Purple loosestrife is a native of Europe and Asia, introduced to North America, where it is now
an invasive species.
I try to stay neutral about terms like "invasive" and even "weed." Not that I don't curse when I'm weeding the landscaping or the vegetable garden. But many of the plants that have acquired these unsavory descriptions were originally introduced intentionally, for their beauty or cultivation potential, sometimes even to control other "invasives." Purple loosestrife is such a plant. "So wait a minute," loosestrife might think, if it could think, "I'm just doing my job. You've thrown me into some new place, and I'm growing, and doing it rather well if I do say so myself. But I'm getting blamed for it. What's that about?"
But I guess I have my limits.
Although oriental bittersweet is probably at the top of my annoying plant list, purple loosestrife is number two. It can turn a thriving beaver pond ecosystem into a barren purple monoculture in a space of about two years—I'm watching it happen to my beaver pond right now.
Identification: Purple loosestrife is a tall, narrow plant, 24-36" (60-91 cm) in height, with a beautiful conical purple flowerhead visible during much of the summer, disappearing at the first frost. It is native to Eurasia; throughout Great Britain, and across central and southern Europe to central Russia, Japan, Manchuria China, southeast Asia and northern India. In the U.S., it is an alien, introduced in the 1800s, now found throughout most of the US and Canada. It rapidly takes over both freshwater and brackish wet areas, even regions that are only seasonally damp, out-competing nearly everything in its path.