In North America, the exotic haplotype M of the common reed (Phragmites australis) is considered a problematic invader of wetlands, negatively affecting marsh plant diversity. In Québec, the extension and paving of the road network at the beginning of the 1960s, provided suitable habitat (roadside verges and ditches) for the successful establishment and growth of this grass, and contributed to its spread across the southern part of the province. However, to what extent the common reed populations that had established along roadsides spread into adjacent wetlands, had not been determined. The present study used a geographic information system and historical aerial photographs to map common reed populations located in 14 marshes adjacent to highways in southern Québec. The exotic haplotype was present in 9 of the 14 marshes investigated. The historical analysis of 3 of these marshes showed that the invasion process was initiated after the construction of the highway, with the plant constantly and rapidly occupying successively larger areas. However, the exotic haplotype of the common reed did not always spread from roads to marshes; occasionally, it established in the marsh and then spread along nearby roads. Nevertheless, preventing the establishment of the exotic haplotype of the common reed in roadside ditches would be a prudent approach to help preserve adjacent marshes from this invasive exotic plant.