Extension Educator, Horticulture
I could be the poster child for SAD - seasonal affective disorder. After a few gloomy days I'm as pale as lawn grass that's spent too much time under the kiddie pool.
It's not just the rain and clouds, but the shorter daylight. We get up in the dark and drive home in the dark. And to further depress us already SAD folks, it's just going to get worse until December 22. The winter solstice marks the shortest daylight of the year. The sun is at its farthest distance from the equator in the northern hemisphere. After December 22 periods of daylight gradually get longer until my favorite day, June 21, the longest daylight of the year. These days go virtually unnoticed by most people today. However to ancient agrarians, SAD support groups, and our plants the change in daylight is a reason to celebrate.
Winter solstice has been celebrated for thousands of years by cultures throughout the world. It was a time to celebrate light. The people of ancient Rome celebrated Saturnalia starting on December 17. The seven day festival was named in honor of Saturn, the god of agriculture, from whose name we get Saturday.
As far as we know, plants don't have any wild parties to mark the winter solstice but they do respond to the length of daylight. The response of plants to the relative length of day and night is called photoperiodism. Plants actually "measure" the duration of darkness rather than of light.
Photoperiodism influences many activities in plants including growth, seed germination, flowering, fruit development, and the onset of winter dormancy. Some plants need a specific period of day light or night length to initiate flowering. The time required can vary among plant species and plant varieties. Some plants require a specific light/dark period to flower or with some plants a change in light /dark period can hasten flowering but is not required.
In plants referred to as short day, their flower bud formation begins when daylight is relatively short about 10 to 12 hours and flowering is suppressed if daylight is longer. A few common plants that require short days/ long nights include poinsettias, chrysanthemums, kalanchoe, celosia, hyacinth bean vine and Spanish flag.
Long day/short night plants initiate flower bud formation once they are exposed to relatively long days and short nights, about fourteen hours or more and won't flower if daylight is shorter. Some of our common vegetables are long day plants such as beet, radish, lettuce, spinach and potatoes. Bulbing in onions is initiated under long days. The amount of daylight required depends on the variety. Other plants that require long days to flower include strawflower, dill, Chinese aster, bachelor's button, fuchsia, gazania, ipomopsis, sweet pea, flax, lobelia, monkey flower, love-in-a-mist, some petunias, Rudbeckia 'Indian Summer', and daisy.
Some plants don't care about day length to flower and are called day neutral plants. Of course all this plant response may change depending on temperature, humidity and maturity of the plants.
So what does all this mean to gardeners? Greenhouse growers have to manipulate light to get some plants to bloom or not bloom at the appropriate time. Poinsettia flowering can be disrupted with lights from parking lots. Short day garden plants may not flower well when planted near outdoor lighting. Hyacinth bean and Spanish flag vines may not bloom well twined around the outdoor light. Some vegetable plants that we don't want to flower may need to be planted later or earlier in the year to get the correct timing. If a plant isn't flowering well for you, include its photoperiod requirement on the list of suspects.
I must be a long-day gardener. I think I will pull up a lawn chair under the grow lights in my basement and wait for spring to shine.