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“Invasive” is a relative term, but our definition is very simple and pragmatic: Any non-native plant species that can escape into any non-cultivated growing site and expand its population by its own volition—fast or slow—and persists, is INVASIVE! Some are more aggressive than others, but any non-native plant that establishes itself is taking space that should belong to a native species.
But rather than focusing on “invasive” as a plant characteristic we should be looking at it from an ecosystem being “invaded”. Any photosynthetic sink that doesn't support the food web and habitat needs of native insects, birds and wildlife does damage. Somehow it can be perceived as something gained if a species doesn't form a massive monoculture, that a field with 20 species of non-natives is somehow not as bad as just a pure stand of buckthorn or Canada thistle. We are getting increasingly “diverse” plant communities made up of mostly non-natives but from a functional ecosystem perspective there really is little difference.
Invasive species destroy habitat and food source for native insects, birds and other wildlife and cost billions of dollars each year in damage and control, and billions more in economic losses. Combine invasives with habitat loss from development and agriculture and it is clear that invasive species are a problem with alarming consequences. The fact that many species known to be aggressively invasive are still sold as agricultural or nursery stock is especially disturbing. That is a practice we would like to see stopped, and it can't happen too soon.
Our lists of non-native offenders are below, some of which are highly invasive, and some officially designated as some level of State or Federal Noxious, Prohibited or Restricted weed. While it is true that some native species are considered noxious, mostly due to toxicity or aggressive growth patterns, our focus is on non-natives. Our categories:
Andrey Fomin has contributed a Polish translation of this page.