A simple model for willow forest growth during the growing season is presented. The model is based on nitrogen productivity, expressed as the daily above‐ground biomass growth per the amounts of nitrogen in foliage and leaf litter. It is shown that the nitrogen productivity can be expressed as a linear function of the amounts of leaves produced. The model, which also includes allocation of biomass between stem and leaf biomass as well as between foliage and leaf litter biomass, is tested against accurate measurements of the seasonal growth of willow of four different plots. Simulated and measured biomass of stem, foliage, leaf litter and the total above‐ground biomass agreed well for all four plots. The model concept should be useful in nitrogen flux models and as a tool to describe the actual growth of a willow stand, knowing only the annual leaf production and the nitrogen concentrations of foliage during the growing season.