Here it is turning red in the fall; note the brown seed scapes, which will last through the winter. Sources agree that this member of the loosestrife family was imported into northeastern USA and Canada in the 1800's from Eurasia for ornamental and medical purposes.

While loosestrife apparently does have medical value, reportedly including for relief of symptoms of Crohn's disease, this lover of wet and sun is a hazard to wetlands. Once imported, loosestrife ran riot in short order, crowding out many other species, including rare ones.

According to the US Park Department (which measures such things), one purple loosestrife plant can produce 2 or 3 million seeds a year, that are then spread by water and wind. Once established, loosestrife also spreads quickly by underground roots. Ecologists object to it because it destroys bio-diversity and, they say, is much less valuable to wildlife than the plants it replaces.

Loosestrife is so widespread, I can't imagine how it could ever be eradicated. It can be controlled by repeatedly cutting it to the ground, which weakens the existing plant (a perennial) and prevents it from going to seed. It can also be dug up which is OK for one or two isolated plants, but the digging disturbs the soil which encourages a whole new array of noxious weeds. There are several kinds of beetles which has been imported by the USDA to munch up the stuff.

At least it's pretty, here are the summer flowers:

Further reading and more pictures:
National Park Service: http://www.nps.gov/plants/alien/pubs/midatlantic/lysa.htm
http://www.botanical-online.com/medicinalssalicariaangles.htm