Sunflowers are annuals, growing only one season. They produce a single, tall, vertical stem with multiple, broad leaves. The stems are usually round when the plant first begins to grow. They become woody and angular as the season progresses. The flower head is actually made up of thousands of tiny flowers surrounded by petal-like bracts. For as small as a sunflower seed is, it produces one of the biggest flower heads of any plant.
In temperate areas, sunflower seeds generally take between 10 and 12 days to germinate and emerge. Within the next 33 days, vegetative growth is significant and the seed head will become visible. Around 27 days later, the first anther will appear, producing pollen. Within 8 more days, the last anther will appear. From that time, it will take another 30 days before the plant is fully mature.
The first stage of growth for a sunflower seed is called vegetative emergence or VE for short. In this stage the seeds germinates and produces a seedling that emerges from the ground. Seed leaves, called cotyledons, that are less than 4cm in length are the first leaves to appear.
The next stages are referred to as the vegetative stages and are identified as V1, V2, V3 and so on. As the plant grows, it will send up a vertical stem that will holds true leaves. Each vegetative stage indicates when a new true leaf appears on the stem. The number of vegetative stages will differ, depending upon the variety of sunflower.
In the reproductive stages of the growth of the sunflower seed, the flower head begins to form. These stages are identified as R1 through R9. In R1, the terminal bud of the stem forms a miniature flower head with tiny specialized leaves, called bracts, that resemble flower petals. In stages R2 and R3, the terminal bud begins to elongate above the last leaf on the stem.
In R4, the flower head begins to open. In the R5 stage the flower is obvious on the plant and continues to develop until fully grown. During this time, the plant will begin to shed pollen, beginning at the outer rim of the seed head and proceeding to the center. Pollination by bees and other insects will then occur. The sunflower seeds will begin to develop. In stage R6, flowering has finished and the real flowers in the seed head begin to wilt. Through R7 and R8 the back of the seed head will turn yellow and the bracts around the rim of the head begin to wilt. In R9, the final stage of sunflower seed growth, the bracts turn brown and the sunflower seeds in the seed head mature. At this stage the plant is considered fully mature and the plant dies.