The common sunflower (Helianthus annuus) is a warm-weather, summer-blooming annual characterised by large, showy flower heads, a stiff green stalk and broad green leaves. Flowers typically appear from May to October, or until the first frosts of autumn arrive. You may propagate sunflowers outdoors in the garden as soon as frosts have finished for the year. Sunflower seeds are mature and ready to harvest about four months after planting.
The sunflower is a native of North America, where it grows from central Canada all the way down to northern Mexico. The plant grows naturally in dry, exposed areas, and can grow in such abundance that it becomes a weed. As a garden plant, sunflowers can grow in bright, full sun locations all over the world. To flower to their fullest extent, sunflowers require at least six hours of bright sunlight. You may harvest flowers as cut flowers as soon as petals begin to lift from the centre.
Sunflower are tough plants that thrive with little care from the gardener provided their basic cultural requirements are met. In the spring when the soil has warmed, sow sunflower seeds in a well-draining, slightly alkaline or neutral soil. Add lime to acidic soils to make the pH more alkaline. Enhance the soil by providing organic matter such as compost, and fertilise and water freely throughout the summer to produce large, healthy blooms.
Sunflowers may suffer if forced to compete with weeds, which steal nutrients from the plants' roots. Limit the appearance of weeds by mulching with organic matter such as grass clippings or compost. Birds may steal seeds in the late summer, and cause a racket. Keep birds from seeds by covering flower heads with a thin mesh cloth or net. Sunflower foliage may cause an allergic reaction on the skin of some sensitive individuals.
Harvest seeds about four months after planting, when the plant's flower heads start to droop downward. The centre of the flower disk will shrivel, and the backside of the flower head will turn a pale yellow. Cut the flower head, leaving about a foot of stem attached, and hang the seed head upside down in a warm, dry place that's not humid or stagnant. Tie a paper bag with holes or a ventilated piece of cloth over the head to catch the seeds.