December’s Weed of the Month is Japanese knotweed (Polygonum cuspidatum), a shrub-like, semi-woody perennial plant that has become invasive in Minnesota. It forms tall, dense thickets and can degrade floodplains, riparian areas, and shorelines.
Japanese knotweed has hollow, bamboo-like stems reaching heights of six to nine feet. It spreads by horizontal stems that can grow 50 feet long. The leaves are large and oval shaped with pointed tips. The plants produce branched clusters of white flowers in late August and September but produces minimal seed.
Japanese knotweed grows quickly and aggressively. It can be found along roadsides, stream banks, and thrives in a wide range of soil types. It reproduces from very small stem or root fragments and spreads by water and human activities such as construction. Japanese knotweed displaces native vegetation and can alter waterways by facilitating erosion and flooding. It can also damage pavement and road edges by growing through pavement cracks.