Landscape Lighting and Plant Dormancy

If you have been in the public horticulture trade for any time at all, the question "What affect does landscape lighting have on plant dormancy?" Has probably come up. I am not sure my answers have always been equal to the question.

Dr. H. Marc Cathey wrote about some ARS research done under controlled conditions that I think addresses this question. He wrote: "Normally, trees and shrubs slow their growth as days shorten in the fall. Leaves of deciduous species shed, and growth above ground stops." The question is, do lights affect this normal hardening off process and dormancy.

" . . . we found that the standard mercury vapor and metal halide lamps used in street lights did not delay the onset of dormancy in plants in the fall of the year. High-pressure sodium lamps, however, supplied the red and far red region of the visible light spectrum, which delayed dormancy and promoted the continued growth of trees such as sycamores, elms, and zelkovas into the winter. . . . By placing deflectors in the street light fixtures, General Electric resolved most of our concerns and greatly reduced the likelihood that plants could be damaged." Symptoms Dr. Cathey lists for lighting problems include:

If you observe signs of light damage that you believe may be caused by incorrectly installed street light deflectors, call your local department of public works and ask if the deflectors can be adjusted."

This article is adapted with permission from the January/February 2003 issue of The American Gardener magazine, published by the American Horticultural Society at If you use it, please give credit to Dr. Marc Cathey the magazine and the society.

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North Carolina State University