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Last updated on April 21, 2014 at 5:21 EDT

Sunflower, Helianthus annuus

The sunflower (Helianthus annuus) is an annual plant in the family Asteraceae. The stem of the flower can grow up to 3 meters tall and the large flower head can reach up to 30cm in diameter. The sunflower is notable for turning to face the Sun, a behavior known as heliotropism.

History

Sunflowers are native to the Americas, and were domesticated around 1000 B.C. Francisco Pizarro found the Inca subjects venerating the sunflower as an image of their sun god, and gold images of the “flower” as well as seeds were taken back to Europe early in the 16th century. Helianthus is from the Greek for “sunflower”.

Description

The term “sunflower” is also used to refer to all plants of the genus Helianthus, many of which are perennial plants.

What is called the flower is actually a head of numerous flowers crowded together. The sterile outer flowers are the ray florets and can be yellow, maroon, orange, or other colors. The flowers that fill the circular head inside the ray flowers are called disc florets. The arrangement of florets within this cluster is typically such that each is separated from the next by approximately the golden angle, producing a pattern of spirals where the number of left spirals and the number of right spirals are successive Fibonacci numbers. The disc florets mature into “seeds”. However, what we commonly call the seeds are actually the fruit of the plant, with the true seeds encased in an inedible husk.

Heliotropism

Most flowerheads on a field of blooming sunflowers are turned towards the east, where the sun rises each morning (hence the sunflower’s Spanish name girasol, and its French name tournesol). Immature sunflowers in the bud stage exhibit heliotropism; on sunny days the bud tracks the sun on its journey along the sky from east to west, while at night or at dawn it returns to its eastward orientation. The motion is performed by motor cells in the pulvinus, a flexible segment of the stem just below the bud. The stem stiffens at the end of the bud stage, and when the blooming stage is reached the stem freezes in its eastward direction. Thus, blooming sunflowers are not heliotropic anymore, even though most flowerheads are facing the direction where the sun rises.

Cultivation and Uses

In order to grow well sunflowers require full sun. They grow best in fertile, moist, well-drained soil with a lot of mulch. Seeds should be 45 cm (1.5′) apart and planted 2.5 cm (1″) deep.

Sunflower “whole seeds” (fruit) are sold as snacks, especially in the United States and Europe, and as food for birds. Sunflower oil, extracted from the seeds, is used for cooking (but is less cardiohealthy than olive oil), as a carrier oil and is used to produce biodiesel, for which it is less expensive than the olive product. The cake remaining after the seeds have been processed for oil is used as a livestock feed. Some recently developed cultivars have drooping heads. These cultivars are less attractive to gardeners growing the flowers as ornamental plants, but appeal to farmers, because they reduce bird damage and losses from some plant diseases. There are also new breeds of sunflowers which are transgenic, so that they are resistant to some diseases.

The sunflower is the state flower of the U.S. state of Kansas, and one of the city flowers of Kitakyushu.

The Jerusalem artichoke (Helianthus tuberosa) is related to the sunflower. The Mexican sunflower is Tithonia rotundifolia. False sunflower refers to plants of the genus Heliopsis.

Scientific literature reports, from 1567, that a 12 m (40′), traditional, single-head, sunflower plant was grown in Padua. The same seed lot grew almost 8 m (24′) at other times and places (e.g. Madrid). Much more recent feats (past score years) of over 8 m (25′) have been achieved in both Netherlands and Canada ([[Ontario]).

Sunflower Helianthus annuus