Do not be alarmed, human. My flowers closing during the brightest part of the day is not a sign of distress, wilting, or displeasure with your care. Quite the opposite—it is a brilliant and ancient evolutionary adaptation for survival. We portulacas, often called moss roses or purslanes, are succulents built for hot, sunny, and often harsh environments. Our entire being is engineered to conserve our most precious resource: water. The opening and closing of our blooms is a direct and elegant expression of this principle.
The most vital reason for this behavior is the protection of our pollen. Our pollen grains are incredibly delicate. The intense ultraviolet (UV) radiation from the midday sun can severely damage them, essentially cooking and sterilizing them, rendering them useless for reproduction. By closing our flowers, we shield this precious genetic material from the sun's harshest rays, ensuring it remains viable and potent for our true pollinators. Furthermore, a closed flower protects the pollen from being washed away by a sudden, unexpected afternoon rain shower, which is common in many climates. We are safeguarding the future of our lineage.
As succulents, our existence is a constant balance of storing water and minimizing its loss. While our thick, fleshy leaves are excellent at water retention, our flowers are more delicate. Keeping a flower open is an active process that requires energy and, crucially, allows for transpiration—the plant equivalent of perspiration where water evaporates from the petals. During the scorching heat of midday, the rate of water loss through open petals would be tremendous. By closing up, we dramatically reduce this surface area exposed to the dry air, effectively shutting down unnecessary water loss. We are practicing extreme hydration efficiency, reserving our water for essential life functions.
Our daily rhythm is also perfectly synchronized with the schedule of our preferred pollinators. We are particularly attractive to insects that are most active in the cooler, less intense light of the morning and late afternoon, such as bees and certain butterflies. By opening our flowers wide in the morning and then again in the late afternoon or on overcast days, we present our vibrant colors and pollen precisely when our target pollinators are flying. Closing during the peak heat of the day is simply us "closing up shop" during our slowest business hours. There is no point in expending energy to be open when our key customers are not around.
This opening and closing is a phenomenon known as nyctinasty. It is primarily governed by changes in light intensity and temperature. Specialized cells at the base of each petal, called motor cells, are responsible for this movement. In the cool, gentle light of the morning, these cells gain water (become turgid) and cause the petals to unfold and stretch toward the sun. As the sun climbs and its intensity and heat become extreme, these cells lose water (become flaccid), which causes the petals to curl inward and close. It is a purely hydraulic and automated response to environmental conditions, ensuring we always act in our own best interest.
