Young sunflowers follow the Sun. Grown ones face east for a reason.
Young sunflower heads track the Sun from east to west each day, then swing back overnight, guided by an internal clock. Once they bloom they stop, and almost all face east. A 2016 study showed why: east-facing flowers warm up in the morning sun and attract more bees.
It is a common belief that sunflowers follow the Sun throughout their lives. The truth is more interesting. Young sunflower plants do track the Sun: their heads face east at dawn, follow the Sun westward during the day, and swing back to face east again at night, ready for sunrise.
In a 2016 study in Science, Hagop Atamian, Stacey Harmer and colleagues at the University of California, Davis, showed how this works. The stem grows unevenly: during the day, the east side grows faster, bending the head westward; at night, the west side grows faster, bringing it back. The rhythm is controlled by the plant's circadian clock, which anticipates the dawn rather than simply reacting to light.
As the plant matures and its flowers open, the tracking stops and the head settles facing east. The researchers found an advantage. East-facing flowers warm up faster in the morning, and pollinating insects prefer warm flowers: in their experiments, east-facing heads received several times more bee visits than those turned to face west. When the scientists warmed west-facing flowers with a heater in the morning, visits increased.
It is a neat example of a plant using timekeeping and growth to shape its world. It also offers a practical gardening tip: if you want to enjoy the faces of your sunflowers, plant them where you can see them from the east.
Source: Atamian et al. (2016), Science
