As Daylength Changes, Garden Plants Respond
Intuitively, most gardeners eventually realize the importance of knowing the where in the year they are. Seeds need to be planted at the right time. Sow too early and improper weather will destroy the crop or foil the flowers; too late and the plants will not have sufficient time to bloom or fruit mature.
So to succeed with many crops, flowers and other plants gardeners have come to appreciate how the variable length of the day can spell either success or doom for one's plants.
The two most important dates are the shortest day of the year and the longest. The shortest day, Monday, tells gardeners that it is midwinter and the longest day, the summer solstice, says midsummer. Two other dates can also be worked out from day length, the spring and fall equinoxes, when days and nights are equal length. In the coming year these dates are:
Winter Solstice (Shortest Day) 21st December
Vernal or Spring Equinox 20th March
Summer Solstice (Longest Day) 21st June
Autumnal or Fall Equinox 22nd September
Garden plants have been following the rules of daylength since long before gardeners ever came upon the scene. Novice gardeners sometimes struggle with this lesson, believing that temperature alone regulates a plants growth and flowering Mistakenly, they believe that keeping their plants a bit warmer in the winter or cooler in the summer will extend their bloom period or harvest. They plant their sweet pea seeds earlier and earlier, in a vain attempt for earlier blooms. During our warm summers, these same enthusiastic neophytes try to cool their spring plants. Vegetables and herbs are where I see this most often; folks planting lettuce, dill and cilantro in summer in cool shady spots, only to watch it immediately go to flower, a result of daylength, not temperature.
Plants that are controlled by daylength can be classified as either short-day or long-day. Short-day plants include many spring and fall flowering plants such as chrysanthemum and poinsettia. Long-day, (short nights) plants include almost all of the summer-flowering plants, as well as many vegetables including beet, radish, lettuce, spinach, and potato.
Different plants react to daylength differently. Onion varieties illustrate this well, forming a bulb only when the number of daylight hours reaches a certain level. Short-day onions, like Yellow Granex (aka Vidalia), Texas Supersweet and White Bermuda, form bulbs when daylight is between 12 and 14 hours. On the other hand, long-day onions, like Walla Walla and Spanish, only bulb when the day length reaches 14 to 16 hours. In other words, planting Walla Walla onions in Orange County isn't a good idea, but Yellow Granex is fine.
So northern gardeners should plant long-day onions. In the North, daylight length varies greatly as you get farther and father away from the equator. Winter days are short, but summer days are very long. Long-day onions will have a chance to produce lots of top growth (hence bigger bulbs) before the day length triggers bulbing. If short-day onions are grown in the North, the onions bulb up too early and are too small.
Southern gardeners, on the other hand, should only plant short-day onions. In the South, there is less variation in seasonal day length. When long-day onions are planted here, they don't get enough daylight hours in summer to trigger the bulbing process.
A little knowledge of the basis of a plants' germinating, growing, flowering, seeding or even dieing can help you succeed in a garden. Daylength, along with temperature, light, water and nutrients, are just a few of the many environmental factors that affect your garden plants.
Ron Vanderhoff is the Nursery Manager at Roger’s Gardens in Corona del Mar.
Questions from Readers December 19th.