Biology · movement
Heliotropism: do sunflowers follow the sun?
Young plants turn with the sun during the day; mature heads face fixed east. How the mechanism works, and why that eastward posture pays off.
In short: Only young, still-growing sunflowers track the sun east to west and turn back at night. This is driven by a daily rhythm that makes the east side of the stem elongate faster by day and the west side at night. At flowering it stops: the head then faces fixed east and warms faster in the morning, attracting about five times more pollinators (Atamian et al., Science 2016).
What mature heads actually do
The most famous story about the sunflower is only half true. A full-grown, flowering sunflower does not follow the sun. Walk past a field in August and almost every head faces the same way: east. They stay there all day, whatever the sun does. The turning people picture happens earlier in the plant's life.
Atamian and colleagues (UC Davis, Stacey Harmer's lab) mapped this systematically in Science (2016). They showed that sun-tracking stops as the stem matures, and that the ripe head settles into a fixed eastward posture. That the plant is botanically not a single flower but an inflorescence of hundreds of disc florets is covered on the page about the anatomy of the flower head.
The mechanism in young plants
While the stem is still growing, the young sunflower does show genuine heliotropism. By day the top leans with the sun, from east in the morning to west in the evening. At night it turns back, so that by sunrise it points east again. This to-and-fro is not because the plant "sees the sun move", but because of unequal growth: by day the east side of the stem elongates faster (tilting the top westward), and at night the west side.
Atamian et al. (2016) showed that this rhythm is partly driven internally by the plant's circadian clock: even in constant light the plant kept the turning pattern for a time. Vandenbrink and colleagues (2014) described the movement of the sunflower stem and the role of growth regulation in it. So it is an interplay of light and a built-in daily clock, not merely a response to the current sun position. How these growth processes are fixed in the DNA touches on the genetics of the sunflower.
Why facing east pays off
That the ripe head settles east is no accident. Atamian et al. (2016) artificially turned some flowering plants west and compared them with the natural east-facers. The east-facing heads warmed faster in the morning from the early sun, and that warmth drew about five times as many pollinators. A warm flower is more attractive to bees, and being active early in the day means early pollination.
The eastward posture is therefore an evolutionary advantage: more pollinators, more pollinated disc florets, more seed. It is not a leftover of sun-tracking but a functional endpoint that the plant locks in once stem growth stops. To get to know the pollinators themselves, see the page on bees and the sunflower; for the practical side of placement and position there is the page on growing sunflowers. The wider overview is in the biology of the sunflower.
Misconception: flowering heads also follow the sun
No. The image of a field all facing west in the afternoon does not hold for full-grown plants — they face fixed east. Sun-tracking is limited to the growth phase before flowering.
Misconception: the flower "seeks" warmth as a goal
The plant does not pursue warmth for its own sake; the eastward posture was selected because the resulting morning warmth brings more pollinators. The benefit lies in reproduction, not comfort.
Sources
- Atamian, H.S. et al. (2016). Circadian regulation of sunflower heliotropism, floral orientation, and pollinator visits. Science, 353, 587–590 (UC Davis, Stacey Harmer lab).
- Vandenbrink, J.P. et al. (2014). Turning heads: the biology of solar tracking in sunflower. Plant Science.