Thanks to a colleague at work who contacted an acquaintance at the Botanical Garden in Leiden, I finally have an answer to my question regarding the Giant Knotweed I
photographed back in 2006.
Yes, it is the giant knotweed: polygonum sachalinense or nowadays fallopia sachalinese; a plant most probably brought to the Netherlands by
Siebold (also, I think, some plants brought to Belgium (shipment arrived in Antwerp in 1830) and probably introduced in England by
Robert Fortune). It is a very invasive plant, which should be contained extensively and the plant should not “just” be planted anywhere. (my translation)
→ Question and problem solved! →
(as for a nice bit of extra info from her)
Last week, I happened to watch a cookery show on the BBC and someone had used the branches to create a dish as you would rhubarb […] She had various people taste the dish (without telling them what they were eating), and people actually enjoyed it! (my translation)