The correct answer is - ornamental garden.
The Japanese knotweed is a plant that was introduced in Europe, initially in the United Kingdom, as an ornamental garden plant. It is a nice looking plant that compliments nicely the aesthetics of a nicely ornamented garden, and on top it it has an advantage that it is always green. This plant is able to sustain high temperatures, but it is especially good in sustaining very low extreme temperatures.
The Japanese knotweed though created a very big problem too. The plant creates a very large, strong, incredibly resilient network of roots. Once planted, it is almost impossible to get rid of it. Even special services for destroying the plant have been formed in order for people to be able to get rid of it.