This beautiful plant turns very, very nasty if you come into contact with its sap. Normally, it is easy to avoid this, because the plant is huge and visible, but if it has been cut back you might not notice it. Last summer I had the misfortune to kneel on some cut hogweed while photographing butterflies and I got the sap all over the exposed parts of my legs and arms (I didn't notice until later). It reacts with the sun and causes blisters like the ones below on my hand - I had similar blisters everywhere the plants had touched (that was a 'shorts' day).
It looks like a massive cow parsley:
![Image](http://www.guypadfield.com/images2006/hogweed061.jpg)
This one was nearly twice as tall as me.
![Image](http://www.guypadfield.com/images2006/hogweed062.jpg)
![Image](http://www.guypadfield.com/images2007/hogweed071.jpg)
Guy