Easy to grow and quite hardy, sunflowers (Helianthus annuus) are a hallmark of summer for many gardeners and nature lovers. Available in bright yellow and orange hues, these large plants reach heights of around 9 feet with flowers up to a foot in diameter. Many of these beautiful giants die after flowering and reaching maturity in the fall, so you'll have to replant them every spring if you want to keep enjoying them. A few perennial varieties do exist, however, including the Maximillian sunflower (Helianthus maximilliani) that grows in U.S. Department of Agriculture plant hardiness zones 3 through 9, and the swamp sunflower (Helianthus angustifolius) found in USDA zones 6 through 9.

Waiting for Greatness

For a time, the seeds of a sunflower lie dormant, waiting for the spring growing season. In the wild, these seeds wait out the cold weather in the ground, while seeds that have been collected and prepackaged sit in warehouses and on store shelves until gardeners liberate them. Dormancy is broken and germination triggered by a combination of soil temperature, water and light, all of which are affected by planting depth. When growing sunflowers from packaged seeds, germination occurs in about five to seven days.

Vegetative Phase

The vegetative phase of a sunflower's life begins after germination. The young plant is considered a seedling for the first 11 to 13 days after breaking through the soil. The seedling moves into the vegetative phase when it forms the first leaf. After that, the young plant is considered to be in various stages of the vegetative phase based on the number of leaves at least 4 centimeters (1.5 inches) long. As the sunflower progresses through this phase, it forms more leaves and grows larger.

Reproductive Phase

The reproductive phase occurs when the sunflower plant actually flowers. This phase starts with the formation of a flower bud. As it continues, the flower bud opens to reveal a large flower. When the flower has fully opened, it will droop downward slightly. This helps the flower itself to gather less rain during rain showers to help prevent fungal infections in the plant.

It is during this reproductive phase that bees visit the flowers and pollinate them, which results in the production of new sunflower seeds. Sunflowers can technically fertilize themselves, but studies have shown significantly greater seed production with pollinators. The sunflower is considered mature and the reproductive phase ends in the fall when the back of the flower turns from green to brown and the small flower petals covering the seeds fall away from the plant easily.

Saying Goodbye

In the fall, after the sunflower has completed its reproductive phase, it will die. As it does so, the plant begins to wither and deteriorate, and the seeds drop out of the flower. Some of the seeds that drop will be eaten by birds, squirrels and other wildlife, but some will also find themselves covered with leaves and soil where they will go dormant and wait for spring to germinate so the life cycle can begin anew.

If you want to harvest seeds for replanting next year or for a tasty snack, cut the flowers off of the plant when they reach full maturity, leaving about 1 foot of stem. Hang the flowers upside down by the stems in a warm, dry place with good ventilation. When the heads are completely dry, you can easily remove the seeds by rubbing two flowers together or running a stiff brush over them.