mantegazzianum, giant hogweed

Heracleum sphondylium, common hogweed

Heracleum sphondylium, common hogweed

I’ll start with the differences because that’s what you’re looking for when you’re out in the country. The common hogweed is a different species of the Heracleum genus, Heracleum sphondylium. As you might expect, it doesn’t grow nearly as tall only reaching around 2m at the most. The other key difference is in the shape and colour of the leaves. I don’t have the botanical vocabulary to describe them precisely so I’ll just try and give a general idea.

The giant hogweed has leaves with sharp edges and they are, usually, quite a bright colour. The Heracleum sphondylium has rather rounded edges to its leaves and the colour is more matt against the giant’s gloss. It’s, probably, a rather odd way to describe it but, I think, the common hogweed just looks a lot less aggressive than the giant.

There is one other difference that may help though it won’t help with individual plants. The Heracleum mantegazzianum has quite heavy seeds that don’t travel far and, like a lot of invasive species, it doesn’t know when to stop and there’s nothing around to make it stop. This means giant hogweed will, often, appear in large colonies. It’s only a partial help with identification because, as I saw on the A697 a couple of months ago, you do get single plants of giant hogweed. What you don’t see is large colonies of Heracleum sphondylium because the common hogweed tends to grow as separate plants.

The similarities come with the look of the flowers and the seed heads. Obviously, there is a difference in the size. Heracleum mantegazzianum seed heads can be over one foot across against the common hogweeds more typical four to six inches. Separating the two is more of a problem if someone shows you a photo and you can’t gauge the scale. To be fair to the unknown representative of Edinburgh City Council, he may have had that problem and veered towards the identification that caused him least problems.

Giant hogweed on the A697

Giant hogweed on the A697

The only similarity is often a surprise to people and can add to the confusion about giant hogweed. Common hogweed can cause burns. The plant is capable of producing the furocoumarins that cause the sensitization of the skin and people have suffered burns similar to giant hogweed after contact with common hogweed.

But, not everybody has that experience and no-one really understands why the common hogweed seems to produce only small amounts and not every plant does this. There has been research suggesting that a root fungus attacks the plant and, in response, it produces furocoumarins. This means that humans getting burns from skin contact with furocoumarins are ‘collateral damage’ in the plant’s war against the fungus.

But that doesn’t explain why the giant hogweed produces uncontrolled amounts whereas other furocoumarins producers do not. It may be that, as with colonisation, the giant hogweed, being non-native, doesn’t know how to deal with the fungus threat in a controlled way but that is only a guess and someone can get the money for further research it will stay simple speculation.

I said ‘other furocoumarins producers’ because it is not just the Hercaleum sphondylium that will, sometimes, cause burns on people who have contact with it. Plants in the Ruta genus are well-known for causing burns though it does seem that it takes very strong sunlight for the chemicals to be activated. One of the Alnwick garden’s gardeners, who had, for years, handled Ruta graveolens without problems, suffered quite serious burns after working on the plant during a period of very sunny weather.

Blisters caused by Pastinaca sativa, parsnip

But the one that causes the most surprise for people is Pastinaca sativa, the parsnip. The story of Mrs Jo Miles was reported in the UK press in 2010 and the plant page has full details and more photographs. This sort of incident is not common for individual gardeners, though market gardeners who are regularly handling large numbers of plants are known to suffer problems. Again, no-one knows if that is because the plant doesn’t always produce furocoumarins or because, typically, parsnip plants are not extensively handled during the height of the summer when the light is strong enough to cause burning on the affected skin.

I must stress that burning from contact with the furocoumarins producers, other that Heracleum mantegazzianum, giant hogweed, is very unusual and, a bit like those TV shows that deal with real crime and ends by telling the viewer not to lose any sleep, I certainly don’t want to start a panic about these plants. But, it is getting to the time of year when more people will be harvesting their garden parsnips and, I’m an optimist, we may, yet, have some sunny weather, so it’s worth just bearing in mind that gloves and long-sleeves are best for dealing with the foliage of the plants.


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