By Katie Jackson
Sunlight is essential to life, especially in the plant world. But some plants appear to not just need sunlight, they seem to worship it. This time of year, when the sun’s rays are most abundant, is an ideal time to watch those plants in action.
Before we talk about those plants, though, let’s talk a little science. As you probably know, the process of photosynthesis allows plants to capture light and use it to turn water, carbon dioxide and minerals into food. Because of this, many plants grow toward a light source, a process called positive phototropism. (By the way, negative phototropism, such as when plant roots grow away from light, is also a thing.)
These aren’t the only ways plants respond to the sun. A number of plants — morning glories, most daisies, tulips, magnolias, altheas, dandelions, some lettuces and many legumes like beans, peas and alfalfa — close their blooms and/or furl their leaves at night. Other plants such as moonflowers, evening primroses, datura, four-o’clocks and several lilies, orchids and cacti open as the sun goes down. Plants in either category are called nyctinastic and are responding to their innate circadian cycles rather than to light as a direct stimulus.
Still others actually pivot their blooms and leaves to track the sun from east to west, a reaction known as heliotropism. This category of plants includes young sunflowers (once sunflowers are mature, they stop tracking the sun and orient toward the east), buttercups, artic poppies and (as their name suggests) heliotropes. Heliotropism is believed to help with photosynthesis but also with reproduction—pollinators prefer warm flowers—and seed development.
Humankind has been fascinated with plants’ responses to sunlight for eons. In fact, Carl Linnaeus, the father of modern taxonomy, even designed a floral clock garden plan using plants that open or close at specific times of day to tell time simply by observing their activity. There’s no record that Linnaeus ever accomplished this feat, though several others tried with only little success because variations in temperatures, rainfall and other uncontrollable factors affected the accuracy of these timekeeping plants.
We, however, can use the fascinating responses plants have to sunlight as inspiration and entertainment in our own summer gardens — a chance to see science and beauty in action. Here are a few ideas.
• Plant sunflowers along fences, in garden beds, interspersed with other sun-loving plants in beds or a meadow setting or in small geometrical outlines to create a sunflower house. Sunflowers are available in an array of heights, flower sizes and colors (from bright yellow to reds, oranges and burgundies) and can be sown in successive plantings so something is blooming throughout the summer. They also provide food for birds and other wildlife, cut flowers for your table and hours of enjoyment.
• Create your own Linnaeus-style floral clock by using a selection of nyctinastic plants that open and close at regular hours such as lilies, primroses, four-o’clocks, marigolds and moonflowers. (Learn more at www.linnean.org, The Linnean Society of London’s website.) Or design a botanical timepiece concept as a garden feature using sun-loving species that can be planted in-ground, in raised beds or in containers in a clock-face or a sundial style arrangement.
• Group a selection of heliotropic and nyctinastic plants in a sunny spot in the yard or mingled in with other summer perennials, including daylilies. (By the way, daylilies are not nyctinastic, they simply produce blooms that last a single day, though reblooming and everblooming cultivars are available.)
• Enjoy the patterns of your own garden by stopping long enough to pay attention to plants at different times of day and night, including the peas and beans in your vegetable garden.
• Worship the sun by planting summer and fall-producing vegetables such as tomatoes, okra, eggplants, peppers, melons, field peas, squash, corn, sweet potatoes and pumpkins.
These are just a few ideas of ways to worship the sun and the remarkable natural rhythms of the plants we adore in our own landscapes. Many ideas are available in books, periodicals and online and through nearby gardening experts such as Master Gardeners, landscape professionals and local garden centers.
Katie Jackson is a freelance writer and editor based in Opelika, Alabama. Contact her at katielamarjackson@gmail.com.
