In temperate climate zones, fast-growing willow species harvested in short harvest rotations of 3 to 10 years are an interesting source of biomass for energy or industrial purposes. The aim of this study was to determine morphological traits and biomass yields of three willow cultivars and three clones cultivated on three different types of marginal soils at two densities. Willow was grown in the Eko-Salix system, with no ploughing, with limited fertilization and cultivation measures, harvested in a 7-year rotation. The experiment showed that willow can be produced in the Eko-Salix system in extensive cultivation; however, the yield was strongly differentiated by the marginal soils and by the cultivars and clones under study and ranged from 4.4 to 17.8 Mg ha^–1 year^–1 DM. The mean yield from all the sites for all the cultivars and clones as well as planting densities in the experiment was 8.0 Mg ha^–1 year^–1 DM. The biomass yield obtained on peat-muck soil and humic alluvial soil was similar and significantly higher than on very heavy clay soil. The Ekotur cultivar gave plants with better morphological traits and the significantly highest mean yield (12.9 Mg ha^–1 year^–1 DM).