Purple loosestrife (Lythrum salicaria) - Flowers have 6 petals, square stems with opposite leaves.
Situation in Kansas
A state quarantine was enacted in 2002 and is designed to prevent the commercial sales of purple loosestrife in Kansas. This plant is a serious wetlands invader and could cause damage to Kansas wetlands if it becomes established. Currently, wild infestations are known to exist in Doniphan, Linn, and Douglas Counties.