The nursery industry has introduced thousands of ornamental plants, but only a small percentage (~1%) has escaped to become invasive and cause economic or ecological damage. Thus, the removal of these known invasives should not present a major economic hardship on the industry. In addition, a pre-screening of potential introductions would be expected to accept most species as possessing low invasive potential, and identify relatively few as having a high probability of becoming invasive.
Currently, several Weed Risk Assessment (WRA) models have been developed, the most common of which was developed by Paul Pheloung and his colleagues in Australia. This model is currently used to evaluate new introductions in other parts of the world. The model consists of 49 questions which require a considerable time commitment to answer. The model has proved to be 90% accurate when comparing known invasive and non-invasive ornamental plant species and is currently used to evaluate new introductions in other parts of the world. Its limitation is that it consists of 49 questions which require a considerable time commitment to answer and it is designed to be conservative so that it is often inaccurate in correctly predicting non-invasive species. Thus, it is very accurate in determining if a species will be invasive, but often rejects non-invasive species. This will likely compromise its practical application in the horticultural industry. Alternatively, Lisbeth Seebacher in Washington has developed an abbreviated version (27 questions) of the Pheloung model which has a higher probability of being accepted for use by the industry. If the accuracy of the Seebacher model, which will henceforth be referred to as the Plant Right model, were greater in predicting non-invasiveness compared to that of the Pheloung model, it would provide a much better and less time demanding method of evaluating invasive potential of ornamental species.
Through the leadership of the California Invasive Horticultural Prevention committee (Cal-HIP) and the Plant Right campaign we conducted an independent study with three biologists (Lizabeth Seebacher in Washington, Christiana Conser at Sustainable Conservation, and Rachel Brush at UC Davis). The goal of this study was to compare the accuracy and time required to complete the evaluation of more than 170 species of invasive (81) and non-invasive (90) plants from California using both the universally accepted Pheloung WRA and the abbreviated Plant Right (Seebacher) WRA. The non-invasive plants were all ornamental species, many of which are on the UC Arboretum All-Star list and the invasive species consisted of both ornamental species which have escaped cultivation to become problematic and other non-ornamental invasive species in California. The Arboretum All-Stars list was an ideal set of species to initially test because it represents a set of species that already contain desirable characteristics that have high marketable value.
Our results show that the Pheloung model is very accurate in predicting invasiveness with all three evaluators correctly rejecting 100% of the known invasive plants. However, the evaluators varied dramatically in there accuracy levels in predicting non-invasiveness, ranging from 15 to 88% accuracy. Thus the Pheloung model appears to be too conservative in predicting invasiveness and rejects far too many species that are not considered invasive. Thus, this model would not likely be accepted by the horticultural industry as a tool for a certification program. In contrast to the Pheloung model, the three evaluators where much more consistent in predicting invasiveness and, particularly, non-invasiveness using the Plant Right model. The evaluators were 97, 97, and 99% accurate in correctly rejecting invasive species. Of the few that were accepted, all these had only limited invasiveness. For known non-invasive species, the three evaluators were 100, 100, and 95% accurate in accepting them as non-invasive. Although we have not evaluated all the data for the time required to complete evaluations using the two models, one evaluator demonstrated a 10 min reduction in the time required to complete an evaluation using the Plant Right WRA (average of 54 min per species) vs the Pheloung WRA (average of 64 min per species).
Thus, the Plant Right WRA may be a more acceptable model to the horticultural industry for use as a tool for certifying current inventories or newly introduced ornamental species. The use of such a pre-screening tool could result in substantial financial savings when likely invaders are identified and prohibited from sale. It is our hope that this screening system could be implemented to protect the economy and environment and, yet, not significantly restrain opportunities for the horticultural industry.