Understanding patterns of shattercane and giant foxtail field emergence during the spring and summer in the Midwest is important to determine the best weed management programs for crop production systems. The primary objective of this study was to determine percentage emergence of shattercane and giant foxtail under field conditions as influenced by soil temperature fluctuations, in 1992 and 1993. A secondary objective was to determine the difference in emergence percentage based on shattercane and giant foxtail seed lots harvested in different years (1985, 1991, and 1992). Soil temperature fluctuations were manipulated by artificially shading the soil surface with plastic netting that provided 30, 60, and 90% of incident sunlight in addition to the full sunlight control (0% shade). Giant foxtail and shattercane seed was planted at depths of 1.25, 2.5, 5, and 10 cm under each shade, and emergence was monitored for 45 d. Soil temperature fluctuations were reduced by increasing depth and amount of shade. Increasing depth of seed significantly reduced percentage emergence by 63 and 54% for shattercane, and 71 and 73% for giant foxtail in 1992 and 1993, respectively. Decreasing light intensity significantly reduced percentage emergence of shattercane by 72 and 73% in 1992 and 1993, respectively, but did not decrease giant foxtail emergence. Seed lots of both species were different from one another both years. These results were consistent over years, even though 1993 precipitation was three times that for 1992. Therefore, shattercane seed uses diurnal fluctuations in temperature to sense proximity to the soil surface and covering vegetation to avoid germination when conditions are unfavorable for growth. In contrast, giant foxtail seed can germinate readily and grow when environmental conditions are within an acceptable range and when seed is not buried too deeply. Differences in percentage emergence between seed lots could play a major role in emergence patterns of shattercane and giant foxtail observed in the field and may be influenced by environmental conditions during seed development on parent plants.