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 although it is the term of art, use of the term day length in reference to most flowering plant's lifecycles is somewhat of a misapprehension — in this respect: Scientific research has confirmed that it's the length of the dark periods, not the length of daylight periods, that predominantly controls plant growth. This fact was discovered long after day length became a widely used term in horticulture, and the term has stuck. Understanding the importance of dark periods can come in handy for the grower, though, because it can be used to modify plants' response to day length, leading them to 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 do not react to day length; they are called day-neutral.

Day length is a function of latitude; all locations having the same latitude coordinate 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 are to the equator, and in the summer, days are longer the farther you are from the equator.

Compare day length in Maine and Florida on the summer and winter solstices:

Most plants do not grow when day length is less than 10 hours. Even if the temperature is kept within the optimum range — for example, in a climate-controlled greenhouse — most plants will just sit dormant until the magic 10 hours of light per day arrives.

Compare dates when day length is less than 10 hours in several US cities:

Because most plants won't grow when they receive less than 10 hours of daylight, winter greenhouse production requires supplemental lighting, as well as, oftentimes, 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 generally has a stronger regulatory effect on a plant's life cycle than the light period: 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 greenhouse producers 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 a crop earlier 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 in the south.
• Basil does not 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 must be weighed carefully against 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 during short days and flowers when days are long. If you plant rudbeckia in early spring, you can grow big, healthy plants that send up bountiful long stems as the days get longer in summer. If you plant them in late summer, however, 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 aspects of food and flower crop production are affected by day length, temperature, or a combination of the two. If you have ever wondered why certain crops do not grow as well for you as they do in other parts of the country, day length may be a contributing factor.

If you are not attuned to the day length in your location, get a sunrise/sunset calculator or app, 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.