Summer is the season for sunflowers (Helianthus spp.). Although some varieties can bloom any time of year, most start blooming in midsummer, and others begin flowering in late summer. Sunflowers also produce edible seeds, and the right time to harvest the seeds also varies among sunflower varieties.
Some sunflowers are annual plants while others are perennials. Annual sunflowers (Helianthus annuus) don't return the following year, but ashy sunflower (Helianthus mollis), western sunflower (Helianthus occidentalis), Maximilian sunflower (Helianthus maximiliani) and other perennial sunflowers sprout again in spring.
Annual sunflowers are frost-sensitive. In areas that experience frosts, their seeds are usually sown in the garden after the final local average frost date in spring. In frost-free areas, the seeds can be sown earlier or later in the year. Spring-sown annual sunflowers bloom in summer. When annual sunflowers are sown in summer, they bloom in fall.
Summer through fall is the blooming season for most perennial sunflowers. Ashy sunflower and the perennial sunflower Helianthus divaricatus, which is hardy in U.S. Department of Agriculture plant hardiness zones 3 through 8, begin blooming soon after midsummer, and the flowers continue to appear into late summer or fall. Maximilian sunflower and western sunflower blooms appear about one month after midsummer and also continue into late summer or fall. Willow-leaved sunflower (Helianthus salicifolius) blooms later. Its flowers appear in late summer and continue into late fall.
Ashy sunflower, Maximilian sunflower and willow-leaved sunflower are hardy in USDA zones 4 through 9, and western sunflower is hardy in USDA zones 4 through 8.
As the petals fade, the central disks of sunflower blooms develop seeds. The right time for harvesting a sunflower's seeds is usually fall, but the best method for determining when to harvest the seeds is to examine the flower and its seeds. When sunflower seeds are mature and ready to harvest, the back of their flower is dry and brown, and the flower petals are withered and may have fallen from the plant.
Ripe sunflower seeds are plump, and, in some varieties, the seed coats are striped black and white.