Identifying key performance traits is essential for elucidating crop growth processes and breeding. In Salix spp. genotypic diversity is being exploited to tailor new varieties to overcome environmental yield constraints. Process-based models can assist these efforts by identifying key parameters of yield formation for different Genotype × Environment combinations. Here, four commercial willow varieties grown in contrasting environments (West and South-East UK) were intensively sampled for growth traits over two 2-year rotations. A sink-source interaction model was developed to parameterise the balance of source (carbon capture/mobilization) and sink formation (morphogenesis, carbon allocation) during growth. Global sensitivity analysis identified day length for the onset of stem elongation as most important for yield formation, followed by various “sink > source” controlling parameters. In coastal climate the chilling control of budburst ranked higher compared to the more eastern climate. Sensitivity to drought, including canopy size and rooting depth, was growth-limiting in the South-East but not in the west of the UK. Light use efficiency increased during perennial maturation of the crop, distinguishing varieties according to canopy size and emphasized quantum efficiency at low light intensity as key to assimilation. However, on average sink parameters were more important than source. The Genotype × Environment pairings described with this new process model helped to identify parameters of sink-source control for future willow breeding.