Growth strategies of the two varieties of Arrhenatherum elatius were studied. The two varieties studied were: onion couch, which produces bulbous swollen internodes and is an arable weed of local importance, whilst tall oat-grass is more widely distributed. An experiment in which the two varieties were grown at different densities in monoculture and were harvested at intervals, highlighted various aspects of the competitive relation between the two varieties and an annual crop. Significant and consistent differences were found between the two varieties for most of the attributes measured during the two growing seasons. Tall oat-grass allocated a greater proportion of its whole shoot dry weight to producing vigorous aerial tillers, whereas onion couch allocated a greater proportion of its whole shoot dry weight to the development of basal internodes. The regulation of tiller number as a reaction to increasing density in tall oat-grass is achieved by high tiller mortality, whereas in onion couch, this stress is expressed by small tiller mortality. Growth pattern shown by the two varieties of A. elatius reflect the adaptive strategies evolved by them in two different habitats and are due to the genetic factors which determine the presence or absence of their characteristic basal internodes.