Do sunflowers really follow the sun?

A field of sunflowers all gazing at the sun is one of summer’s great images. Look at a field of fully bloomed sunflowers at sunset, though, and you might notice something odd. They are all staring firmly in the wrong direction.

The short answer

Sunflowers do follow the sun, but only while they are young and still growing. Each day the young plants face east at dawn, bend west over the course of the day, then swing back east during the night. Once they mature and the flower opens, they stop moving and settle into facing east for good.

The real story

In August 2016, a team led by plant biologist Stacey Harmer at the University of California, Davis, with collaborators including Hagop Atamian and Benjamin Blackman, published a study in Science that explained how this happens.

The secret is uneven growth. During the day, the east side of a young sunflower’s stem grows faster than the west side, which tips the head westward to follow the sun. At night the pattern flips: the west side grows faster, and by dawn the head is pointing east again, ready to start over.

The researchers showed this is driven by the plant’s internal circadian clock, not just by chasing light. When young sunflowers were moved indoors under a fixed overhead light, they kept swinging back and forth for a few days anyway. Their bodies were running on schedule.

Tracking pays off. When the team staked plants so they could not move, those sunflowers ended up with less biomass and smaller leaf area than plants allowed to follow the sun.

So why do the adults stop? As the plant matures, stem growth slows, and without growth there is no bending. The flowers settle facing east, and that turns out to be a clever move. East-facing heads warm up faster in the morning sun, and in the study they attracted five times as many pollinating insects as west-facing ones. As Harmer put it, bees like warm flowers.

Why people get it wrong

The name does a lot of the damage, and the image of a big yellow face tracking the sky is irresistible. Most people only pay close attention to sunflowers when they are in full bloom, which is exactly the stage when the tracking has stopped.

The movement of young plants is also slow and happens partly overnight, so you would need to watch for hours, or film a time-lapse, to notice. The result is a half-true story that everyone repeats: sunflowers follow the sun. The full version is more interesting. They follow it as teenagers, then grow up, pick a direction and stick with it.

“Young sunflowers chase the sun like a puppy. Grown-ups face east, warm their faces and let the bees come to them. Wisdom, that is.”