Tillering is an important morphological component of grain sorghum (Sorghum bicolor L. Moench) development because it affects plant competition, light capture, water use, grain yield, and other physical and biological processes. Our objective was to determine differences in genotype and environmental factors associated with different tillering patterns when grain sorghum plants grown in clumps were compared with uniformly spaced plants (USP). To test the objectives, two studies were conducted in fields at the Texas Agricultural Experiment Station, Bushland, TX during 2005 with two hybrids. In the spacing study, four plants (adjacent to each other, 2.5 cm apart from each other in a square pattern, and 10 cm apart from each other in a square pattern) seeded in clumps were compared to USP (25 cm apart) in rows. In the density study, single plants, or clumps of two, four, and six plants spaced 75 cm apart in 75 cm rows were compared. Leaf number, tiller number, red/far-red (R:FR) light ratio, and temperature were measured in both experiments. Number of tillers plant^-1, leaves plant^-1, and R:FR ratio increased with increasing distance between plants in clumps. At 28 DAP, plants less than 2.5 cm apart had about 1.3 tillers plant^-1 compared to 2.3 tillers plant^-1 in USP. Additionally, the number of tillers plant^-1, leaves plant^-1, and R:FR ratio decreased with increasing plant density. At 28 DAP, single plants had about 2.5 tillers compared to about 1.7, 1, and 0.5 for plants in clumps of 2, 4, and 6 plants. The R:FR ratio for the single plants averaged about 0.32 compared to about 0.20 for clumps suggesting that shading of the main stalks influenced the phytochrome system and was an important factor in reducing tiller formation.