Invasive alien plant species represent an ecological risk to indigenous plants and ecosystems. In this study, the dichotomous system is suggested to assess the ecological risk of invasive plant species. Case studies of noxious alien plants in South Korea were used to develop the system based on environmental risk factors. Assessing ecological risk of invasive alien plants was considered with the following factors: harmful effects to animals or humans, ecological characteristics, invasion time, present state after invasion (dispersion), and availability of resources. Invasive plant risk assessment was based on the methodology for predictive risk assessment used to manage and monitor invasive alien plants in South Korea. Our system enabled us to classify invasive plants in Korea into four grades: (1) noxious species (three categories), (2) monitoring species (five categories), (3) harmless species (innocuous species), and (4) beneficial species. Our assessment system can help land managers monitor invasive alien plant species.

Additional index words: Dichotomous system, ranking system.