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.
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.