How Long Is a Day? Home Gardeners May Need to Know

Ancient people became experts on day length because their crops often depended upon it. Home gardeners these days often overlook the phenomenon, but need to recheck it when puzzled over failure with flowers or vegetables.

Onions are the classic example of how different varieties responded differently to short-day or long-day hours.

But many other plants also vary in their response. Short-day species include chrysanthemum, poinsettia and corn. Long-day examples are cabbage, beets and roses.

So day length needs to be considered, regardless of where you garden.

In subtropical Arizona, where I garden, to secure onion bulbs it’s necessary to plant short-day types (like Granex, Bermuda and Grano) in November and long-day types (Sweet Spanish or Granada) from late January to mid-February.

If planted here in the fall, long-day types will develop a large, thick neck rather than a bulb. They need more daylight than is available. But if short-day types are planted in the fall, bulb development starts in February, when daylight ranges from nine to 12 hours, and continues until they mature in mid-spring. Long-day bulbs mature around mid-July from the January-February planting.

In northern areas, day length may be around 14 hours in February, so the planting process must be reversed.

While the system is not perfectly predictable, it is accurate enough to suggest plants that will do well or poorly under most conditions. Refinements also include, for example, delaying or hastening the flowering of chrysanthemum by covering with black cloth (to shorten the day) or adding incandescent light (to lengthen the day).

Elaborate tables are available showing the amount of daylight hours for key date outdoors in various latitudes. But this was not always the case. The prehistoric Hohokam Indians built a four-story building using Arizona desert materials. Much of it stands today, preserved as the Casa Grande national monument.

Why it was built is subject to some debate. But at certain times of the year, and only then, sunlight floods certain rooms through tiny slits. Most observers believe this was a signal that it was time to plant crops.

The Hohokam seem capable of such engineering. Large sections of the Phoenix metropolitan area’s water distribution system still follow the routes of their irrigation canals.

Some observers believe Stonehenge in England was built for the same reason as Casa Grande.

Robert Johnson Jr., president of Johnny’s Selected Seeds, (Foss Hill Road, Albion, Maine, 04910), offers an intriguing look at day length in the company’s newsletter.

“Every place in the world receives about the same number of daylight hours in the course of a year,” he says. “It is only the seasonal distribution of daylight that is different.”

The number of daylight hours varies according to latitude and time of year except on the equinoxes, March 21 and Sept. 22, when there are about 12 hours of daylight everywhere on Earth.

The farther from the equator, the longer the day from spring through summer. The longest day is June 21, the first day of summer.

“On that day, the farther north (for the northern hemisphere) the greater the number of hours of daylight, up to the latitude of 66.5 degrees,” Johnson says. “From there on to the North Pole, it is continuous daylight, or on Dec. 22, it is rather, continuous darkness.”

The difference between winter and summer daylight varies according to latitude. Buffalo, N.Y., at approximately 43 degrees north “has the sun more than an hour longer than Houston (approximately 30 north) in the early summer and similarly shorter in winter.”

With short-day species, the flowering and reproductive phase of growth is hastened by fall and winter approaching and by growing them closer to the equator (shorter days during the spring and summer growing seasons).

“If the climates are the same at two locations, the same variety of corn grown in the southern United States will mature faster but be shorter with a smaller ear than when grown in the northern United States,” Johnson says.

“In the north, the longer summer days delay maturity but result in a longer vegetative growth period, making a taller plant and longer ear, but a later maturity.”

Local Cooperative Extension Service offices or state land-grant universities should be able to supply--or advise how to find--charts giving duration of daylight for your latitude at representative dates.

They also should have lists of recommended short-day and long-day varieties, which sometimes are referred to as early or late maturing.