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 fundam