On the Clock

  • By Design Publishing
  • 02/19/21

Just like humans and animals, plants have an innate ability to measure time. While they don’t have a central nervous system, they do tune themselves to a circadian rhythm much as we do. These circadian rhythms—or biological clocks—respond to light and temperature signals to regulate essential plant functions such photosynthesis, seed germination and growth, opening and closing their leaf pores, flowering, and senescence. Scientists were surprised to learn that even a plant’s roots actively respond to circadian signals.

While the botanical wonder of internal, synchronized clocks is nearly universal, there are certain plants that are more obvious in their response to changing light and temperature. Many add beauty to your garden and offer fascinating talking points when you entertain guests.

NOCTURNAL PLANTS

Some flowering plants are nocturnal, revealing their floral charms to the world at dusk or after dark. They are ideal to grow if you spend all day at work and tend to your garden in the evening. Often these night-bloomers have pale-colored flowers that make them more visible in the dark for pollinators, while their signature heady aroma is an additional “come hither” enticement to moths and bats.

Excellent picks in the nocturnal group of plants include both Datura and Brugmansia (both go by the common name angel’s trumpet), evening primrose, certain water lilies, tuberose (Polianthes tuberosa), evening stock (Matthiola longipetala), night-blooming jasmine (Cestrum nocturnum), and chocolate daisy (Berlandiera lyrata).

NYCTINASTIC PLANTS: SLEEPING BEAUTIES OF THE GARDEN

It is not fully understood why some plants are nyctinastic, closing their petals and leaves at night in a quasi-dormant state. One theory postulates they do it to keep their pollen dry, protecting it from nighttime dews. Dry pollen keeps its viability longer and is lighter than wet pollen, thus making it easier for insects to distribute. Charles Darwin thought that some plants close up at night to protect themselves from the cold; an unlikely theory as nyctinastic plants still close in the evening in regions where the nights remain warm. Another theory suggests that these plants are conserving energy for the daytime when pollinating insects are most active, or that they are protecting the pollen from unwanted pests and “night robbers.”

Some nyctinastic plants, such as morning glories, tropical hibiscus, and daylilies, flower over an extended period, but each flower lasts only a day. For others, such as California poppies, rose of Sharon, tulips, Osteospermum (African daisies), Oxalis (purple shamrock), spring beauties, crocus, daisies, Portulaca grandiflora (moss rose), sundrops, lotus flowers, and others, the flower reopens in the morning.

Although you won’t want to set your clock by a plant’s behavior, many plants can at least give you a sense of the time of day.

stellar nocturnal flowers

Moonflower (Ipomoea alba): Aptly named, moonflowers have large, round flowers that are typically white. To optimize your enjoyment of these romantic blooms, settle yourself comfortably near the plant in the evening and watch the pointed bud unfurl before your eyes. When at last the petals break free from their confinement, they make an audible popping sound. Originating in the tropical regions of North America, this perennial vine is typically grown as an annual from seed, and flowers best in regions where the daylight hours are short.

Night-Blooming Cereus: This is the common name for several genera of ceroid cacti that flower at night. For fans of the book and film Crazy Rich Asians, the night-flowering tan hua featured in the story is Epiphyllum oxypetalum. The Chinese name translates to “brief moment of glory”—or more idiomatically—“flash in the pan.” A poetic English common name is Queen (or Princess) of the Night. Whatever you choose to call it, the white, five-inch diameter flowers have a cluster of stamens and a sweet aroma. One genus, Selenicereus grandiflorus, blooms just once a year for one night only.

Four O’Clock (Mirabilis jalapa): The botanical name mirabilis means admirable, referring to the fact that different colored flowers, usually pink, white, and yellow, grow simultaneously on the same plant. Sometimes an individual flower will be splashed with different colors. As the name suggests, the fragrant flowers on this bushy perennial open late in the afternoon. If you live in USDA Zones 9 through 11, grow it as a perennial. In colder zones, grow it as an 

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