Sunflowers are big and sassy, sporting heads that look like cheerful children’s drawings of the sun. They punctuate informal gardens with pizzazz and add shine to the landscape.
At a casual glance, the common sunflower seems simple. Look closer, though, and you’ll see that what appears to be a single flower is actually a collection of smaller flowers, or florets–two different kinds of them. Kathy Squiers, horticulturist for Washtenaw County parks, and Mike Palmer, horticulturist at the U-M’s Matthaei Botanical Gardens, educated us about these “composite” flowers.
The big disk in the center is composed of numerous tiny flowers, each complete with pistil, stamen, and the other anatomical features of flowers. Each little flower has an ovary, and each ovary has a single seed. These seeds have been a significant food crop in North America since long before Columbus.
Those iconic yellow rays surrounding the disk are a major part of a second set of flowers. The ray flowers are sterile but they attract pollinators to the fertile florets in the disk. And pollination is at the top of the sunflower’s wish list.
For much of its life a sunflower “follows the sun”: its head faces east in the morning and over the course of the day turns to face the setting sun. Come next morning, it is facing east again. The movement is caused by the elongation and contraction in cells in the plant’s stem, which are controlled by a combination of solar stimulus and an internal circadian clock.
Because the leaves also follow the sun, the movement allows the sunflower to capture more sunshine and so maximize growth. It also keeps the yellow ray flowers illuminated and the head warm, making it even more attractive to pollinators.
Most of the sunflowers we see, in various colors and ranging in size from dwarfs to giants, were created by growers working with the same species–the common sunflower, Helianthus annuus. The jazzy “Mexican sunflower” is a related species, Tithonia rotundifolia. This summer, there’s a nice patch of it in the Children’s Garden at Matthaei Botanical Gardens. It’s popular with monarch butterflies, as our image shows.
Matthaei has very few common sunflowers, but they’re abundant in the Project Grow garden at County Farm Park, where our other photos were taken–it’s worth the walk from the parking lot adjacent to Platt Rd. And for bicyclists, county naturalist Shawn Severance promises lots of sunflowers on the “Pedal to Pollinators Garden Tour” on September 9.
