What does it look like?
It's a member of the Umbelliferae group of plants, the group having cauliflower-like flower heads where the stem forms many branches at the top on which are supported the flowers. Other members are carrots and parsley. The Giant Hogweed, though, is a HUGE thing!!! With very thick stems, almost like a small tree, great pointy leaves, huge flower heads, and 10-15ft tall, it looks like something from outer space!!! And its effects on you are as you'd expect from an alien....
How did they arrive in Britain?
The Giant Hogweed is not a native British plant, but was introduced from the Caucasus Mountains region of southern "what used to be the USSR". The story goes, that a Victorian explorer found it growing there, and thought "what a spectacular plant" and took the seeds back to Kew Gardens, London, to plant. Later, various "fashionable Victorian country gentlemen" ( quoting Peter Gabriel) also fancied the plant for their country gardens, and obtained seeds. However, they were unaware of its ability to spread like mad, and it escaped from the gardens, forming colonies close by, particularly on waste ground and damp places.... The numbers have been steadily increasing since, but it's still generally in well scattered locations.
Flowering and spreading
The Giant Hogweed flowers in late June, early July in the UK. It does not flower the first summer after germination, but the second or third: in the first year the plant will just put up leaves. IT CAN SPREAD LIKE MAD if in a suitable location !!! If someone put one in their garden, and let it seed, and the environment was right, and a strong wind blew while seeding, they could appear all round the neighbourhood!!! Certainly your garden is likely to be filled with them.
The poison
As you may be aware, the Giant Hogweed has a nasty poisonous bite! Touch one on a sunny day, and you're likely to have a nasty blister. Do anything like cut one down and get the sap over you, then you should be prepared for a nasty burn which can blacken the skin afterwards. TREAT THEM WITH RESPECT, should you see one!!!
The poison is not a standard corrosive agent, but a photosensitiser: a mixture of chemicals (furanocoumarins), which react with DNA in skin cells, cross linking the strands and making them susceptible to damage by sunlight... hence the nasty effects.
source: Nick Whitelegg's Giant Hogweed Page