The Effect of Shorter Daylength on Winter Production

Temperature and day length are the two main environmental factors driving plant growth. Temperature is simple: each species has a range in which it grows and an optimum in which it thrives. Day length is trickier, particularly in combination with temperature, and understanding how the two interact leads to better season extension and variety choices.

First, although it's the standard term, day length is a bit misleading when it comes to the life cycles of most flowering plants. Research has shown that the length of the dark period, rather than the light period, is what mainly governs plant development. That was discovered long after "day length" became standard horticultural language, so the term stuck. Knowing that darkness is what counts is useful, though, because growers can use it to change how plants respond and get them to bloom out of season. More on that below.

Many plants rely on day length cues to switch between leafy growth and flowering. These are long-day or short-day plants. Plants that don't respond to day length are called day-neutral.

Day length depends on latitude: every place at the same latitude gets the same amount of daylight on a given date. At the spring and fall equinoxes, day and night are each about 12 hours. The winter solstice brings the shortest day and longest night, and the summer solstice the longest day and shortest night. In winter, days are longer nearer the equator; in summer, they're longer farther from it.

Here's how Maine and Florida compare at the two solstices:

Growth of most plants slows to a crawl when days drop below about 10 hours. Even with ideal temperatures, as in a climate-controlled greenhouse, many plants barely grow until daylight climbs back above roughly 10 hours.

Here's when daylight is under 10 hours in a few US cities:

Since most plants make little growth with under 10 hours of daylight, winter greenhouse production calls for supplemental lighting and often extra heat. Lights can come on shortly before sunset to lengthen the day, or briefly in the middle of the night, a technique called night-interruption lighting.

That's where the dark period comes in again, since it usually has more influence on a plant's development than the light period. Winter's short days mean long nights. When a grower interrupts those long nights with light, some plants respond as if nights were short and days long, just as in midsummer. Many bedding plant and cut-flower greenhouses use night-interruption lighting to bring flowers into bloom in winter.

Day length matters even if you never use supplemental light. Here are a few examples of how it can affect your results:

• Cauliflower is more about temperature than day length: it forms its best heads in cool weather. Northern growers often get that cool weather during heading more reliably than southern growers, which is why cauliflower tends to be easier in the North than in the South.
• Basil grows poorly in the short, dim days of winter, even in a warm greenhouse. Good winter yields require supplemental light and heat, and in most areas the energy cost needs careful comparison with what the crop will earn.
• Flower growers feel the effects of day length most, since many popular cut flowers have day length triggers. Rudbeckia, for instance, grows leaves while days are short and blooms when days are long. Sown in early spring, it makes big, healthy plants that send up plenty of long stems as summer days lengthen. Sown in late summer in hopes of fall flowers, it gives only a few blooms on short stems because days are too short to trigger flowering.

Plenty of other aspects of food and flower growing depend on day length, temperature or both. If you've ever wondered why a crop does better elsewhere in the country than for you, day length may be part of the answer.

If you don't know your local day lengths, use a sunrise/sunset calculator or app and mark the dates when you reach 10, 11, 12 hours of daylight and so on. In time you'll notice patterns between day length and what's happening in your garden.