A plant is considered introduced if it was brought here, on purpose, by humans. Many culinary and medicinal herbs were also introduced to this country by Europeans. A plant that escapes from cultivation and thrives on its own in the wild is considered naturalized. Catnip and motherwort are examples of plants which were introduced by Europeans as medicinal herbs, but which now grow everywhere. Queen Anne's Lace is actually the second-year flower of carrots, escaped from gardens and growing wild.
Wild parsnip is also a garden escapee which does very well in ditches in Minnesota.
Some of these plants are invasive, displacing the native vegetation because they compete well, and are not kept in check by balanced relationships with other organisms. If they displace the native vegetation, lack of the native plants may reduce the populations of other organisms in the community which depend on them, causing displacement or local extinction. Examples of invasive exotic species that are major problems in Minnesota include European Buckthorn, a shrub which was introduced as a hedge-forming shrub and is now displacing native vegetation in woodlands, Leafy Spurge, which displaces native pioneer species in disturbed areas and is very difficult to eliminate, Reed Canary Grass, a grass often planted for hay which invades wetlands, and Purple Loosestrife, which escaped from flower gardens and is now taking over native wetlands and riverbanks, eliminating diversity in these areas. This wonderful resource was authored by two long-time University of Minnesota botanists. It lists all the Minnesota-collected plants in the University of Minnesota herbarium, and provides distribution maps for each species.