Abstracted from a manuscript published in 2009. 


Nesom, G.L.  2009.   Assessment of invasiveness and ecological impact in non-native plants of Texas.  J. Bot. Res. Inst. Tex. 3(2): 971–991.  [pdf]


Non-native species in Texas ––list and documentation.  

      A list of naturalized non-native species in Texas has been developed.  Documentation for the occurrence of these species in Texas is an essential and critical part of both this process and this report –– it begins with literature and other records that have been published mostly after 1970, the date of publication of Correll and Johnston’s “Manual of the Vascular Plants of Texas,” which remains the primary floristic resource for the state.  


Invasiveness and ecological impact of Texas non-native species.

      The system outlined here for use in Texas emphasizes simplicity, allowing assessment of the large number of non-native species (all that are known to occur in the state).  Because essential elements of biology and geography are included in the assessments, the system is termed the Fundamental Invasiveness Index.  




F1   Invasive in both disturbed and natural habitats.  Negatively affecting native species or natural biodiversity by altering native vegetation and habitats or by outcompeting or hybridizing with native species; or, invasive into agricultural habitats and causing significant economic damage.  Aquatic species known to occur in 10 or more counties.  Woody, herbaceous, and aquatic species.  Examples: Arundo donax, Bothriochloa ischaemum var. songarica, Centaurea melitensis, Eichhornia crassipes, Ligustrum sinense, Lonicera japonica, Lygodium japonicum, Nandina domestica, Salvinia molesta, Sorghum halepense, Triadica (Sapium) sebifera, Ulmus pumila. 


F2   Abundant in number and widespread, commonly invasive in disturbed habitats, much less commonly in natural habitats.  


F2-Woody  Trees, shrubs, subshrubs, and woody vines.  Abundant in number and widespread, commonly invasive in disturbed habitats such as roadsides, fencerows, woods edges, and others, sometimes potentially or incipiently damaging in natural habitats (as F1).  Examples: Albizia julibrissin, Morus alba, Nicotiana glauca, Poncirus trifoliata, Pyrus calleryana, Vitex agnus-castus. 


F2-Herbaceous  Annual and perennial herbs and grasses.  Known invasive and/or clearly expanding in geographic range, primarily in lawns, roadsides, and other open, disturbed habitats, sometimes in croplands, widespread and usually abundant but often producing relatively little biomass.  Not significantly affecting native species or natural biodiversity or otherwise strongly altering native vegetation and habitats.  Examples: Avena sativa, Bellardia trixago, Coronilla varia, Cuscuta polygonorum, Daucus carota, Duchesnia indica, Erodium cicutarium, Galium aparine, Gamochaeta coarctata, Hypochaeris microcephala, Lamium amplexicaule, Lolium perenne, Medicago lupulina, Melilotus officinalis, Stachys floridana, Stellaria media, Taraxacum officinale, Torilis nodosa, Trifolium repens, Veronica arvensis, Vicia sativa.   


F2-Aquatic  Aquatic species known to occur in 1 to 9 counties. 


F3   Relatively few in number, known from relatively few localities, usually in disturbed habitats. 


F3-Woody  Trees, shrubs, subshrubs, and woody vines.  Relatively few in number, known from relatively few localities, usually in disturbed habitats, repeatedly introduced or perhaps merely long-persisting at some localities, not showing aggresively invasive tendencies, or perhaps incipiently invasive.  Examples: Ardisia crenata, Buddleja lindleyana, Cinnamomum camphora, Hibiscus syriacus, Koelreuteria elegans, Manihot esculenta, Photinia serratifolia, Pistacia chinensis, Pyracantha koidzumii, Pyrus communis, Rosmarinus officinalis. 


F3-Herbaceous  Annual and perennial herbs and grasses.  Occurrence outside of cultivation known from only one or a few populations, usually in disturbed habitats.  Apparently showing little or no increase of abundance or geographic range since the initial report.  Examples: Cichorium intybus, Gomphrena globosa, Phyllanthus fraternus, Plantago coronopus.  Or in some species, sporadically appearing from repeated introductions and not reproducing.  Examples: Citrullus lanatus, Consolida orientalis, Ipomoea batatas, Luffa aegyptiaca, Narcissus pseudonarcissus, Tagetes erecta. 


F4   Status unknown. 



For accurate evaluation of an individual species, knowledge is required of the following.  


* Nativity.  Is the species native or non-native?


* Approximate date of introduction in Texas (e.g., pre-1970, 1970s, 1980s, 1990s).  Documentation in the current system does not provide specific information for species introduced before 1970 (those included in Correll & Johnston’s Manual of the Vascular Plants of Texas), but for species recorded since that time, dates are evident, as much as possible, in a review posted and periodically updated on the internet (Nesom 2009a).  


* Current geographic distribution.  See distribution maps in Turner et al. (2003), distribution maps generated by data from Invaders of Texas (2009), and records from herbaria (primarily those in Texas). 


* Ecological/reproductive behavior in Texas and in other regions.  Based on field experience of the author and others, published literature, information from herbarium collections. 


* Basic habitat and growth form (aquatic or terrestrial, herbaceous or woody).  Based on field experience, published literature, information from herbarium collections.  


Watch list: non-native species in Texas potentially ranked as F1. 

      Many of the non-native species in Texas are known from relatively few populations.  Among these are a significant number that have been recently recorded for the state and that are known to be both invasive and ecologically destructive in other regions of the United States or other parts of the world.  These features characterize the species included on the “Watch List” –– the ones with high potential to rapidly become destructive in Texas.  A number of additional F3 species, especially the woody ones, and some of those ranked as F4 (“status unknown”) probably belong on the Watch List. 


      The Watch List account probably is the most significant part of the overview of Texas non-native native plants provided here.  The F1 species and many of the F2s are well-known invaders and already so widespread that it is unlikely that they can be eradicated or even controlled except by sustained efforts on local levels.  Research toward the possibility of biological control will be ongoing.  Further, there probably is little hope even of eradicating many of the Watch List species  –– a number of the woody species are widely cultivated, providing abundant seed sources close to natural areas, and it is unlikely that plants will be removed from the cultivated landscape.  Some measure of control might be gained by limiting further planting. 


      From among the Watch List species, a subset is indicated as a “Super Watch List” –– those species that perhaps can be removed from the Texas landscape before they become impossible to control.  These are the ones to which immediate attention should go.  All of the F2 aquatics are included because of their potential for extremely rapid dispersal and growth.  



      The authenticity of this report, especially in the F1 species and Watch List but also in basic concepts, owes to contributions from Jed Aplaca, Bill Carr, Norma Fowler, Laura Hansen, Stephan Hatch, Bruce Hoagland, Eric Keith, Barney Lipscomb, Andy McDonald, Michael McRoberts, Tom Patterson, Jackie Poole, Mike Powell, Nelson Rich, Monique Reed, David Rosen, Jason Singhurst, Bruce Sorrie, Billie Turner, Damon Waitt, and Justin Williams. 


Guy L. Nesom

10 December 2009