Abstract
A descriptive key is provided for eight species of nightshade, Solanum spp., occurring in Canada. The biology for three of four closely related taxa, eastern black nightshade, Solanum ptycanthum Dun. ex DC; black nightshade, S. nigrum L.; and hairy nightshade, S. sarrachoides Sendt., weedy in various parts of the country, is discussed. American nightshade, S. americanum Mill., a name once applied to S. ptycanthum, is actually very rare in Canada and its biology is only partially discussed in this paper.Key words: Eastern black nightshade, black nightshade, hairy nightshade, weed biology, Solanum spp.