Gardening Etcetera: The wonder of sunflowers

In the cool darkness of a recent summer morning, I stared in wonder at the bright faces looking back at me. My young sunflowers (Helianthus annuus) in the grove were standing like soldiers waiting to salute the rising sun. During the previous day, their leaves and flowers tracked the sun as it made its daily voyage from east to west across the sky. And now under the cover of nightfall, they rotated their stems back to greet the sun again.

This remarkable ability of sunflowers to track the sun’s movement is called heliotropism. New botanical research on the subject has found that solar tracking is caused by different sides of the plant’s stem elongating at alternate rates throughout the day and night. In order to track the sun’s movements during the day, the growth rates on the east side of the stem are higher while the increased growth rates at night on west side allow sunflower stalks to turn and greet the dawn.