Fall panicum (Panicum dichotomiflorum Michx.) and witchgrass (Panicum capillare L.) emerged in naturally infested fields as early as April 30 and May 14, respectively. The shortest period between seeding and emergence was during the warmest period of the growing season. The number of days from emergence to heading decreased progressively for later seedings. After emergence witchgrass formed seedheads about 7 days earlier than fall panicum. Fall panicum emerging in the middle of July and witchgrass seedlings emerging July 25 produced mature seeds in the fall, shaded or not. Plant height, tiller number, and yield of both grasses were highest for plants emerging in May and June. Decreased light intensity delayed seed emergence, heading, and maturity of both grasses. Plant height, yields, number of tillers, and number of panicles also were decreased by shading. Fall panicum was a coarser plant than witchgrass and produced more tillers and panicles than witchgrass. Fall panicum and witchgrass seedlings emerging between June 23 and July 7 produced the most vigorous fast growing plants.