Will a Streetlight Affect My Garden?

High pressure sodium lights are among the worst type of lighting for trees.

Streetlights, landscape lighting, even lighting from windows, cars and billboards all add up to a blanket of light surrounding our planet. What some call "light pollution" can adversely affect trees and plants. A single streetlight aimed at a tree can disrupt its entire life cycle. Herbaceous, flowering plants and the insects and animals that pollinate them can also be affected by night light.

Light Pollution and Plants

Plants are highly attuned to their environments. Much of what a plant encounters during its life is variable: Water nutrients, air pollutants, disease and pests don't occur with perfect regularity. The length of days relative to the season, however, is predictable. Many trees, shrubs and herbaceous plants are dependent on cues of light, or lack of light, to induce dormancy or flowering. Even a tiny interruption during what should be a dark period is enough to throw some plants out of sync and cause them serious damage.

Dormancy

Issues of dormancy generally affect trees and shrubs. Short-day trees and shrubs flower in the fall and enter dormancy as a response to shortening days. Long-day trees and shrubs flower in the spring in response to lengthening days and enter dormancy in the fall when days shorten. Day-neutral plants are not affected by day length. Trees and shrubs may be triggered by streetlights to go into dormancy late or leaf out and flower too early, leaving them susceptible to frost injury. Trees and shrubs near a streetlight may also have larger leaves and be more susceptible to damage from air pollution.

According to the American Horticultural Society, most streetlights have deflectors that can be turned away from trees if necessary. They recommend contacting your local public works department to have them adjust the deflector if your trees and shrubs are being adversely affected.

Flowering

Some well-known flowering plants are sensitive to the presence of night lights as well. These plants require a period of darkness to stimulate blossoming. Poinsettias, kalanchoe and Christmas cactus might be affected by a streetlight shining through the window and refuse to bloom. In the outdoor garden, chrysanthemums are the most notable plants requiring dark nights to trigger blooming. Keep your indoor garden in a dark room if you can't control exposure to night light with curtains or shades, and plant chrysanthemums away from any source of light at night.

Pollinators and Night Blooming Flowers

Light pollution, even from a single streetlight, can affect your moon garden as well. Night flowering plants, like night blooming jasmine, cereus and datura, may not bloom at all if exposed to unnatural light at night. This could, in turn, adversely affect nocturnal pollinators like moths and bats. According to Canada's Astro Lab, light pollution caused the decline of numerous flowering plants by altering the migration of their moth pollinators. Ensure night flowering plants are planted away from any night lights that may disrupt their blooming.

References

• The American Horticultural Society: Avoiding Plant Injury From Landscape Lights
• Purdue University: Does Night Lighting Harm Trees?
• Washington State University: The Myth of Night Light
• Iowa State University Extension: Growing Chrysanthemums in the Garden
• Florida Atlantic University: Light Pollution Harms Plants in the Environment
• Astro Lab: Light Pollution