Setaria viridis (L.) Beauv. is a European native and has been present in Canada since at least 1821. It is currently the most abundant weed species in Saskatchewan and Manitoba. Setaria viridis is primarily a problem in late-seeded spring crops and is most competitive under high-temperature and high-light level conditions. Setaria viridis is easily controlled with herbicides but it continues to be a major weed problem due to its rapid growth through the vegetative stage to flowering, the large numbers of seeds produced and its high phenotypic plasticity.Key words: Green foxtail, weed biology, distribution, Setaria viridis