Ambitious targets for reducing emissions of carbon dioxide have created a demand for renewable sources of energy. Short rotation coppice (SRC) willow has the potential for meeting part of this demand. In this study, an experiment including four commercial clones of willow grown on two different soil types in northern Denmark is reported. Annual biomass production was estimated after the first and second growing season in the first rotation using a non-destructive method and total biomass production was measured by harvesting of the willow after the second growing season. The non-destructive method showed a large increase in annual biomass production from the first to the second growing season. Based on the harvested willow, average annual biomass production of the four clones ranged from 5.2 to 8.8 odt ha-1 yr-1 with a significant effect of both soil type and clone. The interaction between clones and soil types was also significant, indicating that different clones may be better suited for different soil types. On average, estimates of annual biomass production obtained by non-destructive estimation exceeded those obtained by destructive methods by 1.2 odt ha-1 yr-1. This bias indicates a need to revise commonly used methods for assessment of biomass production in SRC willow.