Controlled-environment experiments were conducted to determine the effects of soil temperatures of 5 to 20 C, seed depths of 0 to 6 cm, and above- or below-seed cyanazine placement on germination and seedling development of giant foxtail and velvetleaf. Giant foxtail did not germinate at 5 C and failed to emerge from 6 cm deep within 21 days at 10 C. Increasing soil temperature above 10 C increased giant foxtail germination and emergence. Velvetleaf germinated at 5 C but did not emerge within 21 days. Velvetleaf emerged within 21 days from soil depths of 2 to 6 cm at soil temperatures of 10, 15, and 20 C. Giant foxtail and velvetleaf seed germinated on a soil surface kept moist by mulch or frequent watering. Giant foxtail seedling survival was 100% after germination on the soil surface. Velvetleaf often failed to become established; only 28% of the velvetleaf that germinated at 20 C survived. Injury to giant foxtail by cyanazine increased with increasing soil temperatures and decreasing seed depths. Cyanazine placement above or below the seed did not have a consistent effect on giant foxtail injury. Cyanazine placed above the seed was more injurious to velvetleaf than placement below at 15 and 20 C. Differential responses of giant foxtail and velvetleaf seed germination and seedling survivability to initial seed depth appears to be a major factor in weed population shifts when tillage is reduced or eliminated.