The WALA Plant Library
Sunflower

Synonyms for Sunflower:
Scientific Name: Helianthus annuus L.
Family: Asteraceae (Compositae)

Description:

Everyone knows the fields full of bright yellow sunflowers, bobbing heads all pointing in the same direction. With its slim, strong stem, the annual sunflower can grow taller than a person. In shape and colour, its flower head bears a close resemblance to the sun, to which it is connected on many levels. While the flower is still growing, its leaves and buds ‘track’ the position of the sun: a phenomenon known as heliotropism. The buds face east in the morning and west in the evening, turning back east during the night. This cycle only stops when the flowers break out into blossom, the fruits ripen and the power of the sun is harnessed to produce oil. In this case, the flowers generally face east. One of the main reasons for this movement is the growth hormone auxin, which accumulates on the shaded side of the stem, allowing it to grow faster there. In addition, the inner cell pressure on the sunlit side is lower than on the shaded side – another reason why buds reach towards the sun.

The sunflower is the largest member of the aster family. Many tiny individual flowers – known as tubular florets – come together to form a plate-shaped inflorescence, with the outer yellow ray flowers arranged in such a way that the entire head resembles one flower. The diameter of a sunflower head can vary between 5 and 50 centimetres and consists of anything from 100 to 8,000 individual blossoms. In its native America, this striking flower thrived above all in the prairies of North America, where it grew wild among the high prairie grass, itself reaching heights of over two metres.

For gardening enthusiasts, the variety of sunflower cultivations blooming between July and September holds an endless fascination: some are small enough to fit in a flowerpot, and some are up to five metres in height; some have yellow, orange, red or brown ray flowers; some are brown and some are yellow inside. With 70 or so different types, the sunflower family is a large and varied one.