Japanese knotweed is a large herbaceous perennial that spreads aggressively by rhizomes, forming large clones where unimpeded. It has been widely planted as an ornamental and has become an invasive species in some areas, mostly on sunny sites. The reference to "bamboo" in some common names arises from the large, hollow stems shown above. The stems are green when young (with purple or reddish speckles) and orange-brown when mature. Leaves may be up to 6 inches long and the plants may grow to about 10 feet tall. In Wisconsin it does not often reproduce by seeds, but once established it is extremely long lived and difficult to eradicate. Notice in the photo of the "winter population" above that plants in the foreground have been repeatedly mowed off by a lawn mower. The reddish-brown plants farther back have invaded the adjacent forest.