Japanese Knotweed is a major nectar source for bees. The honey made form this plant is very dark, and has a much different taste. However it is very invasive and is often sprayed. I have been told that they way to control Knotweed is to spray while it is flowering. I don’t know if that is true (although the guy who told me seemed certain) but that would likely cause massive damage to honey bee colonies and other pollenating insects.