Exotic vascular plants are increasingly numerous, and decision support systems identifying the most problematic species are needed to help environmental managers to develop control strategies. The fundamental tool in this respect is a list of weeds, or a weed risk assessment. We propose here a list for the province of Quebec constructed using an innovative approach based on 1) well-defined criteria, 2) consideration of all potential problems associated with exotic plants, 3) use of credible scientific data, 4) assessment by a panel composed of experts with diverse expertise and who are also potential users of the list, 5) use of a decision support approach, and 6) a debate among experts in order to reach a verdict concerning the status (weed, no weed) of each candidate plant. The list contains 87 of the 908 taxa of exotic vascular plants that are naturalized in Quebec, i.e., 9.6% of the total. About two thirds of the weeds are problematic for agricultural or ornamental plant production or for forestry; the others are, in decreasing numerical order, problematic for biodiversity or natural ecosystem functions, health, landscaping or home gardening, and recreational activities. Evaluating the threat posed to biodiversity by individual species was a challenge, because few relevant studies have been published. The use of well-defined criteria greatly reduced (by a factor of 3) the number of weed species from an initial list based on individual uncensored expert opinions. The resulting list is not definitive, and should be used with caution. However, we estimate that our approach is more rigorous than the other qualitative approaches developed to date, with performance that matches the semi-quantitative or quantitative tools frequently used for assessing invasive plants, such as the Australian Weed Risk Assessment.