Concern raised against using highly competitive, exotic, large-statured, perennial grasses with fast growth rates as bioenergy crops has led to calls for risk assessment before widespread cultivation. Weed risk assessments (WRAs) are decision support tools commonly used throughout the world to determine the invasion risk of new plant taxa—primarily used as a pre-entry screen. Here, we compare the common Australian (A-WRA) and newer U.S. (US-WRA) models to evaluate the invasion risk of 16 candidate bioenergy crops and to compare their WRA scores to 14 important agronomic crops and 10 invasive species with an agronomic origin. Of the 40 species assessed, the A-WRA and US-WRA ranked 34 and 28 species, respectively, as high risk, including the major crops alfalfa, rice, canola, and barley. Surprisingly, in several cases, both models failed to effectively parse weeds from crops. For example, cereal rye received scores above (US-WRA) or comparable to (A-WRA) kudzu, a widespread damaging invader of the Southeastern United States introduced as forage. Our results indicate that these models are unable to accurately address broad, intraspecific variation and that species introduced for agronomic purposes pose special limitations to WRAs. This further supports other calls for postborder evaluation (e.g., field testing) following WRA screening. We should be cautious of the role of WRAs in setting policy, as illustrated by this relative evaluation of novel crops.

Nomenclature: Kudzu, Pueraria montana var. lobata (Willd.) Maesen & S.M. Almeida; cereal rye (Secale cereale L.); alfalfa, Medicago sativa L.; barley, Hordeum vulgare L.; canola, Brassica napus L.; rice, Oryza sativa L.

Management Implications: The U.S. bioenergy industry seeks to cultivate dedicated energy crops to meet increasing demands for bio-based energy sources. Many of these potential crops are not native to the United States and are large-statured, perennial species with fast growth rates that effectively compete with resident vegetation—all traits shared by many invasive plants. Therefore, there have been repeated calls to prevent the introduction and wide cultivation of invasive species for bioenergy. One method widely used to identify the invasion risk of new species are weed risk assessments, with the Australian (A-WRA) and United States (US-WRA) versions being the most widely used. To identify the invasion risk of bioenergy crops, we compared their A-WRA and US-WRA scores to those of the 14 of the most-common agronomic crops and 10 invasive species originally introduced for agriculture. This allowed us to compare the biofuels to crops, which we expected to have low WRA scores and to known invaders, which we expected to have high WRA scores. Both WRAs found most species to be high risk, including many crops. The WRAs suffer from many limitations, including being unable to deal with species that include high intraspecific variation, like Sorghum bicolor, which is both a crop and weed. Therefore, WRAs should be used cautiously in setting policy, even when only serving as the first tier of a multistep, risk-assessment process.