Purple loosestrife or purple lythrum is a member of the Lythraceae or loosestrife family. The plant often sends up multiple stems that can range in height from 6 to 8 feet. The stems are four to eight sided and can either be smooth or pubescent. The erect stems are tough and often appear to be woody at the base of the plant. Leaves are simple, entire, and can be opposite or whorled. Purple loosestrife flowers are arranged on a spike that is from 2 inches to 3 feet long. Individual flowers have five to seven petals that arise from a cylindrical green tube. Petals of the flower are typically purple but can range from white, to pink, or to red. Purple loosestrife seeds are light tan, angular, and 1/32 of an inch in size.

Purple loosestrife is considered a noxious weed under North Dakota state law, thus landowners are required to eradicate or control the spread of the plant.