Have heard this pernicious and evasive plant has been found in Willesden. It is said to be the biggest problem facing the construction industry today (though I suspect government policies and recession must be close runners). Japanese Knotweed is a tall perennial plant. The shoots start to emerge in late March/early April, are asparagus-like and are a red-green colour. As it grows through the summer the red colour turns into red speckles on an otherwise green stem. The weed can grow a metre a month and can cause heave below concrete and tarmac, coming up through the resulting cracks and damaging buildings and roads. Studies have shown that a 1cm section of rhizome can produce a new plant in 10 days. Rhizome segments can remain dormant in soil for twenty years before producing new plants. It has to be removed by specialists.