Breeding for higher grain yield in maize (Zea mays L.), utilizing increased selection densities, has produced varieties that are adapted to grow at higher population densities. The effects of increased planting density on grain yield and final phenotypes are well known, but the effects of density on plant growth across the growing season have been less widely characterized. The objectives of this study were: 1) examine the effects of high planting density on growth rates, growth timing, and biomass partitioning of the ear, stalk, and tassel; 2) characterize the difference in density effects in maize populations before and after recurrent selection for grain yield; 3) characterize heterosis and hybrid performance in plant growth and biomass partitioning; 4) characterize the effects of recurrent selection on heterosis levels in plant growth. Four populations, Iowa Stiff Stalk Synthetic (BSSS), Iowa Synthetic Corn Borer #1 (BSCB1), and the populations derived from the 17th cycle of reciprocal recurrent selection, BSSS(R)C17 and BSCB1(R)C17, along with the BSSS/BSCB1 and BSSS(R)C17/BSCB1(R)C17 population crosses were utilized in this study. The populations were growth at 3.23, 6.46, 9.69, and 12.92 plants m-2 at six locations near Ames, IA over 4 years. Increased density lowered maximum growth rates for all plant organs, but reduction in ear length, plant height, and ear and stalk biomass growth rates occurred at higher densities in the selected populations and population crosses compared to the unselected populations. Increased planting density affected plant organ growth timing differently in BSSS and BSCB1. High density delayed stalk biomass accumulation in BSCB1 but not BSSS, and delayed ear length growth and ear and tassel biomass accumulation in BSSS but not BSCB1. Growth delays the cycle 0 populations were not present in the cycle 17 populations, or in the cycle 0 population cross. BSCB1 and the BSSS/BSCB1 population cross partitioned lower levels of biomass to the ear and had smaller harvest indices at high density, while biomass partitioning and harvest index were not affected by density in the selected population and population cross. Heterosis was present in final phenotypes, growth midpoints, growing period length, and maximum growth rates. Heterosis levels for final phenotypes and maximum growth rates increased with selection. Increases in heterosis levels were due to depressed per se population performance and slight increases in selected population cross phenotypes. BSCB1 was often the dominant parent in population crosses in regards to growth midpoint timing, maximum growth rates, harvest indices, and density response.