Giant or Japanese knotweed is an upright shrub-like perennial that can grow up to 10 feet tall. It is native to Asia. This knotweed is very invasive, and can form large dense colonies. The alternate leaves on the stems are up to 6 inches long and 3-4 inches wide, triangular or heart shaped.
Giant or Japanese knotweed has greenish-white flowers that bloom in late summer. Knotweed can spread from the seeds, or from rhizomes, runners, roots and stems. Stem segments that are cut will re-grow if they contain viable nodal buds. Once established, knotweed spreads via large rhizomes, which can reach up to 30 feet in length.
Small knotweed plants can be dug up and disposed of. Cutting the plant as close to the ground as possible reduces the viability of the rhizomes, but must be done several times during the growing season to be effective. The EC-130 Guide for Weed Management in Nebraska recommends two different herbicides for the control of knotweed, using a cut stump treatment, or a foliar treatment. Research shows that it may take three to five years of herbicide application before the knotweed is effectively controlled.