The use of oats as phytometers for seasonal studies of microclimatic favourability and nutrient limitations is reported. The effect of age and development on growth parameters in long-term experiments is illustrated by comparison with data from related short-term experiments. Regression analyses were carried out on seasonal growth data to discover patterns of growth and development. Intersite differences were more clearly demonstrated by plants grown in nutrient-rich media than by those grown in soil and were most marked under poor weather conditions. Highly significant differences in growth existed between seasons for the nutrient-treated vermiculite experiments and between treatments for the soil and vermiculite experiments. The use of regression analyses to explain dry matter accumulation is discussed, and the usefulness of phytometers in growth assessment is considered.