Are you a person who gets really hungry at 10 a.m. on the dot? You’re not unusual. I used to think this happened to me because I get up and eat breakfast at an insanely early hour in the morning, so 10 o’clock really seems a lot later. Fortunately for me, scientists are constantly in pursuit of making people feel better about why their bodies do strange things. Research has shown that digestion is actually strongest between 10 a.m. and 2 p.m.

What in the world does all this have to do with gardening? I am just trying to point out that all natural organisms respond to life in cycles. We call these cycles circadian rhythms. It’s perfectly natural for your body to become hungry at 10 a.m. every day because that is when digestion is really kicking in. That’s when your body can most efficiently digest food, so that’s when it wants you to eat. This is a rhythmic occurrence; that’s why it happens on a daily basis at exactly the same time.

Plants, like us, respond to life with circadian rhythms. Their physiological processes are constantly changing, responding to light, temperature and moisture. These changes, however, are not chaotic. They are systematic and rhythmic. Rhythmic changes occur on a daily and seasonal basis. Many interesting rhythmic changes in plants are brought about by light.

Light has three primary characteristics which affect plant growth — the quantity, quality and duration of light. Quantity refers to the intensity or concentration of light. It is understandable how plants respond in this case. More growth occurs (up to a certain point) in the presence of more light. Light quality refers to the wavelength (and hence, color) of light which reaches the surface of plant leaves. The duration of light refers to the photoperiod which a plant is exposed to.

When photoperiodic responses in plants were first studied, researchers believed changes (such as flower development) were brought about by the length of time plants were exposed to sunlight. It has since been discovered that plants respond to the length of uninterrupted dark periods. We still maintain the original terminology, however, when we refer to plants as “short-day,” “long-day” or “day neutral.” A more accurate description would be to call them “long-night,” “short-night” or “light-neutral” plants.

At this time of year it feels as though the sun hardly creeps above the horizon before it dips back down, leaving us in darkness. Most organisms (including us) are very sensitive to its short-lived presence, however. Some plants, like poinsettia, chrysanthemum and Christmas cactus, produce flowers this time of year when the day length is less than around 12 hours in duration. These are all classified, therefore, as short-day plants. On the other hand, plants like phlox, daisy and potato are long-day plants — they flower when day length exceeds 12 hours in duration. Yet other plants, such as geranium and petunia, flower regardless of day length. Generally, however, these plants flower more profusely during long days.

Knowing how plants respond to day length can be of practical use to gardeners. This information can come in handy when planning a landscape. Planting a variety of plants will allow you to have flowers the whole season long. Take a look around throughout the season and watch your house plants carefully to see if you can determine how your special plants respond to light.

Diana George Chapin is the NEWS garden columnist.