Why Plants Do That

Why Do Young Sunflowers Follow the Sun?

A young sunflower faces east when the sun comes up. All day it turns, a little at a time, so that by evening it's facing west, where the sun goes down. Then, during the night, it swings all the way back to face east again, before the sun has even risen. Following the sun like this has a name. Scientists call it heliotropism.

How does a plant turn without muscles?

You turn your head with muscles in your neck. A sunflower doesn't have any muscles, so it turns by growing.

Think of the stem as having an east side and a west side. During the day, the east side of the stem grows longer than the west side. When one side of a stem gets longer than the other, the stem bends away from the longer side, so the top of the plant tips toward the west and follows the sun across the sky. At night it switches. The west side grows more, and the stem bends back toward the east.

The scientists who studied this, a team from the University of California, Davis and the University of Virginia, checked which genes were switched on in each side of the stem. Genes are the instructions inside every living cell. The genes that help a plant grow toward light were switched on differently on the east side and the west side, which fits with one side of the stem growing faster than the other. Their study was published in the journal Science in 2016.

How does it know to turn back in the dark?

At night there's no sun to follow, and the sunflower still turns back to face east before morning. It can do that because it has a clock inside it. Something in the plant keeps time on a cycle that's about a day long, and that cycle tells the stem when to grow on which side, even when the plant can't see the sun at all. That kind of inside clock is called a circadian clock.

The scientists found that the sunflower's circadian clock controls the daily turning, and that this daily turning was linked to the plants growing more leaves and stems.

Why grown-up sunflowers stop

Once a sunflower is fully grown, it stops turning, and its flower stays facing east.

Facing east turns out to help the flower. In the same study, east-facing flowers got more visits from pollinators, the animals that carry pollen from one flower to another so the plant can make seeds.

So the same inside clock does two jobs. It turns a young sunflower toward the sun every day while it's growing, and it keeps a grown sunflower facing east, where its flower gets more pollinator visits.

Try it together

If you can find a young sunflower, in a garden or in a pot outside where it gets sun all day, check which way its top is pointing at breakfast and again at dinner. Draw an arrow in a notebook each time. After a few days, look at your arrows and see whether the morning ones point one way and the evening ones point the other. Once the plant is fully grown, keep checking, and see whether it stops turning and keeps its flower toward the east.

My first children's book, Sunny and the Sunflower Squad, is coming soon.

Start a garden together

If you'd like to start a garden together, my Field Guide to Common Medicinal Herbs has a beginner foundations section and starter-garden guidance: kbaco.net/downloads/field-guide

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