Sunflower Phenology—Year, Variety, and Date of Planting Effects on Day and Growing Degree‐day Summations¹

Six sunflower ( Helianthus annus L,.) varieties were planted at seven dates ranging from April 24 to June 28, 1967 through 1969. Day and growing degree‐day (GDD) summations were calculated for consecutive growth periods. Varietal differences in relative maturity were apparent at head‐visible stage. Differences among varieties in length of growth periods after first‐anther stage were small. Day summations in the prebloom periods declined from early to late planting in contrast with relatively small and random variation in GDD summations. The decrease in day summations from early to late planting in the emergence to head‐visible and emergence to first‐anther periods was not attributed to photoperiod but to increase in GDD per day. GDD summation to either head‐visible or first‐anther stage was an effective rating of varietal maturity and did not vary greatly under differing environments. Both day and GDD summations declined from early to late planting in both last anther to maturity and planting to maturity periods. Both day and GDD summations varied among years for all growth periods, but the magnitude of variation in some periods was not great.