Invasive plants are alien species that show a tendency to spread out of control. Although not synonymous with "exotic plants," the "invasive" label is now typically reserved for plants that have been introduced from other regions and spread like wildfire in their new habitats. Indigenous plants that spread rapidly and overpower the competition tend to be referred to now by the experts as simply "aggressive," "thuggish" or "ill-behaved," in common parlance.
But confusion over terminology remains, because the "invasive" versus "aggressive" distinction has not yet become cemented in our language. For example, some American gardeners will refer to sumac shrubs as "invasive," because of their tendency to spread, while others will point out that they are native to North America, therefore, they are more properly termed "aggressive" in that region; by definition, where they are native, they cannot be considered invasive, technically.
It is thought that the tendency of invasive plants to spread so much may be due in part to the fact that the insects and diseases that plague them in their native lands are often absent (or exist in lower numbers) in their new homes, where the invasive plants thus enjoy a "free rein," relatively speaking.
Helping invasive plants spread in some cases are extensive underground networks of root-like plant parts called, "rhizomes." The rhizomes are so widespread that attempting eradication by digging them up is usually fruitless.
Invasive plants compete so successfully against other plants that they can crowd out their competitors, thus producing a monoculture that discourages the growth of other plant species. These exotics often specifically crowd out indigenous plants in this manner -- a fact that makes the "invasives" issue a hot topic in some circles (especially in the native plants movement).
Classic cases of invasive plants forming such a monoculture can be seen in entrenched stands of Japanese knotweed and purple loosestrife (picture), both of which have notoriously vigorous rhizomes. While Japanese knotweed and purple loosestrife have become "poster children" for invasive plants because they are so widespread, there are many other, lesser-known examples of rhizomatous, alien plants that form monocultures, such as common butterbur.
Landscapers need to act aggressively to eradicate invasive plants that invade the lawn or garden. Many would classify this activity as "weed control," but note that the terms "invasive plants" and "weeds" are not synonymous. Some noxious weeds happen to be invasive, but not all are. Nor are all invasives weedy-looking. Some are quite beautiful; I show examples in my photos of invasive plants.
One common misconception is that if a plant is considered invasive in one U.S. state, it must be an invasive plant in every state. This simply isn't true. Conditions vary wildly in a country as big as the U.S.A. An exotic plant capable of swallowing the South may be incapable of spreading very far in the North, due to the colder climate. In fact, there are a number of factors to keep in mind before prejudging a plant for invasiveness.
Sometimes, the species plant will be invasive, while a cultivar of that plant will be relatively well-behaved. For example, while spotted loosestrife is a known invasive, it's 'Alexander' cultivar, sometimes called "variegated yellow loosestrife," is far less of a problem.
My full article on invasive plants will also help you identify some of the worst offenders.
Also Known As: invasive species (there are invasive species in both the animal and vegetable kingdoms)
Examples: Some species formerly installed by highway crews to control soil erosion on the edges of roads are now considered "invasive plants."