A general-purpose definition for the commonly used term "native plant" is offered here for consideration for use in floristics and biogeography, conservation and habitat management, horticulture, environmental education, legal and regulatory documents, and other purposes.  This presentation draws not only on my decades of personal floristic and biogeographical research experience (beginning in the 1950's), but also on definitions and practices of numerous other researchers, organizations, land-management agencies, and other parties, as well as numerous personal and internet discussions, and consultation of published and shared unpublished works, all far too many to attempt to list here.  However, it is no mere coincidence that the definition and discussion offered here correspond closely to longstanding usage of the U.S. National Park Service, The Nature Conservancy, and Dr. John T. Kartesz.

Terminology.  The term "native plant" has become quite firmly established in the realms of botany, horticulture, conservation, and law, as well as in general-public environmental education and natural-diversity awareness.  Furthermore, dozens of state-based U.S. organizations have named themselves as a "native plant society," or otherwise note their interest in and concern for "native plants" in their mission statements.

However, as has been noted by others from time to time, both words in the "native plant" term have their problems.  "Native" in some other contexts implies birth or origin in a specified place or area, with the frequently used alternative term "indigenous" also having a similar alternative meaning.  Also, the word "plant" is quite widely and commonly used to refer to an individual organism, probably moreso than its use here to refer to a species or other entire taxon.

As has occasionally been noted elsewhere, if these original literal views of "native" and "plant" are followed, then a "native plant" would be any individual plant, of any species whatsoever, that germinated or otherwise established itself in exactly the spot where now found, .  For example, if these views are followed, the now-mature Asiatic-origin mimosa tree (Albizia julibrissin)  that sprouted about 20 years ago from dispersed seed in my back yard on Capitol Hill in Washington, D.C., would be considered native to my back yard, and hence native to the District of Columbia, to the United States, and to North America, much as I am personally considered a native to Dayton, Ohio (where I was born), and hence native to Ohio, to the U.S.A. and to North America.  While leading to some interesting thoughts and consequences, this strange concept of botanical nativeness is useless in most scientific, conservation, legal, and other conventional contexts, and is not pursued further here.

One might think that the term "native plant species" would be an improvement on the term "native plant" for present purposes.  This replacement term works well if only species-level taxa are being commonly considered in a particular context, for example in some instances in environmental education or when labelling nursery material, when rare exceptions can be addressed if encountered.  However, more generally, especially in scientific, conservation, and legal contexts, the distinction between native and non-native is usefully applied at any taxonomic level, not just the species, for example in the often important separate consideration of native or non-native subspecies or varieties of a particular species.

Taking a step further, one might then consider "native plant taxon" to be the more appropriate general term, but  that involves a quite specialized technical word virtually unknown to the general public, including many native plant enthusiasts.  Furthermore, the word "taxon" has the additional complication of following Greek rather than English conventions in generating its plural ("taxa"), a phenomenon with few if any parallels in familiar everyday American English.  I therefore consider it totally unrealistic to expect or encourage widespread substitution of the technically better phrase "native plant taxon" for the solidly established but admittedly doubly questionable term "native plant"  -- is anyone ready for an "Oregon Native Plant Taxa Society"?  "Native plants" it will continue to be!

The related term "naturalized" also needs brief mention.  Naturalized plants are non-native plants that have become thoroughly established within one or more non-managed ("wild") habitats or ecosystems in one or more places within an a region of interest.  While descendants of a nation's naturalized citizens may in turn be natives to that nation, under the definition of "native plant" offered here, the same is not true of a nation's (or other area's) naturalized plants (despite some published claims to the contrary), since descendants of non-native plants still have direct or indirect human intervention somewhere in their history.  Mere passage of time does not turn a non-native into a native in the botanical world.

Geographical scoping.  In all cases when considering whether a particular plant taxon is native or non-native, or in any other discussion of "native plants," the geographical region being considered must always be explicitly or implicitly stated.  For example, some U.S. government policies consider "native plants" to be those plant taxa considered native in at least one place anywhere in the United States.  Under this nationwide geographical scoping, the yellowwood (Cladrastis kentukea)  that I planted a decade ago in my front yard would be a "native plant," since it is native elsewhere in the United States (although not in the mid-Atlantic region), while my backyard mimosa, despite its spontaneous appearance from alleyway seed, would (appropriately, in my view) not be considered a (nationally) native plant. When the geographical scale is tightened to the District of Columbia, then my  yellowwood becomes non-native to my local state-level jurisdiction, while such recurrent yard-invaders as black walnut (Juglans nigra)  or willow oak (Quercus phellos), which represent locally native species, are rightly called "native plants" in D.C.  If no geographical limitation is provided, then the "native plant" term could be applied to any plant from anywhere on our planet, with the traditional Aztec view of corn or maize (Zea mays)  as a "Gift from the Gods" being a possible exception.

Limitations in floristic knowledge.  Note that the tighter the geographical scoping, the more difficult it generally is to determine whether or not a particular regionally native plant species (or other plant taxon) should be considered native to the particular area of interest.  Early observations or collections of a species demonstrably from within the actual area of interest of course answer the question quickly, but for most localized places, such knowledge is thin or nonexistent.  Known current or historical presence as a native nearby (within routine dispersal range), coupled with presence of appropriate native habitat within the area of interest, is also persuasive evidence (but not proof) that the species is native within that area.  If the area of interest is located well within the presumed native range of a widespread and readily dispersed species, and appropriate habitat is furthermore naturally present within the area, then nativeness in that area can generally be assumed (but again not proven), in lack of evidence to the contrary.  Floristic information provides the baseline of geographical distributions and habitat preferences necessary to such lines of reasoning.

In the United States, the presence of nationally native trees, wildflowers, and other vascular plant species is documented with accuracy approaching 99% at the state level, providing a firm national foundation for presenting state-by-state distribution lists and maps.  Distribution data for vascular plants is dramatically less well known at the county (or equivalent) level, varying substantially from state to state, with this knowledge disappointingly weak in Georgia, Maryland, and a few other places.  On the other hand, county-level plant records are surprisingly complete for state-native vascular plant species for hundreds of particular individual counties across the nation, generally those with universities, major museums, scenic attractions, field stations, regionally unusual topography, substrates, or habitats, or otherwise now or at some time of high personal interest to one or more field botanists. 

The state and the county have long been the geographical units customarily recorded for the great proportion of botanical specimens and locality reports.  Discovery of a new state record for a species usually leads to publication.  Newly documented county records for a species are also frequently published, or nowadays submitted instead to editors of state-based or national county-level distribution atlases or databases for inclusion in these works with appropriate attribution or acknowledgement.

On the other hand, for a variety of reasons, physiographic provinces, ecological regions, major watersheds, or similar tessellations of the landscape are rarely recorded in consistent form (if at all) with individual specimens or other documented observations.  Distribution summaries by such tessellations are accordingly rarely produced directly from site-based primary documentation.  However, such distribution presentations can now be readily approximated (on a yes/maybe/no scale) from distribution data for the pertinent fully and partially included counties.  Further resolution of 'maybe' cases can then be made through herbarium and literature review as well as focused field research.  On the other hand, floristic documentation at localized, within-county levels requires the luxury of intensive field study, generally involving samplings in various seasons for more than a single year.

Not all regionally native plants are locally native everywhere in their regions.  Nativeness at scales smaller than states is rarely addressed systematically, although presumed native ranges have been worked out for a fair number of continentally native U.S. species that have been introduced into areas beyond their original distributions, or that have become established in places within their natural dispersal ranges that lacked appropriate habitats or substrates before modern human activity provided niches for them (such as limestone ferns on old mortar, aquatic plants in farm ponds, or saline-habitat plants along heavily salted highways).

Challenges in determining native ranges of certain species.  Many mysteries remain regarding the locally native status of some of our most familiar and otherwise well-known plants.  As one dramatic example, the black locust (Robinia pseudoacacia),  a tree native to the Appalachians, was widely planted in the Atlantic states, the Midwest, and various other areas where it soon became established and often naturalized.

In the mid-Atlantic region around Washington, D.C., there is no question that black locust is native in the mountainous region from the Blue Ridge westward.  Within the Piedmont and Coastal Plain areas near Washington, this abundant species is generally considered non-native, although obviously often at least well naturalized.  Black locusts along and near the Potomac River, however, pose a further question of nativeness, still unresolved.  Most of the Potomac's headwater streams arise deep in Appalachia, in the region where black locust is unquestionably native.  The Potomac watershed regularly experiences tropical storms or hurricanes as well as occasional quickly melting deep snowfalls, with the river readily flooding grandly following such broad-scale meteorological events.

Given the quantity of plant material, including whole trees, dispersed downriver by these major Potomac floods, it would be expected that seeds, seed pods, and viable roots of the black locust would be frequently flood-dispersed and deposited in numerous places on the river's floodplain or riparian shore as far downriver as the Coastal Plain well southeast of Washington.  Trees arising from these dispersed propagules would be expected to flower and fruit and spread locally in this riparian habitat, and perhaps eventually disperse naturally further from the river corridor.  My current personal view is that many (but not necessarily all) of the black locusts near the Potomac are native, at least as far downstream as Charles Co., Maryland, and King George Co., Virginia.  What if any upland mid-Atlantic black locusts away from the Potomac are also native remains a mystery.

Note that the black locust question posed here is not where the species currently occurs -- that is quickly determined as needed -- but in which places the species' presence is not due solely to direct or indirect human activity.   Since black locust trees are conspicuous and distinctive in flower, historical accounts may prove useful here (as has been the case in documenting the presettlement distribution of black locust near the Ohio River), making this a true question of "natural history" and not merely natural science.  On the other hand, high-tech modern science (such as biochemical comparisons) might instead be applied to try to resolve the affinities of the black locusts in various places in the Potomac watershed.

Toward an improved definition.  Given the lively discussion stimulated by most attempts to define "native plants" or related terms, I hope and expect that this presentation will before too long need significant revision, perhaps by November.  Questions, suggested revisions or corrections, and any other comments are all quite welcome.  In particular, the current rather detailed consideration of various kinds of direct and indirect human intervention is clearly quite preliminary, and deserves substantial further attention before the next version can be considered ready.

Definition:

For purposes of this definition:

1. A plant is any living organism that is:

2. A geographical region of interest is an unambiguous, bounded, contiguous or non-contiguous geographical area specified by a legal or other commonly recognized name and/or by a precise description, for example: 

• West Virginia

• Montgomery and Greene Counties, Ohio

• The schoolyard of Horace Mann Elementary School, Dayton, Ohio

• Coastal Plain of Alabama and Mississippi

• Yellowstone National Park

• Chesapeake Bay Watershed

• U.S. Caribbean Territories and Possessions

• Ridge and Valley Region of Virginia and West Virginia as bounded by the center lines of highways U.S. 50, W.Va. 28, U.S. 250, and U.S. 220

Note that a map, or a comparable representation within a geographical information system, can be helpful in visualizing the extent and boundaries of a geographical area, but does not substitute for a written name or description.

3. A taxon (plural, taxa) is a taxonomic group of any rank (such as a kingdom, phylum, class, order, family, genus, species, subspecies, or botanical variety) as recognized in a particular scientific classification.  While most taxa have Latin-form scientific names, unnamed taxa are occasionally recognized, for example some interspecific hybrids that are identified by formulas rather than by names. or newly discovered species not yet formally named.

Note that hybrids between two or more taxa may themselves be considered taxa.

Note also that a named plant cultivar is not itself a taxon, but is instead a member of a botanical species or other botanical taxon.

4. A plant taxon is considered present within a geographical region at a specified time if it is known to occur in at least one place there then as a rooted or otherwise established living individual (and not, for example, occurring within the region solely as dispersed seeds, spores, pollen, or other non-established propagules, or as dispersed nonviable or dead material such as flood deposits, tornado debris, or beach drift).

5. Historical presence of a plant taxon within a geographical region is here taken to include presence in at least one place there at any time since the advent of modern traditional biological classifications (hence approximately the year 1500 A.D. onward), but  here excludes presence within the region only in earlier times, as for example indicated by historical, archaeological, or paleontological evidence.

For example, the dawn redwood [Metasequoia glyptostroboides, and/or related species] occurred widely in the United States and Canada many millions of years ago, as shown by numerous fossils, but that fact does not make the genus Metasequoia  native to North America for present purposes;  all modern North American Metasequoia  plants are instead direct introductions from its modern native range in China, or descendants of such introduced plants, even when established here outside cultivation.

6. Human intervention (direct or indirect) includes:

Note that widespread, diffuse environmental changes due or apparently due to cumulative effects of diverse human activities are generally ignored in determining the local nativeness of various plant taxa, including for example any that may be due to such factors as:

• regional or widespread air, water, or soil pollution or contamination

• regional or large-scale hydrological changes due to dams, channelizations, diversions, deliberate water releases, drainage, groundwater withdrawal, or similar causes

• changes in frequency or intensity of floods or other hydrological events due to regional land-use change such as agricultural clearing or substantial introduction of impervious surfaces

• long-term changes in upper-atmosphere gas proportions

Note also that a plant taxon resulting from artificial or naturally occurring hybridization between a plant taxon native to a region of interest, and another plant taxon not native there, must be considered non-native because all of its individuals include genes having direct or indirect human intervention in their ancestries.  Furthermore, all descendants of such hybrids, including any backcrosses, polyploids, or other derived taxa, are correspondingly also non-native to the region of interest.

On the other hand, since the taxon and not the individual or population is the entity of assessment under this definition, the determination of nativeness of a taxon to a specified geographical region applies to that taxon in its entirety.  For a particular taxon that is considered native to the region of interest, plants of the same taxon from elsewhere that are brought into the region of interest, or otherwise become present there, are also considered native in that region (at the level of the specified taxon, even if not as individuals populations, or genotypes), and their presence there does not affect the native status of other members of that species there, even if some or all of these plants interbreed or otherwise co-mingle.  However the nature and characteristics of individual plants or propagating stock of a native species that originate from outside the region of interest may nevertheless be a legitimate consideration when planning restorations, reintroductions, or other habitat-management needs, or when obtaining material of a native species for landscaping or educational displays.

Any questions, comments, or suggested revisions or clarifications may be directed to the author.

Permission is hereby granted to reproduce or further distribute this work, provided that it is presented in its entirety (without any alterations, deletions, revisions, or significant changes to flow or formatting), including this concluding notice regarding copyright, permissions, and suggested citation.

Suggested citation: 

Morse, Larry E.  2006.  Definition of a "Native Plant" -- 24 August 2006 Version. L.E.M. Natural Diversity, Washington, D.C.