Shrub willow triploid hybrid cultivars obtained the greatest aboveground biomass compared to diploid and tetraploid cultivars, and greater biomass was highly correlated with leaf traits and rapid early stem elongation. Shrub willow (Salix L. spp.) is a strong candidate for use as a dedicated bioenergy crop in moist, temperate climates due to high growth rates, excellent regenerative properties and relatively low nutrient demand. Large discrepancies exist in the literature as to the benefits of fertilization for improving biomass production. Controlled environment fertilization studies can remove some confounding edaphic and climatic factors present in field studies. Ten top-performing commercial or pre-commercial cultivars, mostly bred in the US, were assessed for response to five fertilization levels by measuring 20 biomass and growth traits. Triploid hybrid Salix viminalis × S. miyabeana cultivars had the greatest total aboveground biomass, as well as high stem biomass. There was a strong relationship between early stem growth and final aboveground biomass, but only under adequate fertilization. Different strategies for high biomass production among cultivars are discussed in the context of nitrogen investment in leaves. Comparing field trial results and this greenhouse experiment indicates that while the yield ranking is generally preserved, the greenhouse results diverged greatly for two cultivars.