On the move: How sunflowers follow the sun
- Meghan McMahon
- Sep 30
- 3 min read
Have you ever heard that sunflowers will follow the sun across the sky? They do in a way.
The flowers are firmly rooted to the ground where they grow, but the flower heads will move throughout the day so they are always facing the sun. If you look at a young sunflower in the morning, it will be facing east. But if you return at the end of the day, as the sun begins to set, that same flower will be facing west.
Sunflowers are well-known for this ability to move through the day to face the sun. There's even a word for it — heliotropism. It is also sometimes called solar tracking. The term heliotropism comes from the Greek words "helio," which means sun, and "tropism," which means movement of an organism away from or toward an external stimulus.
Only young sunflowers track the sun. As the plants grow and the blooms get larger, the tracking movement diminishes until the flower heads are left facing east to get the morning sun.
Words to know
Circadian: Recurring naturally on a 24-hour cycle even in the absence of changes in daylight.
Diminish: To make or become less.
Efficiently: In a way that achieves maximum productivity with minimum wasted effort or expense.
Elongate: To make something longer.
Facing east is an advantage for the plants because sunlight warms them up more quickly in the morning. This helps them attract bees and other insects. Facing east also allows them to reproduce more efficiently.
Sunflowers use their circadian rhythm, sort of like an internal clock, to know when the sun will rise. But the question of how sunflowers are able to move with the sun was a mystery until researchers from the University of California Davis made great strides in understanding this phenomenon in 2016.
The scientists discovered that the plants' stems allow them to track the sun. During one part of the day, one side of the plants' stems can elongate. Then in another part of the day the other side of the stem elongates. This allows the plants to move with the sun throughout the day.
As part of their study, the researchers tied up sunflowers so they weren't able to move with the sun and also exposed some sunflowers to differing levels of artificial light. The plants that were tied up did not grow as well as the ones that were not.
Sunflowers are the most well-known heliotropic flowers, but there are others that track the sun too. Some poppies, including Arctic poppies, follow the sun, as do buttercups. Even some crops track the sun, including alfalfa and soybeans. Dandelions do as well.
The most common and familiar sunflower is the common sunflower, or annual sunflower. It is the flower that is grown for seed and oil, and it comes in many shapes and colors. You’ve probably seen tall, yellow sunflowers growing in gardens and fields. These are common sunflowers. They can grow to be 10 feet tall! That’s about as tall as a one-story building.
The common sunflower is one of dozens of sunflower species, most of which are native to the United States. Other sunflower species you might see growing in Will County forest preserves and in other grasslands and prairies include the sawtooth sunflower, woodland sunflower and prairie sunflower. All of these have bright yellow flowers like common sunflowers, but they don’t get as big.
