Do sunflowers really move their flowerheads to face the sun throughout the day? Are they really tracking the sun across the sky? Let’s have a look at what these beautiful plants are up to.
If you are interested in more garden facts and folklore, also see Popular Garden Myths We’ve All Fallen For.
Sunflowers and The Sun
Do Sunflowers Really Track the Sun Across the Sky?
Yes, sunflowers are heliotropic—moving in response to sunlight—but not the way we’re often told.
Have you seen those popular garden memes showing sunflower flowerheads greeting the dawn and gradually turning to follow the arc of the sun throughout the day? That’s not accurate.
The flowerheads do not follow the sun, but, before the plant is blooming, the stems and leaves are responsive to light.
This heliotropic movement only occurs during the vegetative plant growth stage.
How and why this happens is still debated. One of the best explanations involves hormones.
Hormones and Heliotropism
Whatever part of the stem is in the shade during the day will also get the most auxin hormones—what we also know as rooting hormones— triggering more growth than on the sunny side of the stem. In other words, the amount of growth within one stem can be lopsided. And it’s that growth difference that tilts the stem tip toward the sun.
And this is why sunflowers are only heliotropic prior to flowering—the tilt can only happen when the stem is actively growing and those hormones are shifting around.
Once the plant enters the reproductive stage—budding, flowering, and going to seed—the heliotropic movement stops.
Sunflowers At Night
So what happens at night? How does the (not yet flowering) stem return to its east-facing position by dawn?
This part involves circadian rhythms, those internal daily changes in metabolism or physiology that we all have, repeating roughly every 24 hours.
Without sunlight present, the auxin returns to where it was before daybreak. And that shifts the stem back to an east-facing position. As the sun rises, the process begins all over again.
Why Do the Flowerheads Face East?
If you’ve ever seen a field of sunflowers, you’ve probably noticed the flowerheads all face the same direction—often toward the eastern sky or close to it—and this does not change.
In my own garden I’ve noticed that if the sunrise is blocked by a building or other obstacle, the flowers will form facing whatever direction provides first direct light of the day.
We’ve seen fields of dwarf sunflowers face various directions including north, but sunflowers with larger heads seem to be more east-centric.
There are a number of different hypotheses about their final sun-facing position.
Facing the morning light, sunflowers may be more attractive to pollinators.
The sunlight may also help evaporate morning dew, reducing susceptibility to fungal disease.
Or the warmth of the sun might bring more optimal plant temperatures.
These are all possibilities but we just don’t know yet.
Summary
Sunflowers are heliotropic but it’s only the stems that move and only during their vegetative growth stage.
Once budding and flowering, the flowerheads are stationary and usually east-facing or close to it.
More About Sunflowers
Sunflower Podcast Episodes
More on heliotropism and sunflowers:
Some history of sunflowers and interesting facts:
Ready to grow? Let’s get started:
Sunflower Tip Sheet
Read More
- How to Grow Sunflowers and What to Avoid
- 12 Tips for Growing Giant Sunflowers
- 19 Gift Ideas for Sunflower Lovers
These include recent and historical studies mentioned in the podcast episodes.
- Phototropic Solar Tracking in Sunflower Plants: An Integrative Perspective (2015) | Annals of Botany
- Sunflower inflorescences absorb maximum light energy if they face east and afternoons are cloudier than mornings (2020) | nature.com
- Observations on the Nutation of Helianthus annuus (1898) | journals.uchicago.edu
- Observations on the Nutation of Sunflowers (1890) | jstor.org
- The Herball, or Generall Historie of Plantes (1597) | Chapter 247: Of the flower of the Sunne, or the Marigolde of Peru | archive.org
Theses articles are behind paywalls:
- Following the Star: Inflorescence Heliotropism (2017) | sciencedirect.com [paywall]
- How Do Sunflowers Follow the Sun—and to What End? (2016) science.sciencemag.org [paywall]
- Turning Heads: The Biology of Solar Tracking in Sunflower (2014) sciencedirect.com [paywall]
- Athanasius Kircher’s Sunflower Clock (1995) degruyter.com/ [paywall]
~Melissa the Empress of Dirt ♛
