Landscape Lighting

Occasionally questions come to Hortline regarding the use of security or landscape lighting. Clients are concerned about the effects this lighting may have on plants in the landscape. To understand how these types of lighting may affect the plant we need to understand how plants use light. Light influences plant growth through quality (wavelength or color of the light spectrum), intensity (irradiance), and lighting duration. Light visible to the naked eye occurs in the wavelengths from 380 to 760 nanometers (nm). The wavelength of 380 nm is visible to us as violet. The wavelength of 760 is visible to us as red. Wavelengths in between are seen as the colors of the rainbow, the light spectrum.

Light controls many plant processes. For example, light in the 400-450 nm (blue) and 625-700 nm (red) wavelengths is required for photosynthesis. Photoperiod responses occur in the red wavelengths. Phototropic (growth of plants toward the light source) responses occur in the blue wavelengths. Plants are grouped into three areas based on photoperiod responses; short-day, long-day, and day-neutral. Short-day plants initiate flower buds when days are shorter than a critical point. A more appropriate name may be long-night plant. Long-day plants continue in a vegetative state until day length is longer than a critical point. A more appropriate name for this group of plants may be short-night plant. Flowering for day-neutral plants is not controlled by photoperiod. When given light for 24 hours, short-day plants continue to grow vegetatively and flowering is inhibited. Long-day plants are induced to flower earlier and continue to put on vegetative growth. Day-neutral plants continue to grow vegetatively. A concern with landscape lighting is that plants will continue vegetative growth late in the season. This late growth will not have the opportunity to harden off properly before winter and may experience severe winter injury.

The most common types of lighting include incandescent, high-pressure sodium, metal halide, and mercury vapor. A study by Cathey and Campbell, published in the mid-70's, compared five light sources and their effects on promoting vegetative growth of woody plants, delaying flowering of short-day plants, and promoting flowering of long-day plants. Sources were ranked from most effective (those able to promote vegetative growth) to least effective. Incandescent High Pressure Sodium Metal Halide = Cool White Fluorescent Clear Mercury. Thus, for landscape lighting we would want to avoid those lights (like incandescent) that promote vegetative growth.

Incandescent lights produce all wavelengths of light and are closest to natural sunlight. Incandescent is the least expensive light source to purchase, but the most expensive to operate because of short lamp life and low efficiency. Oddly enough it still remains the most popular type of lighting for residential landscapes. Metal halide is more efficient to operate and has wavelengths in the blue-green-red area. It has better color emission than mercury, but not quite as good as incandescent. Mercury vapor lamps emit light in the blue-green range which accentuates the green color of plants. High-pressure sodium lights are highly efficient. They emit more yellow and red wavelengths which doesn't do much for the color of anything. They are best used strictly for security lighting rather than enhancement of a landscape feature. Fluorescent lights are seldom used for landscape lighting.

The important thing to remember is that light isn't the only factor involved in plant growth. Temperature, water, fertilization, and soil type can also influence plant growth. Light given off by street and landscape lighting, in most cases, is not intense enough to affect plant growth.

Some things can be done to avoid possible problems with landscape lighting. Select the proper light for the area. For security purposes, high-pressure sodium lights are the best choice. Metal halide would be preferred over incandescent in residential areas, malls, parks, etc., where true color is important. Lights that may affect plant growth can be shielded to direct light away from plants. Select plants tolerant of security lighting. Many plants, long-day and day-neutral species, are better adapted to these locations. A brief list of suggested plant species for security lighted areas includes:

Carpinus japonica	Hornbeam
Ginkgo biloba	Ginkgo
Malus sargenti	Sargent's crabapple
Pyrus calleryana	Bradford pear
Quercus palustris	Pin oak
Tilia cordata	Littleleaf linden
Phellodendron amurense	Amur corktree
Ostrya virginiana	American hophornbeam
Gleditsia triacanthos	Honeylocust
Cercis canadensis	Redbud

Plants sensitive to security lighting include:

Acer ginnala	Amur maple
Acer platanoides	Norway maple
Catalpa bignonioides	Catalpa
Cornus sericea	Red-osier dogwood

These are not all inclusive plant lists. Unfortunately, there hasn't been any published data in the area of plants and light sensitivity since the mid-70's. With more and more homeowners installing landscape lighting, it is definitely an area of growing interest.

This article originally appeared in the February 10, 1993 issue, pp. 10, 1993 issue, pp. 10-11.