Sunflower (Helianthus annuus L.) is an alternate cash crop in the northern humid USA. Sunflower responds variably to planting date and plant population in different production areas. Field experiments were conducted for 3 years to determine the effect of planting date and plant population on sunflower seed yield and other plant characteristics under the humid growing conditions in the north central USA. In Exp. I, three planting dates, from early May to late June, were established with two hybrids at 30,000, 45,000, 60,000 and 75,000 plants ha−1 at Arlington (1978 to 1980) and Spooner (1978 to 1979), Wis. In Exp. 11, two hybrids were established at three populations, 30,45, and 60,000 plants/ha, under irrigated and dryland conditions at Hancock (1978 to 1980), Wis. The soil types were silt loam (Typic Argiudoll) at Arlington, and sandy loam (Typic Udipsamment) at Spooner and Hancock. Planting in early and mid−May at Arlington, and mid−May at Spooner maximized seed yield, seed weight, seeds head,−1 oil concentration, oil yield, and test weight. Sunflower seed and oil yields were not influenced by plant populalions ranging from 28,700 to 73,200 plants ha−1. Relatively constant yields were a consequence of reduced seed weight and seeds head−1 as plant population increased. The yield response of sunflower to increasing plant population, among planting dates and hybrids was more variable under the northern, cooler climate at Spooner. Yield and plant height were much higher under irrigated conditions on the sandy soils at Hancock. Although seed oil concentration under the dryland conditions at Hancock was 1.8% higher, the oil yield was 172 kg ha−1 lower, than under the irrigated conditions.