Chicago Botanic Garden

Invasive Plant Science and Policy

Invasive Plants

PHOTO: Conium maculatum (Poison hemlock)What is an invasive plant?

Although invasive plants are almost always not native to a region, it is important to note that most nonnative species are not invasive. In addition, some native species can become invasive. The following definitions are used by the Chicago Botanic Garden:

  • Native (indigenous) — a species that was present in North America prior to European settlement or has arrived since through natural means of dispersal.
  • Nonnative (exotic, alien, introduced) — a species that was brought to North America by humans, either deliberately or accidentally.
  • Naturalized — a nonnative species, or native species from another region of the country, that has become established in disturbed areas and/or native communities.
  • Weedy — a species that readily spreads, especially in disturbed areas, but generally does not pose a threat to the integrity of native plant communities.
  • Invasive — a species, usually nonnative, that is able to establish itself within existing native plant communities and is posing a threat to the integrity of the community.

What Makes a Plant Invasive?

When plants are introduced to a new location, either intentionally or accidentally, they can spread prolifically, outcompete native species for resources, and eventually dominate the landscape. Biologists are studying the mechanisms underlying a taxon’s ability to become invasive, but for now it is still difficult to predict whether or not a species will become invasive in a new habitat. Some factors common to many invasive plants include the following:

  • Ability to escape from natural enemies
  • Rapid growth and early maturity
  • Production of many seeds
  • Ability to reproduce vegetatively
  • Seeds that are dispersed widely (such as by birds or wind) and seeds that germinate quickly (do not have long dormancies)
  • Production of seed asexually

People have introduced the vast majority of invasive species, either accidentally or deliberately. For example, kudzu was introduced to control soil erosion. Many nonnative species came to the United States with the colonists, who wanted familiar food and garden plants. Some invasive species were, or still are, popular ornamental plants used in landscaping. The rapid expansion of global trade and human mobility has led to many species arriving accidentally. Seeds can hitch rides to new locations in cargo or even stuck to the bottom of hiking boots.