Tall sunflower varieties might need to be staked.

Phototropism in Sunflowers

by Julie Christensen

Sunflowers (Helianthus) are native to Central and North America and were cultivated by Native Americans as early as 2600 B.C. Early tribes ground sunflower seeds into flour and oil, and used the leaves for baskets. In your garden, you can use sunflowers to teach your children about phototropism -- the tracking of the sun.

1. Phototropism

Only young sunflower plants exhibit phototropism, which is the ability to track the sun. Young stalks and buds face east each morning and twist to follow the path of the sun from east to west during the day. During the night, they move back to face the east. Researchers believe they might do this as a protective defense. If the young buds faced south or west, the summer heat might scald the newly forming seeds, according to the National Sunflower Association. After the stalks mature and the flowers open, the plants do not move but face east continuously.

2. Explanation

Phototropism affects both the leaves and flowers of young sunflowers. The small leaves of a young seedling turn toward the sun. As the plant matures, the leaves on the shaded side of the plant grow more quickly than the leaves in full sun. This uneven growth pattern causes the plant to shift. Young buds also move in response to the sun's path across the sky. After the buds open, though, the flowers become frozen in one position.

3. Extra Fun

Children can learn about phototropism by watching sunflowers in the garden or on your windowsill. Plant a few sunflowers in a sunny garden area. As the sunflower plants grow, track their movement during the day. Take pictures, draw illustrations and take notes. If you lack space in the garden, try growing sunflowers in a sunny windowsill. They won't track the sun's course, but they will stretch toward the window.

4. Varieties

Although wild sunflowers don't exhibit phototropism, all cultivated varieties do. A few types of sunflowers are perennials, including the Jerusalem artichoke (Helianthus tuberosus), found in U.S. Department of Agriculture plant hardiness zones 2 through 10, and the Maximillian sunflower (Helianthus maximillanii), which thrives in USDA zones 3 through 9. Most garden varieties, though, are annual plants. Children seem to especially love growing either the mammoth varieties, such as "Lyng's California Graystripe," (Helianthus annuus "Lyng's California Graystripe), which grows up to 7 feet tall, or dwarf varieties, such as "Big Smile" (Helianthus annuus "Big Smile"), which remains under 2 feet tall. "Autumn Beauty" (Helianthus annuus "Autumn Beauty") and "Moulin Rouge" (Helianthus annuus "Moulin Rouge") have unusual dark red flowers.

Photo Credits

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