It is essential that we improve our ability to predict which nonnative species will become invasive in order to prevent their introduction and spread. Past attempts to foresee invasions have met with limited success, but increased computing power, increased availability of information about exotic species, and comprehensive evaluations of invasion potential are improving our ability to predict which species are likely to invade most successfully. We used data from Colorado and other states to develop an effective means of predicting the spread of invasive plant species among states. Qualitative criteria were used to develop a numerical threat index, which rates potential invaders based on distribution and abundance with respect to climate, biological characteristics, and preferred habitats of the species. Out of a compiled list of 388 species, we identified six invasive nonnative plants that are highly likely to invade Colorado, 10 with medium invasive potential and five with low potential. Species found to be likely to invade Colorado included garlic mustard, smooth distaff thistle, and Syrian beancaper.

Nomenclature: Garlic mustard, Alliaria petiolata (Bieb.) Cavara & Grande ALAPE; smooth distaff thistle, Carthamus lanatus L. CAULA; Syrian beancaper, Zygophyllum fabago L. ZYGFA