Reed canarygrass (Phalaris arundinacea) is one of the most invasive plant species in North American wetlands, but there are few historical data regarding the spread of this species, particularly at its northeastern distribution limit. We reconstructed the spread of reed canarygrass in Québec using herbarium specimens and a method that accounts for some of the biases associated with this type of historical record. We hypothesized that the beginning of the spread of the species coincided with the introduction in Canada of reed canarygrass cultivars for forage. Reed canarygrass specimens collected during the 19^th century in places as remote as Lake Mistassini and Anticosti Island provide strong evidence that this species is native to northeastern North America. The spatial distribution of specimens collected before 1925 suggests that reed canarygrass probably occupied most of its present-day area at that time. There is no strong evidence that reed canarygrass expanded its distribution limits in Québec during the 20^th century. However, the species colonized several new sites within its distribution limits in recent decades, especially from 1963 to 1978. The spread was probably associated with nitrate pollution and road construction in southern Québec, and with water level fluctuations of the St. Lawrence River. Although herbarium specimens suffer from sampling biases, their use may provide new spatio-temporal insights into the spread of invasive species and facilitate the identification of probable causes of invasiveness.