I agree, Japanese Knotweed.
Horrible invasive stuff because it regenerates so easily from only
tiny amounts of suitable tissue (thankfully, the seeds are sterile) - although it's edible and good (harvest young shoots) it's an offense to cause its spread so you'll need to be
very careful handling any live parts of it:
http://www.netregs.gov.uk/netregs/processes/367836/?version=1&lang=_e
I have to say I'd only touch Hemlock for tinder if I were truly desperate and couldn't find anything else. I'm sure someone will accuse me of being over cautious, or reel off a list or resons why it's perfectly safe when dead and dry but I'm not willing to make a liar out of my Grandmother or knock the lessons I learned at her knee.
There's plenty of other stuff to play with if I want to make a fire.