As their name suggests, sunflowers do love the sun. Young sunflowers face east at sunrise and turn west throughout the day. This motion allows growing sunflowers to soak up maximum sunlight that they can then convert to energy through photosynthesis.
It might look like your sunflowers move because of the sun’s light, but this motion is actually caused by an internal circadian clock.
According to UC Davis sunflower stems grow so tall thanks to a plant hormone called auxin. This hormone is tied to sunflowers’ circadian clock; it’s like a 24-hour timer that syncs the plant with light and temperature changes.
For example, a circadian clock can tell flower petals to close up at night. But the clock is solar powered so a few days without sun will mess up its rhythm.
In the sunflower’s case, its’ internal clock tells the stem to grow faster on one side and then the other. The uneven stem growth is what tilts the sunflower’s face left and right throughout the day.
You can tell sunflowers follow their clock, not the sun, by watching how they move at night. Young sunflowers end the day with their faces pointed west. Overnight, they make a 180-degree turn in order to be facing east for sunrise.
Fully-grown sunflowers don’t always face the sun. Instead, they will settle with their faces pointing east. As their growth slows sunflower plants focus on fruiting, and sunflowers’ circadian clocks switch to focus on morning sunlight only.
Why east in particular? Sunflowers that face east heat up faster in the mornings, and warm flowers can attract five times as many pollinators. Mature sunflowers settle facing east to encourage bees to visit during the all-important pollination season.
Do Sunflowers Face Each Other On Cloudy Days?
Maybe you’ve heard sunflowers turn to face each other on cloudy days. In reality, this is just a myth. Sunflowers don’t turn to “reflect” light toward each other. Even when it’s overcast, their best source of energy is still the sun.
Fully-grown sunflowers can’t turn their faces at all, since their stems are done growing. They’ll always face east to capture the morning sunshine.
Young sunflowers might face each other for a few hours when it’s cloudy, but that’s because their internal clocks are set using direct sun.
Without clear skies that let them track the sun, sunflowers will try to maintain their circadian cycle for a few days. After about 48 hours with no cues from direct sunlight, a sunflower’s movement cycle slows down and eventually stops.
At this point sunflowers are conserving energy since their faces can’t soak up the sun’s rays directly.
When you see two young sunflowers facing each other on a cloudy day, the real explanation is simply that one flower adjusted to lack of direct sun faster than the other.
Check your sunflower patch after another overcast day; chances are, both of those flowers will be pointed toward the eastern sky in anticipation of the sun’s return.
Do Sunflowers Share Energy When There Is No Sun?
Unfortunately, sunflowers can’t share energy with each other in place of the sun. There’s just no substitute for sunlight when it comes to photosynthesis, the process that turns solar energy into plant fuel.
Remember that even if two sunflowers are turned with their faces pointed toward each other, this is just their circadian clocks adjusting after a couple of overcast days.
Your sunflowers do need full sun to grow their best, though. In fact, the majority of their nutrition comes from absorbing sunlight. This is also why sunflowers’ internal clocks follow the sun so closely during their growing period.
Although common sunflowers can’t reflect the sun to share light, some sunflower species share energy in other ways.
The Jerusalem artichoke is a tuberous-type sunflower that grows perennially. Unlike annual sunflowers, these spread and multiply by underground rhizomes.
Since one large root can support 6 stems, this sunflower variety uses their roots to share energy. Click the link to read more about the Jerusalem artichoke where the plant is listed as a weed by Ohio State University Extension.
What Do Sunflowers Do When There Is No Sun?
Sunflower leaves can still photosynthesize on cloudy days or in the shade. These plants also have a built-in protection system that prevents damage from too much direct sun.
If sunflowers absorbed sunlight faster than they could convert it into energy, their leaves would get damaged or even die.
When clouds pass over the garden, sunflowers will just switch this shade system off until the sun comes out again. This internal shade protection does take a while to adapt (a lot like their circadian cycles). Lots of clouds between sunny days can slow your sunflowers’ growth as they struggle to adjust.
Too much cloud cover will usually force growing sunflowers into energy conservation mode. After several days, their circadian movement stops and stem growth pauses. They’ll stand with their faces straight up or to the east until the sun reappears.
Since sunflowers rely on direct sun, planting them in the shade will stunt sunflower height. Mature sunflowers will also miss out on the sun warming that attracts pollinators like bees.
For the tallest and healthiest sunflowers, you’ll want to plant them in the sunniest part of the garden. Sunflower containers are a flexible option for sunny porches or patios.
Wacky Tip – There’s even a solution for cloudy spells during pollen season: a portable heater pointed toward your mature flowers will bring back pollinators in droves.
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