Early-season weed control is essential to prevent yield loss in corn not only because of direct competition for resources, but also because weeds change the light quality environment of developing corn seedlings. Field experiments were conducted from 2003 to 2005 at the Arkell Research Station, Guelph, Ontario. Corn was grown hydroponically at a density of 40 000 plants ha-1. Weeds were present from corn emergence until 4, 6, 8 and 10 leaf tips and silking. Root:shoot decreased, root and shoot (leaf) dry weights decreased, rate of leaf appearance was delayed, and height increased relative to the weed-free control in the corn sampled prior to silking. As well, stem and ear weights were decreased at silking. These responses were typical of shade avoidance responses and show that low R:FR due to the presence of weeds during the critical period of weed control is detrimental to the growth and development of corn.