Margaret Lauterbach: Day and night matter to plants

Special to the Idaho StatesmanApril 4, 2014 

Solar lighting is an inexpensive way to illuminate the garden, but artificial lighting can play havoc with some plants’ growth and blooming cycles.

Solar lights are fun and inexpensive. If you have a power outage at night, you can bring one inside. Winter days are too short and winter sun isn't strong enough for your solar light to last long, though.

It's tempting to try to repel destructive wildlife by placing these lights in the garden, but be alert to a failure to bloom or thrive on the part of plants.

Most plants need a dark period of rest that may be upset by artificial lighting. This phenomenon, first observed in tobacco plants, is known as "photoperiodism." That also governs bulb set in short day onions, fruit set in strawberries, as well as time for blooming in many ornamentals and fruit trees.

It's startling to see apple blossoms in fall, knowing they won't be followed by fruit, but daylight hours of equal length do occur twice each year, spring and fall.

Our late spring and summer nights are long, too long for setting bulbs on onions such as Vidalia or Granex, for instance. Walla Walla Sweet onions were originally short-day onions, but have adapted over the years to form bulbs in long-day areas.

Nunya beans, or popping beans, grow in Peru, and only flower on short days. Trying to grow them here is futile, since they're trying to bloom in late fall, just in time to be killed by frost. Our lower altitude, compared to the Andes, may have contributed to their demise when I tried to grow them in my greenhouse.

Plants such as asters, chrysanthemums, and other fall-blooming plants rely on long nights (i.e., short daylight hours) to spur blooming. Adding supplemental lighting and covering plants to simulate darkness can force blooming at specific times and are common practices in nurseries.

Some folks have found remarkable growth occurs at night, measuring girth of melons each morning and each evening. Changes are only observed mornings. When girth reaches a plateau, the melon is ripe.

Nighttime darkness may govern growth in other crops as well. During very hot weather, for instance, stomata (pores) close during the heat to conserve moisture, and only open when temperatures moderate. The plant goes into semi-dormancy for the duration of scorching heat. Our cool nights are a distinct advantage over parts of the country that have hot humid days and nights, but lights may erase that advantage.

There's not much you can do about a street light if that's the source of light contamination, other than move the affected plants, but you can easily pull out and move solar lights. If you use landscape lights for security, perhaps putting them on a motion detector basis would be preferable to constant light.

Some plants will just fail to bloom if continuously lighted.

Another problem with lights in the garden is that they may repel night-flying insects such as some moths that would serve as pollinators if they weren't trying to avoid lights. As I recall, the miller moths that are parents of cutworms are attracted to lights, and that may cause a different problem for you in the garden. Nobody needs more cutworms.

There are tiny solar-powered lights available. A woman in Florida said she watched a doe step carefully into her garden in fading light, then startle and bolt at the sight of a tiny hummingbird light. Deer usually return and become accustomed to repellents, but one could move the hummingbird light from one place to another to fool them.

Send garden questions to melauter@earthlink.net or Gardening, Idaho Statesman, P.O. Box 40, Boise, ID 83707.

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