The Coastal Gardener:

On Monday morning, at precisely 9:47 a.m., our annual winter solstice will occur. At that exact hour, our sun will appear at its lowest point in the sky, and shadows will be longest. It’s also the day when our daylight hours will be shortest. From here on, the days get longer. What does this matter to a local gardener?

Day length is critical to the growth and life cycle of many plants.

Intuitively, most gardeners eventually realize the importance of knowing 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; sow too late and the plants will not have sufficient time to bloom, or fruit to mature.

So to succeed with many crops, gardeners have come to appreciate how the variable length of the day can spell success or doom.

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. In the coming year, these dates are:

Winter Solstice (Shortest Day), Dec. 21.

Vernal or Spring Equinox, March 20.

Summer Solstice (Longest Day), June 21.

Autumnal or Fall Equinox, Sept. 22.

Plants have been following the rules of day length since long before gardeners ever came on the scene. Novice gardeners sometimes struggle with this lesson, believing that temperature alone regulates a plant’s 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; planting lettuce, dill and cilantro in summer in cool shady spots, only to watch it immediately go to flower, is a result of day length, not temperature.

Plants controlled by day length 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 day length 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 lasts 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 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. Day length, along with temperature, light, water and nutrients, are just a few of the many environmental factors that affect your garden plants.

Ask Ron

Question: My plumeria leaves are still green and healthy. Should they have fallen by now? Should I be concerned?

Judy

Newport Beach

Answer: It’s not unusual for many plumerias in Orange County to retain their leaves well into winter, sometimes even longer. Don’t worry, they’ll drop their leaves whenever they are ready. In the meantime however, you should be watering your plumeria very little during the cool winter months. Overwatering in the winter is a popular cause of plumeria failure. Established plants in the garden probably need no water at all beyond rainfall. Potted plants may get a splash or two during a particularly dry period, but not much more. Resume watering and feeding in late spring when the new leaves begin to expand.

ASK RON your toughest gardening questions, and the expert nursery staff at Roger’s Gardens will come up with an answer. Please include your name, phone number and city, and limit queries to 30 words or fewer. E-mail stumpthegardener@rogersgardens.com, or write to Plant Talk at Roger’s Gardens, 2301 San Joaquin Hills Road, Corona del Mar, CA 92625.

RON VANDERHOFF is the Nursery Manager at Roger’s Gardens, Corona del Mar.