Ask Mr. Smarty Plants

The most invasive/noxious plant in the U.S. is the one in your yard, or in a nearby nature preserve, or waters where you like to fish and boat. They are almost invariably introduced,  non-native plants that were brought in from Asia or Europe with good intentions, for groundcover or lumber or just as an ornamental. Sometimes the invasive arrives in your area as a stowaway seed in a shipment of seeds, perhaps grass seeds, from a foreign country. If the seeds or imported plants find favorable climate but no animals or insects that will graze those plants they will prosper. Left in their natural habitat, over the millennia, the plants and the grazers or the humans searching for fuel will reach some sort of natural balance. Invasiveness or noxiousness is in the eye of the beholder. We will try to find some references to noxious and/or invasive plants in the New York area, and perhaps some others in other parts of the U.S. that just haven't managed to spread to your area yet, or for which your climate is not favorable.

An article from the United States National Arboretum details the following plants as being outstanding invasives: Lythrum salicaria (Purple Loosestrife), native to Europe, Asia, and Australia, Ailanthus altissima (Tree of Heaven), native to China and Taiwan, and Hedera helix (English Ivy), native to Asia and Europe, as being outstanding leaders in the North American invasives field. Each plant link will take you to additional information on that plant.

To focus in a little more on invasive plants in your area, here is an article from the Cornell University Ecology and Management of Invasive Plants Program.