The early growth of two hybridPopulus×euramericana(Dode) Guinier clones and a species clone each ofP. deltoidesBartr. ex Marsh, andP. nigraL. in controlled environments was studied in terms of growth of organs, partitioning of dry weight among parts, and growth analysis parameters. The hybrid clones grew more vigorously compared with the species clones with regard to most of the parameters measured. From 7 weeks after planting onward, the hybrid clones had greater plant dry weight, leaf dry weight, leaf area, and root dry weight than did the species clones. The hybrids developed a larger root system and greater leaf area per unit weight earlier than did the species clones. The greater leaf area was associated with lower specific leaf area. At all times the hybrid clones had significantly greater relative growth rates which were associated with early differences in unit leaf rate and later differences in leaf area ratio among the clones.