On the territory of Russia, ragweed (Ambrosia artemisiifolia L.) is included on the list of quarantine plants. It is an invasive species that distributes to the north. Currently, the northern border of ragweed distribution in Europe passes through the north of Germany, Poland and the southern border of Belarus, then along the European part of Russia (Kursk and the south of Lipetsk and Saratov regions). In order to plan effective protection against A. artemisiifolia invasion, it is necessary to know the prospects for its further expansion. Information on the northern distribution limits in its homeland in North America, particularly in Canada, is of special interest in this respect, since it allows to estimate the ecological limits of ragweed and to give a more accurate forecast of its distribution in invasion zones. It is logical to assume that in its homeland ragweed has the biggest ecotypic diversity, and a longer history of habitation could allow American ragweed populations to more fully occupy its fundamental ecological niche. Therefore, the aim of the research was to identify the northernmost self-sustaining ragweed populations in its homeland in Canada to estimate the ecological amplitude and define the potential northern ecological and geographic boundary of ragweed distribution. Data for the analysis were obtained from various sources: herbarium data, information from the Global Biodiversity Information Facility, aerological observational data and literary sources. To identify the sites of A. artemisiifolia naturalization, we developed 4 criteria. The criteria for identifying naturalization points were: density of ragweed detected points; pollen index of the ragweed complex; the suitability of landscape for the growth of the species; latitude of location. Ragweed naturalization at the point of detection was calculated as the sum of the scores for all four criteria (See Table 1). Naturalization points were identified on the basis of the four developed criteria before carrying out ecological and geographical analysis, which allowed us to define the quantitative ecological limit of A. artemisiifolia in the north (See Table 1). The layers of ragweed distribution according to GBIF and points of pollen monitoring stations were overlaid to calculate the complex estimation of ragweed naturalization. Comparison of ragweed locations in Canada (See Fig. 1a and 1b) shows that the western provinces of Canada are characterized by a much lower occurrence of A. artemisiifolia than the eastern ones. We analyzed ragweed locations in each province of Canada. The highest value of integrated assessment suggests the highest probability of naturalization of the species at the point. The naturalization of ragweed at a point is considered proven if it is possible to find additional indications of the presence of ragweed populations at the point in literature. 11 agglomerations of ragweed location points were selected for a comprehensive assessment of the probability of A. artemisiifolia naturalization. We found that the four agglomerations have the highest integrated rating of “7” (Winnipeg, Naming, Saguenay-Lac-Saint-Jean, Rimouski) (See Table 2). Another proven point of naturalization is the area near Medicine Hat. For the remaining territories, the naturalization of the species is currently doubtful. For ecological and geographical analysis of the potential A. artemisiifolia spreading to the north, we used a global raster computer map of the Day Degrees above 0 °C in the period from the transition of the day length after 14 hours to the first autumn frosts. This indicator is taken because ragweed is a short-day species and shortened day length is a signal factor for flowering and the beginning of ragweed pollination. The reduction in the length of the day to 14 hours at the end of summer is the threshold value for the start of pollination, and, hence, the start of seed development. Seed ripening in the north continues from the start of seed development until the first autumn frosts, and the possibility of moving the species to the north is determined by the sufficient heat supply of this particular period. A map of Day Degrees taking into account the threshold value of the photoperiod was created on the basis of the layers of monthly average temperatures of the WorldClim 2 set. Points of ragweed naturalization in Canada were overlaid on a map of temperature sums, and temperature values from the northernmost locations were extracted (See Fig. 2). Potential ecological boundary for the distribution of ragweed to the north was drawn taking into account the quantitatively determined ecological limit in its homeland in Canada. Ecological and geographical analysis showed that the current actual naturalization border of A. artemisiifolia in the primary distribution area does not extend beyond territories with temperatures above 660 °C (See Fig. 2). This quantitative value can be taken as the ecological and geographical border of the currently existing A. artemisiifolia ecological niche and ecological boundary of its distribution in high latitudes. The isoline of temperature sums of 660 °C for the period from the transition of the day length after 14 hours to the end of the growing season describes the actual distribution of northern populations of ragweed in its native land in Canada with high accuracy. This confirms the predicativity of the selected environmental factor and allows using it to predict the expansion of the secondary range of ragweed in invasion zones. The paper contains 2 Figures, 2 Tables and 40 References.