The individual mean seed (caryopsis) mass of 34 natural populations of Leymus arenarius widely distributed in Iceland and one each from Scotland and England showed a six-fold variation. Phenotypic plasticity was a factor in the variation between populations in different environments but there was also likely to have been genetic differentiation. Heavy-seeded populations tended to come from early successional sites, where they also produced longer spikes and more seeds per spike. Individual seed mass was also very variable on spikes within populations and within spikes; lighter seeds were found consistently in the lower third of the spike and in the florets distal to the rachis in the spikelets. Germination was examined using the powerful methods of Survival Analysis. There were striking asymptotic relationships between seed mass and germination: an inverse relationship between median germination time and seed mass and a positive relationship between total germination percentage and seed mass. Samples from small-seeded populations typically achieved half the total germination of large-seeded ones, with up to ten-fold longer median germination times. Similar trends emerged from analysis of large- and small-seeded spikes within populations and large- and small-seeded florets within spikelets. Natural stands of Leymus arenarius are harvested on a large scale in Iceland for sowing in reclamation programmes to combat unstable and eroding sands. The harvest period is short and susceptible to autumn gales and grazing. Mean individual seed mass would provide a rapid and reliable measure of seed quality, in terms of rapid germination and high germination rate. Stands should not be selected for harvest unless the individual seed mass averaged for whole spikes is at least 5 mg and such stands are most likely to be found in early-successional dune sites.