Cottonwood stem growth and biomass accumulation are altered by changes in the red to far-red light ratio (R:FR). Genetic variation in this response might be exploited to improve yield. The first objective of this study was to detect the R:FR signals that affect important morphological traits and biomass accumulation under field conditions. The second objective was to compare the effect of R:FR signals on growth traits under greenhouse conditions. The third objective was to compare genotypic variation in responses to R:FR ratio changes. We focused on morphological traits (such as height, diameter, stem taper, and branching), biomass deposition, and non-structural carbohydrate accumulation. Two approaches were used to achieve different R:FR ratios. In the field, two spacings (40 cm and 3 m) were used. In the greenhouse, Plexiglass RTM chamber filters filled with copper sulfate solution or water were used along with a no-filter control. Six clones (one Populus deltoids Bartr., one P. trichocarpa Torr. & Gray, and four of their F2 hybrids) were used in the studies. The R:FR ratios inside copper sulfate filters were 1.2, water - 0.6, and no filter - 0.6. The R:FR ratios under both narrow and wide spacings changed dramatically from 1.2 immediately after planting to 0.4 after 42 days from planting at narrow spacing. The results of both greenhouse and field studies showed that trees exposed to low R:FR ratios increased height 10 to 15% ( P < 0.05) and accumulated 7 to 10% more biomass than trees subjected to high R:FR ratios. However, in the greenhouse trees under the "no-filter" treatment had the shortest stems and least biomass accumulation. They did have increased stem tapers which might be the result of responses to wind flexing. Total leaf areas were lower for trees subjected to high R:FR signals. Clonal effects were significant for most metric traits. Some clones did appear to be less sensitive to the R:FR light.