By simple definition, invasive plants are alien species that show a tendency to spread out of control. The label "invasive" is generally reserved for plants that have been introduced from other regions and spread like wildfire in their new habitats. This definition distinguishes invasive plants from other plants that more accurately are called "aggressive."
Invasive vs. Aggressive Plants
The key characteristic of an invasive plant is that is not indigenous to the area in question. 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 technical definition, where they are native, they cannot be considered invasive.
What Makes Invasive Plants So Invasive?
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 (non-native plants) 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).
A Few Examples of Invasives
Classic cases of invasive plants forming such a monoculture can be seen in entrenched stands of Japanese knotweed and purple loosestrife, 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 such as burning bush 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, like the tansy, are actually useful and quite beautiful.
Invasiveness Is Not Universal
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. 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, spotted loosestrife is a known invasive, while it's 'Alexander' cultivar (sometimes called "variegated yellow loosestrife") is far less of a problem.