The St. Lawrence River (North America) and its banks are becoming increasingly colonized by invasive exotic plant species. One of these, the Japanese knotweed (Fallopia japonica), is gradually blocking access to water and disrupting riparian ecosystems. To help prevent its spread, the Jacques-Cartier ZIP (Zones d’intervention prioritaire — Areas of Prime Concern) Committee studied its growth strategy within the metropolitan area of Montreal. During the study period (2012 and 2013), stands of Japanese knotweed showed an annual increase in area and density, and growth started earlier in riparian areas than in the other habitats types studied. The species richness of herbaceous plants within Japanese knotweed stands decreased over time, becoming monospecific after a few years. A decrease in the species richness of arthropods in these stands was also detected, indicating a change in the ecosystem food web. These impacts are threatening the ecological integrity of wetlands along the St. Lawrence.