Invasive terminology for nursery producers



The nursery and landscape industries are often viewed as major pathways for introduction of invasive plants. Whether this recognition is deserved or not, other states and government entities are passing restrictive legislation that has impacted their nursery industries. In an effort to be proactive, the Oregon Association of Nurseries Natural Resources Committee (OAN-NRC) is developing guidelines for its members in hopes of avoiding similar legislation. One of the first hurdles identified by this committee is the lack of a consistent and accurate terminology to describe invasive plants.

Ecologists have been debating invasive terminology for 20 years. They still do not agree precisely what constitutes a native plant, much less what constitutes an invasive plant. I will not resolve the debate in this article. The purpose of this article is to give nursery professionals a refresher course on relevant terminology. This article will frame the debate surrounding terminology, offer additional reading for anyone interested, and finally make practical suggestions for definitions that could be used by the nursery industry.


How do we define the term ‘native’? This single question has been the focus of many debates in ecology journals. While opinions differ among ecologists, a consistent theme among most writings is that an accurate definition of native must articulate a geographic reference and a time reference.

To what geographic scale do we assign a plant’s native range? Wilson et al. (1991) provides a thorough discussion of this topic using examples specific to Oregon (I highly recommend this article). Wilson points out that Oregon stonecrop (Sedum oreganum) grows native along sunny boulders of the Santiam River in the western Cascade Mountains, but is never found under old growth forest tress just a few feet away. It could be said that Oregon stonecrop is native to Oregon, but considering the specificity of this plant’s geographic range and habitat, this would be far too vague. While it is commonplace and convenient to assign a plant’s native status in reference to political borders, remember that locality and habitat are also important.

There is also the question of time. How far back into history must we go to determine which plants occurred naturally and which ones were introduced by man or nature? Since plants first inhabited land over 500 million years ago, native ranges for virtually all plant species have been changing. Glaciers, wind, floods, animals, and other forces of nature were initially responsible for the spread of plants across the globe. Ecologists point out that man has increased the rate of spread by several orders of magnitude.

So if a plant's native range is changing constantly, with or without human interference, at what point in time do we draw the line and say anything occurring naturally before this time is native? Speaking solely for the British Isles, Webb (1984) suggested that native plants are those which evolved on the island or arrived there by natural means prior to the Neolithic period (7000 to 10000 years ago, a period characterized by the development of agriculture). Not surprising, some ecologists find fault with this definition (Shrader-Frechette, 2001), and either way it’s not much use for the U.S.

In reference to the Pacific Northwest, Wilson et al. (1991) recommends that any species occurring in a particular ecological habitat before Euro-American settlement is a species native to that habitat. There are flaws with this definition. It implies that prior to Euro-American settlement, North America was ecologically pristine, that indigenous peoples were not responsible for movement of plants (although we know that they were), and that introduction of plants by indigenous peoples does not constitute an introduced species. Furthermore, it implies that plant introductions by Euro-Americans were bad while those by indigenous peoples were good or natural. Despite its flaws, nursery producers operate in the real world and require a practical and unambiguous definition for ‘native’. Using the arrival of Euro-Americans to North America is a clearly defined point in history, and one with relatively clear botanical records of pre-existing plants and those introduced from Europe. It makes practical sense to use this time frame and thus the definition by Wilson. Referring only to the Willamette Valley of Oregon, Euro-American settlement began in the 1830’s.


Plants that are not native have been called many things: non-indigenous, alien, exotic, introduced, waif, and many others. The term ‘alien’ has been associated with racially insensitive attitudes towards immigration (Peretti, 1998). Immigrant workers are often described as ‘illegal aliens’ or ‘undocumented aliens’, labels that some consider offensive. Regardless of their racial connotation, terms like exotic and alien conjure negative attitudes towards a species. Most ecologists are now using the term non-indigenous species with the acronym NIS to neutrally describe plants that are not native.

Note that the terms native or non-indigenous do not imply a plant’s potential for spread, persistence, or ecological impact. Japanese maple and Japanese knotweed are both NIS, but this alone does not adequately describe these plants nor differentiate the hazardous potential each has on Oregon ecosystems. Native and NIS merely describe a plant’s place of origin. Many NIS are extremely beneficial to man and have no adverse economic, environmental, or human impact (wheat and soybeans, for example). Other terms and concepts are necessary to thoroughly describe potential harm of NIS.


There are four phases in the invasion process (adapted from Richardson et al., 2000). The first step is introduction. Many plant collectors, nurserymen, arboreta directors and gardeners scour the world looking for new plants. Introduction is often an intentional act, although without malice. Introduction can occur accidentally. Some plant species are introduced along with other non-invasive plant propagules, fruits, machinery and a multitude of other imported goods. Again, an overwhelming majority of introductions, or NIS, are harmless or even beneficial. We are concerned here with those NIS that demonstrate measurable economic and habitat harm..

The second phase of invasion is establishment, which occurs when plants survive (but not necessarily reproduce) in their new environment. The third phase is colonization and occurs when plants in the founding population reproduce and increase in number to form a self-perpetuating colony. The fourth and final phase is naturalization. Naturalization occurs when the species establishes a new self-perpetuating population, undergoes widespread dispersal and becomes incorporated in the resident flora to the point that it is often mistaken as part of the resident flora.

Richardson et al. (2000) discusses the nuances over how ecologists define ‘naturalization’. He found that most ecologists (29%) through their writings identify naturalization and invasion as the same phenomena. Another large group (25%), including Richardson, believe that naturalized plants are those ‘that reproduce and sustain populations without direct intervention by humans, often producing plentiful offspring, mainly close to parent plants’. Richardson adds that invasive plants be defined as ‘naturalized plants that produce reproductive offspring, often in very large numbers, at considerable distances from parent plants and thus have the potential to spread over a considerable area’. So the distinguishing criteria between a naturalized population and an invasive population is one of distance from the original introduction.

One criteria that is glaringly missing from Richardson’s definition is that of economic or environmental impact. Most ecologists recognize that an invasive species is one that causes or has the potential to cause harm to people or natural systems. In 1999, President Clinton, in the much publicized Executive Order 13112 regarding invasive species, states that an invasive species is an ‘alien species whose introduction does or is likely to cause economic or environmental harm or harm to human health’. Richardson argues that impact should not be part of the invasion definition, that other descriptive terms are already used to imply negative impact such as ‘weed’ or ‘pest’. He offers many examples of what he calls benign invaders whose environmental impact is below any practical detection. Mouse-ear cress (Arabidopsis thaliana) is one of his examples, a very common but arguably insignificant weed throughout the Willamette Valley of Oregon. Despite his rather convincing argument, it is of practical sense that nursery producers acknowledge that most ecologists and governing agencies believe impact is an important component of invasion.

The term invasive is often used as a taxonomic description rather than to describe an ecological phenomena. However, Colautti and MacIsaac (2004) remind us that non-indigenous species are actually non-indigenous populations of species. That is, the same species that are non-indigenous, naturalized, or invasive in one area are native somewhere else. It is inaccurate to refer to an entire plant species as ‘invasive’.

Legal status

The term noxious weed is used often but has no ecological basis. This term normally is used as a local classification for problematic plants. Many entities have noxious weed lists. There is a federal noxious weed list, as well as many municipal, county and state noxious weed lists. Some non-profit organizations also create their own lists. These lists are specific to the region they represent, they are subject to the opinions of those who create them and they are rarely similar. These weed lists, especially those created by government entities, usually carry some sort of regulatory or planning review action.

The Oregon Department of Agriculture (ODA) defines noxious weeds as ‘any plant designated by the Oregon State Weed Board that is injurious to public health, agriculture, recreation, wildlife or any public or private property’. The ODA further classifies noxious weeds as either A, B, or T weeds, depending on their perceived importance and the reasonable ability of local and state government to provide control.

Noxious weed lists are developed with political and budgetary considerations. Some mandate control of listed plants. Weeds that are truly noxious may or may not be placed on the list if the government entity cannot afford to control the plant. Some of the worst weeds are not listed because their level of infestation may be too high to make eradication or control feasible.

A plant can be labeled as noxious in one region, but not another. For example, butterfly bush (Buddleja davidii) is classified a noxious weed in Oregon, but not in Alabama and not by the federal government. I recommend nursery professionals use this term sparingly when describing plants. If you are not familiar with the status of a plant in another region, you may be falsely labeling it. And as discussed above, labeling or not labeling a plant as noxious often has little to do with the plant’s actual invasive potential. It pertains more to the specific legalities and budgetary constraints of the entity that assigned the classification.


Ecologists attempt to explain plant invasion with science and theory, although even they do not agree on terminology much less the process. Well-intentioned people inject emotion and bias and the issues become even more unclear. Members of the U.S. and Oregon nursery industries have the expertise to guide the general public in this controversial and poorly understood topic. This article provides only brief highlights of relevant terminology. For more in-depth discussion and greater understanding, I suggest further reading. I provide a list of literature used to write this article. Any Oregon nurseryman interested in these articles can contact me via email; I will send copies free of charge (via email).

Key Points

• Use the term native to describe plants that naturally occurred in a particular habitat or region prior to Euro-American settlement.

• Use the term non-indigenous species (or NIS) to describe plants that are not native. Avoid using the term ‘alien’.

• Use the term invasive to describe populations of plants that produce reproductive offspring, often in very large numbers, and distribute those offspring at considerable distances from parent plants and thus have the potential to spread over a considerable area and cause serious economic or environmental impact.

• Be careful using the term ‘noxious’. It is not an ecological description, but a label applied by government entities whose criteria vary greatly.

Further reading

  • Colautti, R.I. and H.J. MacIsaac. 2004. A neutral terminology to define ‘invasive’ species. Diversity Distrib. 10:135-141.
  • Meyers, J.H. and D.R. Bazely. 2003. Ecology and control of introduced plants. Cambridge University Press, Cambridge, UK.
  • Peretti, J.H. 1998. Nativism and nature: rethinking biological invasion. Environmental Values 7:183-192.
  • Richardson, D.M., P. Pysek, M. Bejmanek, M.G. Barbour, F.D. Panetta, and C.J. West. 2000. Naturalization and invasion of alien plants: concepts and definitions. Biodiversity Res. 6:93-107.
  • Shrader-Frechette, K. 2001. Non-indigenous species and ecological explanation. Biology and Philosophy 16:507-519.
  • Webb, D.A. 1985. What are the criteria for presuming native status? Watsonia 15:231-236.
  • Wilson, M.V., D.E. Hibbs, and E.R. Alverson. 1991. Native plants, native ecosystems, and native landscapes. Kalmiopsis 13-17.








































Many NIS, such as wheat, have been very benefical to our society without causing any noticeable ecological harm.











Butterfly bush is a non-indigenous speces that has been documented to establish invasive populations in Oregon. It is classified by the Oregon Dept. of Agriculture as a noxious weed, however, it may not receive the same classification in other states or regions.



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