What the lifespan of a sunflower?

Answer 1

Short answer: the sunflower lives during the warmth of late spring, summer and early fall. It dies when the seed head starts to drop seeds, or when the cold of late fall starts to bite.

The sunflower (Helianthus annuus) is a flowering annual dicot. It starts life as a seed. Germination will not begin until the soil temperature is between 55F & 60F, as the plant is sensitive to cold. The young plant will grow vegetatively for 6-8 weeks before a flower appears. Actually, the flowers are where the seed head will form, each floret representing a possible seed. It takes about a month for the seed head to mature and begin to drop its seeds. About the time of seed drop, the plant will start to wither and die. This might be mid summer, or it might not be until the cooling days of autumn.