Hemlock water dropwort

This tall green umbellifer, oenanthe crocata or hemlock water dropwort, grows beside virtually every stream, river, and canal in southern England. It looks a bit like cow parsley or celery, but is extremely poisonous to animals and humans - in fact I think it may be the most toxic plant in Britain.

This is a typical location, growing beside a stream, often with similar-looking plants like cow parsley and hogweed. Don't even touch it! A tiny amount will make you feel very ill indeed.

Here is a close-up of the flower.