The sunflower (Helianthus annuus) is one of the more cheerful flowers to grow in your garden, with its big flowers with yellow petals. An annual, or plant that grows quickly to maturity within one growing season, the sunflower is killed by drought or the fall frost. It produces hundreds of seeds that ensure sunflowers grow again in the next year. It is a member of the aster or daisy family, Asteraceae.
When danger of frost is gone in spring and when soil temperatures warm above 65 degrees Fahrenheit, the large, wedge-shaped sunflower seed germinates. First cracking open to allow the seed root to grow downward into the soil, the seed coat is soon shed as the cotyledons, or first leaves, emerge. The small seedling has light green, rounded leaves with tiny hairs and soon begins to grow.
Relishing the warm, moist soil and abundant sunshine, a sunflower seedling grows quickly. The upright, sturdy stem is lined with sandpaper-textured leaves. Depending on the variety of sunflower, the plant reaches its mature height of 3 to 12 feet anywhere from six to 12 weeks' time.
In the height of summer, the tip of the sunflower's stem will form a large, star-like flower bud, surrounded by many small leaves. The bud opens to reveal the ray petals, usually a shade of yellow, that then enlarge and fold outward to reveal the disc floret. The disc is the flat, broad central plant of the sunflower blossom. Attracting bees, the disc is pollinated. After the flower is pollinated, the colorful ray petals wither and drop away, leaving just a broad seed head that resembles a shower head.
After insects pollinate the small disc florets in the core of the blossom, the ovaries in each floret begin to swell and mature. They become firm, beige and tan, with dark stripes on the seed coats. As the the big seed head matures and dries, the individual sunflower seeds jar loose in the breeze and drop to the ground below. The seeds remain dormant through the autumn and winter, awaiting the right conditions next spring to germinate.
After the seeds ripen and begin to shed, the sunflower plant slowly degrades. Leaves once green now yellow and no new growth occurs. If the growing season is long, the sunflower may be fully dead well before the killing fall frost. Sunflower plants that die before flowering and setting seeds succumb to excessively dry soils or a rare plant disease.