Tuesday, July 03, 2012

Giant Hogweed (Heracleum mantegazzianum)

Giant Hogweed (Heracleum mantegazzianum) found growing in the middle of Frome, in a small close surrounded by houses. This amazing plant, native to the Caucasus, was introduced to the UK in the nineteenth century and is evidently here to stay. (From the UK it has spread to much of W. Europe and N. America)

The photo below gives a better view of the plant, but without such an obvious scale comparison.

Contact with the plant can lead to a nasty rash, or rather burn, with scarring, hyperpigmentation and long-term hypersensitivity to sunlight. This happens if the plant's furanocoumarins get under your skin. They absorb photons and react with them, so every time you expose the affected area to sunlight, then back comes the inflammation.  (Similar to parsnips, both wild and cultivated, but these usually present more of a risk to farmers, growers and walkers, and less of a risk to children in town.)

A younger plant nearby. The plants are monocarpal, that is, they live for 5-7 years but only flower in their final year.

This one must have been damaged earlier in the year, and consequently is only the same height as a normal hogweed. It did give me a chance to get closer to the flowers.

You can still tell it isn't normal hogweed, mainly because the leaves look so different. Hogweed (H. sphondylium) is hispid, i.e. harshly bristly-hairy, on nearly all surfaces.Giant Hogweed is, at most, softly pubescent (I told you not to touch it!!), and the upper surface of the rather lax-looking  leaves is pretty much hairless.

Compound umbel, above.

Below is one of the individual umbellules. 

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At 3:35 pm, Blogger Vincent said...

I had a close encounter with a hogweed this time last year as written up at the time.

I'm wondering why this plant has evolved such deadly weaponry and who its intended enemies were as it became a species. (How anything becomes a species at all seems to be a very odd mystery, but that's too big a question to ask you I suppose.)

At 10:25 pm, Blogger Michael Peverett said...

That's a very nice post. Pessoa and you in symbiosis.

I suppose a lot of lowland plants seek to defend themselves against grazing. That would be particularly important when the flowers take five years to produce. But I realize that this isn't a complete explanation, because it begs the question why not all such plants have the same properties.


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