eNature.com says of Purple Loosestrife:
"absent only from South Carolina, Georgia, Florida, Louisiana, Arizona, and New Mexico",
but here it is, in the Bonnet Carre Spillway in Montz, Louisiana.
eNature.com further states:
There is no doubt that when this showy, magenta-flowered perennial is in bloom it provides a truly spectacular sight. However, this is an aggressive species that covers acres of our wetlands and tends to crowd out native aquatics valuable to waterfowl and other wildlife. Purple Loosestrife is widely considered a noxious weed and the planting of it is prohibited in many states.
Update 7/25/2011: I saw it last week in the Home Depot garden section. Is it prohibited in Louisiana?