The two primary environmental factors that affect plant growth are temperature and day length. Temperature is easy enough to understand: Every plant species has a temperature range in which it will grow, and optimum temperatures in which it will thrive.
Day length is a little more complicated, especially in combination with temperature. Understanding the relationship between the two can lead to more successful season extension and variety selection.
The first thing to know is
that the term day length is a misnomer. Scientific research has confirmed that it's actually the length of the dark periods, not the length of daylight periods, that controls plant growth. This fact was discovered long after day length became a widely used term in horticulture, and the name has stuck. Understanding the importance of dark periods can come in handy for the grower, though, because it can be used to trick plants about day length and force them into bloom outside their normal season. More on that later.
that the term day length is a misnomer. Scientific research has confirmed that it's actually the length of the dark periods, not the length of daylight periods, that controls plant growth. This fact was discovered long after day length became a widely used term in horticulture, and the name has stuck. Understanding the importance of dark periods can come in handy for the grower, though, because it can be used to trick plants about day length and force them into bloom outside their normal season. More on that later.
Many plant species have day length triggers that determine when they grow vegetatively and when they bloom. They may be long-day plants or short-day plants. Some plants don't react to day length; they are called day-neutral.
Figuring day length
Day length is a function of latitude all places on the same latitude have the same amount of daylight on any given day. Day is equal to night 12 hours each at the two equinoxes, which mark the beginning of spring and the beginning of fall. The winter solstice marks the shortest day (longest night), and the summer solstice marks the longest day (shortest night) of the year. In the winter, days are longer the closer you get to the equator and in the summer, days are longer the farther you get from the equator. Compare day length in Maine and Florida in summer and winter:
| Location | Day length at summer solstice | Day length at winter solstice |
| Portland, Maine | 15 hrs., 26 min. | 8 hrs., 55 min. |
| Miami, Florida | 13 hrs., 45 min. | 10 hrs., 32 min. |
Most plants don't grow when day length is less than 10 hours. Even if the temperature is kept within the optimum range in a climate-controlled greenhouse, most plants will just sit dormant until the magic 10 hours of light per day arrives. See the chart below for examples of the period when day length is less than 10 hours.
| Location | Dates when day length is less than 10 hours |
| Atlanta, Georgia | December 8 to January 4 |
| Washington, D.C. | November 19 to January 26 |
| New York, New York | November 14 to January 30 |
| Portland, Maine | November 8 to February 4 |
Because most plants don't grow with less than 10 hours of daylight, winter greenhouse production requires supplemental lighting as well as supplemental heat. Lights can be turned on shortly before sunset to extend the length of the day. Or they can be turned on in the middle of the night for a short period of time, a procedure called night-interruption lighting.
This brings us back to the fact that it's the dark period that's important to plants: The short days of winter have long nights. If a grower breaks up those long nights by turning on lights in the middle of the night, some plants will act as though the night is short (and therefore the day is long) and behave just as they would in the middle of summer. Many bedding plant and cut-flower greenhouses use night-interruption lighting to force flowers to bloom in winter.
Even for growers who don't use supplemental lighting, the facts about day length are pertinent. Here are some examples of why day length may be a factor in gardening success:
- Cauliflower starts to develop a head when days get shorter. That happens sooner in northern than in southern regions, which means cauliflower will produce sooner in Maine than in Virginia. Cooler temperatures during head development also lead to better flavor. So cauliflower is an easier crop in the north than the south.
- Basil doesn't grow during the short days of winter, even in a tropical greenhouse. Supplemental lighting and heat are required to get good winter yields, and in most places the energy costs are not justified by the income from a basil crop.
- Flower growers are especially subject to the rules of day length because many of the most popular cut flowers have day length triggers. Rudbeckia, for example, grows vegetatively in short days and flowers under long days. If you plant them in early spring, they will grow big, healthy plants and send up bountiful long stems as the days get longer in summer. But if you plant them in late summer hoping for a fall crop, you will get few flowers on very short stems because the day length is too short to trigger blooming.
Many other facets of food and flower production are affected by day length, temperature, or a combination of the two. If you have ever wondered why certain crops don't grow as well for you as they do in other parts of the country, day length may be a contributing factor. If you aren't attuned to the day length in your location, get a sunrise/sunset calculator and mark your calendar with the dates when you have 10 hours of daylight, 11 hours, 12 hours, and so on. Over time, you will begin to notice correlations between day length and your garden's activity.
By Lynn Byczynski
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