Genotypes with enhanced rooting ability are crucial for deployment of intensively cultured Populus plantations. Rooting of dormant, unrooted cuttings is the first biological prerequisite to stand establishment. The primary objective was evaluating three developmental systems affected by site conditions: lateral and adventitious root ontogenies, root/shoot growth rates, and temperature dependent physiologies. The study tested rooting ability of 21 clones from five taxonomic backgrounds ((Populus deltoides x Populus trichocarpa) x P. deltoides 'BC1'; P. deltoides 'D'; P. deltoides x Populus maximowiczii 'DM'; P. deltoides x Populus nigra 'DN'; P. nigra x P. maximowiczii 'NM') at Ames, Iowa; Waseca, Minnesota; and Westport, Minnesota, across three planting dates during 2001 and 2002. The experimental design was randomized complete blocks with 12 blocks per planting date and a single 20 cm cutting per clone per block. Spacing was 1.2 x 2.4 m, with two border rows of unrooted cuttings. Trees were harvested 14 days after planting. Dry weights, dimensions, and counts were determined for lateral roots, callus roots, callus, shoots, leaves, and cuttings. Height following establishment was also determined. Growing degree days (GDD) were estimated from aboveground and belowground temperatures. Precipitation data were compiled. Tree growth and environmental data were analyzed using univariate and multivariate approaches according to the Statistical Analysis System. The percentage of cuttings rooted among clones ranged from 21.8--86.1%. Broad-sense heritabilities for root and top dry weight ranged from 0.09--0.11 and 0.31--0.38, respectively. BC1 clones exhibited the highest root dry weight, while NM and DM clones produced the highest top dry weight. Clonal and taxonomic group shoot:root ratios ranged from 5--77 and 11--71, respectively. Clonal performance was stable over all year x site combinations, except for Westport 2002 where root growth was relatively poor. Rooting increased with increasing belowground GDD for all taxonomic groups. Least-squares regression models indicated very similar taxonomic group responses for each rooting trait. A minimum of four days with at least four GDD per day, along with precipitation dispersed throughout the growing period, were needed for above-average rooting. Clone NM6 performed well relative to others, while clone DN34 performed poorly.