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."
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 and insects tend to spread invasive plants by dropping wayward seeds that take root. Helping invasive plants spread even more are the extensive underground networks of root-like plant parts, rhizomes. The 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 invasives a hot topic in some gardening circles.
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.
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 the 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
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.
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.
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.
"The Problem With Invasive Plants." Environmental Services, The City of Portland, Oregon.