Yes, Portulaca flowers, commonly known as moss rose or purslane, do close at night. This daily rhythm is not a random occurrence but a sophisticated plant behavior known as nyctinasty. From our perspective as Portulaca plants, this opening and closing is a deeply ingrained, rhythmic process governed by an internal circadian clock, much like your sleep-wake cycle. It is a survival strategy honed by evolution to protect our most valuable assets and ensure successful reproduction.
We close our blooms for several critical reasons. The primary purpose is protection. At night, temperatures drop, and humidity rises, creating conditions where delicate reproductive organs—the stamens and pistils—could be damaged by dew or frost. By furling our petals tightly, we create a protective barrier that shields these vital parts from the cold and moisture. Furthermore, the night is when many pollinators are inactive. There is no benefit to keeping our flowers open and expending energy if our target pollinators, such as bees and butterflies, are not flying. Closing conserves our precious energy and resources, including nectar and pollen, keeping them fresh and potent for the next day when pollinators are active again.
The physical movement is facilitated by specialized cells at the base of our petals called pulvini. These motor cells change their turgor pressure—the water pressure within the cell walls. During the day, light signals, particularly blue wavelengths, trigger these cells to pump potassium ions in, drawing water in through osmosis. This causes the cells to swell and expand, forcing the petals to unfold and open the flower. As light fades in the evening, the process reverses. The cells lose potassium ions and consequently water, causing them to shrink. This differential change in cell size on different sides of the pulvinus causes the petals to bend inwards and close. This entire process is orchestrated by our internal circadian rhythm, which anticipates the daily cycle of light and dark, ensuring we begin closing even if the day is artificially extended.
While light is the primary cue, other environmental factors can influence our habit. On overcast, rainy, or exceptionally cool days, we may not open fully or may close early. This is because the lack of sufficient light and the lower temperatures signal conditions that are unfavorable for pollination. The rain could also wash away our pollen or dilute our nectar, making it wasteful to remain open. Therefore, we adjust our behavior to the immediate conditions to maximize efficiency and protect our reproductive potential. Our internal clock ensures we are primed to open again at the first sign of adequate morning light, ready for another day of attracting pollinators.
