Japanese Knotweed?
Answers:
Apparently it was brought in by Edwardian and Victorian plant hunters who thought it would make a nice addition to their country gardens as it looked quite exotic, like bamboo. In Asia it has natural pests (little bugs, bacteria etc) that keep its growth in check. Unfortunately over here, it just grows like crazy, is pretty much indestructible, is fairly resistant to even industrial strength weedkillers, and is invading fields, hedgerows, riverbeds, etc and killing off our natural plants.
Did you know it's now classed the same as 'toxic waste' if you cut it down and take it to your local tip? That's because even a tiny little 1cm piece thrown into a landfill site will start another out-of control invasion! If you cut some down and dump it by a roadside you can go to prison! How crazy is that! For a plant! Try googling 'japanese knotweed cornwall' for a good site about how serious they take it in cornwall... its mad.
Other answers:
It was brought over as an ornamental plant
It was brought over as an ornamental plant
must have been brought over by accident!!cant imagine anyone would have created such a horrid problem on purpose!
It was imported by Victorian plantsmen as an exotic garden plant.
They didnt know what a voracious weed it would become
Same thing happened with the rhododendron
2 points.....kerching!
In addition to salforddude's answer, they also introduced bindweed!!
It was brought into the country by the Victorians who thought it would make a nice ornamental plant :(
It was bought as an ornimental plant for tubs it was not supposed to be put in the garden ? As you see now mostly in Cornwall it has gone beserk it kills all other plants that get in its way the only way to get rid of it is to cut it to about 6inches high you will see it has a tube left fill the tube with insecticide and leave until it dies , this will be a boring task but is the only way to get rid .
they birds brought it in
I personally don't know why Knotweed was brought over from Japan, however if my maternal grandfather was alive then I could have the right answer as he was Japanese!