What Are Invasive Plants?: Defining Invasive

Calling something a weed is rather ambiguous. One person’s weed is another’s vegetable, medicine, or ornamental. A weed is something that gets smoked, or pulled, admired, or cursed. They are the plants unintentionally spread by an array of creatures or avoided by grazers. When we call a plant an “invasive”, we need a stricter definition than “weed”. Better precision in a definition may not prevent an invasive plant from being scorned or adorned by two different people, but some boundaries will keep the word “invasive” from being diluted and losing its ability to relay a true threat. All invasive plants may be a weed, but not every weed is an invasive plant.

Invasive plants are plants that have a negative impact on ecosystems. In most cases they come from somewhere else (generally another country or continent); they are termed a non-native or an exotic. But not all exotics are invasive! The exotic must first be able to reproduce, then it must be able to spread, and I would add that it must be able to spread quick and aggressively, thus having a harmful impact on native vegetation. Even with a stricter definition for invasive plants there are plenty of them. Invasive plants are outcompeting native plants for space, sunlight, and other resources; they are putting native plants and ecosystems under stress, inhibiting ecosystem functions. Invasive cattails and reed canarygrass don’t have to completely eliminate native swamp milkweed to be detrimental; even by reducing the milkweed population in half, the space for Monarch Butterflies  to lay their eggs will have been drastically reduced (Monarch caterpillars only eat milkweed plants).

milkweed (Asclepias species) and Monarch

People are impacted by impaired ecosystems too! Exotic Phragmities, a tall wetland grass, invades coastal areas in the Great Lakes and blocks scenic views or disrupts fish spawning grounds, which negatively affects the fishing industry. In New England states, Japanese barberry, an invasive shrub, has taken over forest floors potentially preventing recruitment of the next generation of forest trees for timber or maple syrup. We spend millions of dollars combating invasive plants in an effort to preserve ecosystems.

Invasive plants came to our lands accidently on ships or in shipping containers; they came from mistakes, thinking they would make wildlife friendly windbreaks ( for instance autumn olive), or they came intentionally for ornamental or agricultural purposes. Remember, not every plant brought over from another country, accidentally or intentionally, should be labeled as an invasive. Lists of plants considered to be invasive are made and kept by various non-governmental and governmental organizations (some examples below).

I attempted to make the term “invasive” more objective above, now I want to propose at this stage that an “invasive” be more subjective. Just because mullein is an exotic on your state’s invasive species list doesn’t mean that you should also consider it as an invasive on your property (not everyone will agree with me!). In another blog post I will explain how to practically approach invasive plant management on your property and suggest a protocol to follow, but in this more philosophical expose, I want to voice that it’s ok to be very specific (or to an extent, subjective) in identifying the invasive species at the small spatial scale of your individual property.

I must note that some species may not spread aggressively at one site while at a nearby site they do or they may live at an acceptable level for several years before emerging as a nuisance. Also, there are some species where subjectivity is not as neighborly and I highly recommend taking action for the greater good of ecosystems across your region. These species are often very aggressive invaders only known from a few places and/or in small populations and there is a chance of eradicating them from your region. Japanese knotweed and black swallowwort (photo below: Leslie J. Mehrhoff, University of Connecticut), in the Midwest, are two such species of highest concern that, for everyone’s good, you should keep off your land. Just how to do that will be the subject of other posts! 

Black swallowwort, Vincetoxicum nigrum (photo: Leslie J. Mehrhoff, University of Connecticut)

Some lists of invasive plants or “weeds”: USFS, invasive.org, and Midwest.