Why Do Mature Sunflowers Usually Stop Following the Sun?

A sunflower in full bloom usually does not track the sun all day. The conspicuous tracking mainly occurs in growing young plants and buds. As the head matures and stem growth slows, movement diminishes and the head generally settles facing east. Sun-tracking describes behaviour at a developmental stage, not an unchanging action throughout the plant’s life.

Nor does the head actively twist like a neck. Researchers marked stems and filmed them, finding that the eastern side elongated faster during the day, pushing the tip westwards. At night, faster growth on the western side brought it back east. Light signals and the internal circadian rhythm jointly regulate this unequal growth. Plants moved into a growth chamber with a stationary light continued swaying for several days, showing that they were not simply chasing the brightest point at each moment.

Why settle in the east? As growth slows, responses to morning and afternoon light become asymmetric, and westward bending weakens. Facing east also has measurable consequences: heads warm more quickly in the morning and, in the experiments, receive more pollinator visits. Warming west-facing heads restored some of that attraction.

Mechanism and function should be distinguished. Unequal growth and circadian timing explain how the movement occurs; temperature and pollination experiments reveal consequences of orientation. Neither means that a sunflower ‘knows bees like warmth’, nor that every open flower behaves like a young plant.

https://www.ucdavis.edu/news/sunflowers-move-clock
https://www.ucdavis.edu/curiosity/news/why-sunflowers-face-east