Lythrum salicaria

Purple Loosestrife is a wetland plant that was introduced to the east coast of North America from Europe and Asia in the 1800's. It has spread into lakes, rivers and waterways and was distributed as an ornamental through nurseries. This exotic species chokes out native vegetation, such as cattails and other wetland plants.

An adult plant is between 2 and 7 feet tall and can disperse over a million seeds per year.
The plant is able to resprout from roots and broken stems. Purple Loosestrife plants form dense, often impenetrable stands, which are not suitable for wildlife cover, food or nesting sites for many wetland native species. The lack of effective predators in North America has allowed Purple Loosestrife to proliferate. Because the flower is attractive, the plant was sold by nurseries and planted in many home gardens. It is now found in ditches, along roadsides, wetland areas, and rivers and streams throughout our area.

Control measures include hand-pulling, herbicides, and biological controls, but because of the enormous number of seeds in the soil, eradicating an established stand is very difficult.

more info: http://www.aphis.usda.gov/lpa/pubs/invasive/3purploo.html