Tubers of water yam (Dioscorea alata L.) are an important source of carbohydrates for millions of people throughout the tropics. Planting of the crop in the Caribbean and Central America is usually carried out between March and July. As water yam is very sensitive to small variations in photoperiod, changes in the planting date may greatly affect crop development and growth. The aim of this study was to assess the change in the radiation use efficiency (RUE) as a function of the planting date, and its effect on yam growth and yields. For this, we adapted the CropSyst model to simulate yam development and growth. The model was modified, calibrated and tested using two independent data sets from field experiments carried out in Guadeloupe (French Antilles) under non-limiting conditions for water and nutrients and over a wide range of planting dates and photoperiods (e.g. from 12.9 h in March to 11.3 h in October). The planting-harvest period varied from 4.4 months for the late planting dates to 9.1 months for the early planting dates. The RUE was relatively stable in the vegetative phase, increased after tuber initiation (TI) to reach a maximum value during the exponential phase of tuber growth, and then decreased towards the end of growth. The mean value of RUE increased from the early (March, 1.4 g MJ−1) to the intermediate planting dates (July, 2.7 g MJ−1), and then decreased for the late planting dates (October, 1.3 g MJ−1). Maximum RUE was negatively correlated with photoperiod. This particular behaviour of RUE was induced by a source–sink interaction which was controlled by crop development and photoperiod. The model described yam development and growth satisfactorily for all the planting dates: e.g. R2 > 0.92 for the correlation between modelled and measured tuber biomass. Yields varied little from the early to the intermediate planting dates because of an offset between the length of the vegetative phase, which determined leaf area, and the level of RUE after TI, which determined the capacity of the crop to fill tubers. Growth and yield drastically decreased for the late planting dates because photoperiod induced fast TI which strongly affected the leaf area. The model was a powerful tool to identify the underlying mechanisms affecting yam yields for some early planting dates, and to assess the factors involved in the high year-to-year variability currently observed in water yam yields.