By simple definition, invasive plants are non-native 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." The introduction of invasive species can cause economic or environmental harm, or can even harm human health. They compete with native plants for water, light, nutrients, and space. Invasive species contribute to the decline of 42% of the U.S. endangered and threatened species. They degrade wildlife habitats and water quality, increase soil erosion, and displace native food sources for wildlife.
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."
But confusion over terminology remains because "invasive" and "aggressive" are often interchangeable words. For example, some American gardeners will refer to sumac shrubs as "invasive" because they tend to spread, while others will point out that they are native to North America.
What Makes Invasive Plants So Invasive?
It's believed that birds, insects, and even humans tend to spread invasive plants by dropping wayward seeds that take root. Some invasive plants spread through extensive underground networks of root-like plant parts called rhizomes. Some plants' rhizomes are so widespread that attempting eradication by digging them up is usually fruitless.
Invasive plants compete successfully against other plants, crowding them out and discouraging the growth of other plant species. These exotics (non-native plants) often specifically crowd out indigenous plants, which makes invasive plants a concern for gardeners who care about the environment and native plants that support wildlife.
Invasive plants spread successfully because they produce large quantities of seeds, thrive on disturbed soil, often have deep root systems that smother roots of surrounding vegetation, or may produce chemicals in leaves or roots that inhibit the growth of plants around them.
Examples of Invasive Plants
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. But invasive plants don't have to spread by rhizomes. Seeds easily spread from invasive plants, hitching a ride on clothes, shoes, pets' fur, and even cars.
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, but not all, noxious weeds happen to be invasive. In addition, not all invasives are weedy-looking. Some, like tansy, are actually useful and quite beautiful.
In addition to the species listed above, here are common North American invasive plants to watch for:
- Black locust
- Butterfly bush
- Common buckthorn
- Dame's rocket
- English ivy
- Giant hogweed
- Japanese honeysuckle
- Norway maple
- Chinese wisteria
- Bradford Pear tree
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 many 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 its 'Alexander' cultivar (sometimes called "variegated yellow loosestrife") is far less of a problem. Additionally, while butterfly bushes are considered invasive in many areas, breeders have introduced new cultivars with sterile seedheads, allowing gardeners to enjoy this pollinator magnet without fear of spreading seeds.
Know Before You Plant
Before planting your garden, find out if a plant is invasive in your particular area. Be aware when you shop garden centers, because some invasive species may be lurking in the pots of native plants. Check with your local university extension office or website for a list of plants considered invasive in your area.
It can be tough to spot an invasive plant in your garden. If you have a gut feeling that a rambling plant may be invasive, do your homework to see if it can be identified. Ask experts at your local cooperative extension office, or show photos or samples to a knowledgeable gardener.
You may realize you already have invasive plants in the area you want to cultivate. Encourage growing native plants to edge out the invasive varieties. Removing the invasive plants before replanting will take some elbow grease and a diligent, watchful eye. Pull roots of invasive plants in the spring or the fall when the soil is moist yet still crumbly to allow you to dig deeper to disturb and pull roots while leaving soil behind. Bag up invasive plants, and replant immediately with native plants.
Here's what you can do to prevent the spread of invasive varieties:
- Don't pick, gather, and bring home wildflowers that you can't identify.
- Remove weeds and seed packets from shoes and clothing after a hike and before arriving home.
- Try to keep your automobile off of weed-infested roads and trails.
- Check your pants and shoes, as well as your pets' fur, for seeds after a hike.
"The Problem With Invasive Plants." Environmental Services, The City of Portland, Oregon.