Do Sunflowers Follow the Sun? Yes, But Only When They Are Young

Its name says it all: the sunflower, in many languages a flower that turns with the Sun. In Turkish it is the “day-turner”, in French tournesol, in Spanish girasol. But walk past a field of sunflowers in full bloom on a summer evening and you may notice something odd: they are all looking the wrong way, towards the east. So do sunflowers really follow the Sun?

Yes, while they are young

The answer is yes, but only for part of their lives. Young sunflower plants, before their flowers open, move remarkably. In the morning their heads face east, towards the rising Sun. During the day they slowly turn, tracking the Sun across the sky to face west in the evening. Then, during the night, they swing back to face east again, ready for sunrise.

This sun-tracking is called heliotropism.

How a plant turns without muscles

Sunflowers have no muscles or joints. They move by growing unevenly. In 2016 a team led by Stacey Harmer at the University of California, Davis, studied the plants closely. They found that during the day the cells on the east side of the stem grow faster, bending the head towards the west. At night the west side grows faster, bringing the head back to face east.

Crucially, the timing is controlled by the plant’s internal circadian clock, the same kind of 24-hour rhythm that governs our sleep. When the researchers moved plants indoors under a fixed overhead light, they kept turning back and forth for a few days. When they tried artificial “days” that were much longer than 24 hours, the tracking fell apart. And young plants were already turned east before dawn, anticipating the sunrise rather than reacting to it.

Why mature sunflowers face east

As the plant matures, the stem stops growing, so the movement stops. The clock also changes its behaviour so that the head settles in its morning position. That is why almost every mature sunflower faces east.

The researchers wanted to know whether facing east brings any benefit. They turned some potted plants to face west and compared them with plants facing east. East-facing flowers warmed up faster in the morning sun, and bees and other pollinators visited them about five times more often. When the west-facing flowers were warmed with a heater in the morning, they attracted more visitors too. Warmth, it seems, makes flowers more attractive to insects that like to start the day warm.

Why track the Sun at all?

Tracking helps young plants grow. In the study, plants that were prevented from moving, by being staked in place or turned around so that they faced the wrong way, had smaller leaves and less biomass. Following the Sun appears to help young sunflowers capture more light while they are building up their leaves.

In short

• Young sunflowers follow the Sun from east to west and swing back east at night.
• They move by growing faster on one side of the stem, not with muscles.
• The rhythm is set by the plant’s internal circadian clock.
• Mature sunflowers stop moving and face east.
• East-facing flowers warm up faster and get about five times more pollinator visits.
• Tracking helps young plants grow bigger.