What Defines an Invasive Plant?

How invasive species are introduced, how they threaten entire ecosystems, and what can be done about them, are issues of great concern. While invasive plants are only a miniscule percentage of the plant species in North America, they have become a major nuisance. Billions of dollars are spent annually attempting to control them. The long-term consequences of an unwitting introduction of non-native plant species can be disastrous. This is why learning what makes a plant "invasive" and how that term differs from other plant-related classifications is paramount. Below, we break down the terminology and analyze the impact certain invasive plant species have had on their ecosystems.

Invasive and Other Plant-Related Definitions

Not all non-native species are invasive. Tulips and apple trees, both originally from Central Asia, can be found all around the habitable world, but on their own they are not destructive of the ecosystems in which they grow. Kudzu (various plants of the genus Pueraria), introduced to the American South from Japan, and purple loosestrife (Lythrum salicaria), a Eurasian native overrunning habitats in New Zealand and North America, are invasive species. Sumac shrubs (plants of the genus Rhus), while labeled “aggressive” because of their ability to easily spread, are not invasive in North America because they are natives. And while Baby's breath (Gypsophila paniculata) may be invasive on the West Coast of the United States, it is not in New England.

The National Invasive Species Information Center (NISIC) defines an invasive species as a non-native species “whose introduction does or is likely to cause economic or environmental harm or harm to human health.” “Noxious” is often used by horticulturalists as a synonym for “invasive.”

NISIC considers a native species to be any species that, “other than as a result of an introduction, historically occurred or currently occurs in that ecosystem.” In North America, "non-native species" generally refers to plants brought to the continent with the arrival of Europeans, Africans, and other non-indigenous Americans. As members of the most impactful invasive species, however, the first humans to arrive in North America also brought non-native plants with them, including gourds, corn (maize), and barley.

"Domesticates" is the name given to non-native species that have been “naturalized” and have developed symbiotic, non-harmful relationships with other flora and fauna within an ecosystem. The European honey bee (Apis mellifera), so vital to pollination, is a North American domesticate.

What Is the Impact of Invasive Plants?

Many invasive plant species are transported accidentally. Global commerce has transported plant and animal species aboard planes and ships. Seeds can attach themselves to the clothing of international travelers or be embedded in the soil of harmless non-native plants imported from other habitats. 

Other invaders brought intentionally for aesthetic, medicinal, or functional reasons may escape from gardens and landscapes and grow out of control. Among America's most noxious invaders, purple loosestrife was introduced in the early 1800s for medicinal uses. Kudzu and Japanese honeysuckle (Lonicera japonica) were planted for erosion control. The Norway maple (Acer platanoides) was planted as a shade tree as early as 1756. Japanese barberry (Berberis thunbergii) was imported to the United States as an ornamental in 1875. And English ivy (Hedera helix) was planted by early English colonists as a ground cover.

Invasive species are not harmful in their own native habitats. But in new habitats, they often lack natural controls such as herbivores or parasites. Their unchecked growth leads to a loss of biodiversity by blocking sunlight, altering the nutrient level, chemistry, and microbiology of the soil, depriving waterways of oxygen, hybridizing with native plants, transporting pathogens, and germinating earlier than seeds from competitor plants. In worse-case scenarios, invasive plants can hasten the local extinction of native species. There are, however, no documented examples of native plant extinctions exclusively attributed to plant invasions.

Only an estimated 0.1% of non-native plants become invasive, yet they can do enormous damage — for example, purple loosestrife alone has been estimated to cost $45 million annually in control costs and forage losses. Doing your part to avoid introducing invasive species to local ecosystems can be as simple as checking with your local garden center before buying any unfamiliar plants.