The growth progression of the sunflower (Helianthus annuus) is divided into three stages: vegetative emergence, vegetative growth and reproductive growth. Sunflowers have a relatively short maturation period, requiring between 90 and 100 days from germination to full seed head maturity in most parts of the north and central United States. It is grown in these areas as an oilseed and snack food crop.
Vegetative emergence is the shortest stage of the sunflower’s growth progression. It consists only of the time between seed germination and the emergence of the seed leaves, or cotyledons. Seed leaves are not true leaves. In fact, any leaves less than 4 cm that grow on the plant at this time are still considered part of the vegetative emergence stage.
Germination can take up to eight days under optimal temperatures and moisture conditions. The seed leaves provide energy and nutrients to the developing plant for up to 10 days as the first true leaves develop and emerge.
The vegetative growth stage is charted in terms of leaf emergence as the plant grows. Each phase of the vegetative growth period is marked as a leaf appears. A 6- or 7-foot tall plant with 24 leaves would therefore have 24 growth phases. This stage lasts about 60 days, or until the flower structures begin developing.
Nine distinct stages comprise the reproductive growth period of the sunflower plant. The first several of these phases comprise flower growth and development, but the main portion of this growth stage is devoted to the blossoming and pollination of the flower structures. The end portion of the reproductive growth stage is marked by the wilting of the flower structures and seed ripening, at which time the sunflower reaches physiological maturity.
After physiological maturity, the sunflower plants are allowed to stand in the field for up to two full months to allow the plants to dry out. Harvesting sunflower seed heads before the plants have adequately dried complicates the harvesting process. Excess moisture causes the seeds to cling to the fleshy portion of the seed head and reduces yield.