Early planting extended the period from transplanting to heading and maturity of both cultivars by 8–10 days in both years. However, final dry weight tended to decrease, especially in ‘Hitomebore’, with a marginally significant planting time × cultivar interaction. Slower leaf expansion and reduced canopy radiation capture during early growth caused the dry weight decrease in both cultivars, but the longer growth duration of ‘Koshihikari’ compensated for this reduction until the heading stage