University of Missouri Extension

G6515, Reviewed May 2002

Lighting Indoor Houseplants

David Trinklein
Department of Horticulture

Houseplants are popular indoor decorations. Attractive and constantly changing, they add a softness of line and provide a bit of nature indoors. However, the ideal location of a plant for decoration may not be the ideal spot for plant growth. Lack of adequate light is the most common factor limiting the growth of plants in many areas of the home. Supplementary electric lighting is usually the easiest and least expensive way to provide enough light for plants that do not receive adequate natural light (Figure 1).

Artificial lighting Figure 1
Artificial lighting, if properly designed, allows plants to be grown indoors in nearly any setting.
 

Why do plants need light?

Light provides the energy plants need to make the food required for them to grow and flower. Plants are the only organisms able to use light to produce sugars, starches and other substances needed by them as well as by other living organisms.

Is light color important to plants?

Certain colors in light rays are important for proper plant growth. Leaves reflect and derive little energy from many of the yellow and green rays of the visible spectrum. Yet the red and blue parts of the light spectrum are the most important energy sources for plants, and plants require more rays from the red range than from the blue.

Plants growing outdoors, in greenhouses or close to windows are exposed to a natural balance of the blue and red light rays that plants need. Where plants receive little or no natural light, you must provide additional light from artificial sources.

Which types of lights are best?

As a single light source for plants, incandescent light bulbs are not particularly good. They are a good source of red rays but a poor source of blue. They produce too much heat for most plants and, if used, must be located some distance from the plants, thus reducing the intensity of the light the plants receive. They are also about one-third as efficient as fluorescent tubes in converting electrical energy to light. Furthermore, a standard incandescent bulb's life is often only about 1,000 hours, whereas a fluorescent tube's life is normally 10,000 hours or more.

Fluorescent tubes provide one of the best artificial light sources available for plants in the home. Other light sources such as sodium-vapor and metal halide lamps may be used but are not as readily available or adaptable for home use.

Fluorescent tubes are made in many sizes and shapes: circular, U-shaped, square or straight. Straight tubes in 2-, 4- or 8-foot lengths are used most frequently.

What is the best balance of artificial light?

Many indoor gardeners use cool-white fluorescent tubes. Warm-white fluorescent tubes also seem fairly effective, but fluorescent tubes listed as white or daylight are less desirable for indoor plant growth. Cool-white tubes produce a small amount of red rays in addition to orange, yellow-green and blue rays. However, the red light produced usually is not enough for plants unless windows or other artificial lights produce additional red rays.

A few incandescent bulbs in the growing area can furnish needed red rays. A general ratio of incandescent to fluorescent light is about 3 to 10, so for every 100 watts of fluorescent light, you should provide about 30 watts of incandescent light for a better red-to-blue light balance.

Special fluorescent tubes also have been developed for growing plants. These have a higher output in the red range to balance the blue output. Many home gardeners have found that these tubes can be used in combination with cool-white tubes. Use one special plant-growing tube to each one or two cool-white tubes. This method is more economical than using all special tubes, since cool-white tubes cost less than the special plant-growing tubes. Also, fluorescent plant-growing tubes use less electricity and produce less heat than incandescent bulbs, and you will not have to provide fixtures for both incandescent bulbs and fluorescent tubes.

May I use spotlights or other special bulbs?

Yes, although they are less effective than fluorescent lights and the combinations described previously. However, fluorescent fixtures may not be suitable for some locations. Reserve these special light sources for situations where supplementary light is essential. Self-reflectorized spot lamps coated to emit more blue light are now available.

How much light should plants receive?

The amount of light necessary varies with each plant. In general, the light fixtures available for home plant lighting make it practically impossible to produce too much light for most plants.

Plants that can adapt to interior settings usually are divided into three general categories: those suitable for low, medium and high light intensities. (These three groups are referred to in the discussion of individual plants in the last section of this publication.) The categories generally indicate the minimum light required. Growth is often best at the higher end of these suggested light ranges.

Table 1
Light output (in foot-candles) measured at various distances below fluorescent lamps. Output in parentheses is measured 1 foot on either side of a line directly below the lamps. All lamps are standard 40-watt tubes.

Distance Type of fixture
 
0.5 foot 500 700 900
1 foot 260 (200) 400 (260) 600
2 feet 110 (100) 180 (150) 330
3 feet 60 (60) 100 (90)  
4 feet 40 60 100

How far from the light should I place plants?

Most plants should be located with the tips of the plants 6 to 12 inches from the light source. The intensity of light drops rapidly as the distance from the light bulbs or tubes increases. Table 1 shows this reduction of light intensity with distance below and to the side of tubes. Fluorescent tubes also do not produce as much light at the ends as they do in the center. Therefore, the brightest spot under a fluorescent fixture is directly beneath the center of the tubes.

The light fixture's position should be adjustable so you can keep the distance between the light and the plant fairly constant. Fluorescent shop or workroom fixtures often are hung on chains with S-hooks for easy adjustment. These fixtures are easily raised or lowered from link to link. If the fixture is not movable, you may make some adjustment by raising plants on stands, shelves or boxes.

How long should I use lights?

In most cases, plants receiving no outdoor light should be lit from 16 to 18 hours each day. If some additional light is received, 12 to 14 hours each day may be adequate. Lights should be used at the same time that plants receive window light. Using lights at the beginning or end of the day will not usually be as effective as using lights during daylight unless natural daylight is quite bright.

How can I get the most from artificial light?

Reflectors and reflective surfaces can maximize the available light. Bulbs with self-contained reflectors are helpful.

Porcelain-coated reflectors are excellent and require little maintenance. Keep reflectors clean and free of rust or any coating that reduces their effectiveness. White paint or aluminum foil beneath or around the growing area helps reflect light and makes it more efficient.

Space plants far enough apart to allow light between them. Arrange plants so they do not shade each other. Keep tubes clean and replace old tubes promptly.

Homemade vertical lighting system Figure 2
This homemade vertical lighting system with adjustable shelves can accommodate small or large plants.
 

How should I light the bottom of a tall plant?

You may want to supplement light placed above the plant with spotlights around the base of the plant and directed on the lower leaves. You can also use fluorescent tubes in a vertical position to provide side lighting from the top to the bottom of a plant. This vertical position also can be used for smaller plants arranged on shelves (Figures 2 and 3).

African violetsFigure 3
Small, low-light plants (e.g., African violets) are well suited for light gardens.
 

Do I really need to measure light?

The eye is a poor judge of light intensity because it automatically adjusts to different light levels. Light measurements are helpful in setting up a plant growing area but should be used only as a guide.

Light meters that measure foot-candles may be used if available. Photographic light meters normally do not read in foot-candles, but some manufacturers supply a conversion table. When available, these may also be used.

Calculating wattage per square foot of growing area is a useful and easy way to estimate light required. With this approach, light measurements are unnecessary unless problems develop.

How can I tell if there is enough light?

The growth pattern of the plant can be a good indication. No growth may indicate poor light but could be a sign of other problems as well. A healthy plant under poor light may develop long internodes (length of stem between leaves). Leaves may develop smaller than normal on some plants. Color of many plants in poor light may be pale green, and lower leaves may yellow and drop.

What window produces the best light?

Consider size, direction, overhang and shade from trees or buildings. Large windows provide the best growing conditions and allow plants to be placed fairly far back into a room. But even low-light plants usually do not receive enough light at distances greater than 10 feet from an average window. The best windows for plants are those not shaded by a large overhang, trees or structures.

Windows facing south provide the brightest light conditions for the longest duration. In winter, any houseplant benefits from the light of a south window. However, plants that do not need bright light may be sunburned by the bright light at south windows in late spring, summer or early fall. Place plants requiring less light, such as African violets, at a north window or to the side or interior of a large south window during these times. South windows are most appropriate for plants requiring bright light and some direct sunlight.

East and west windows are well suited to many plants in the medium light range, while north windows are satisfactory only for plants requiring the lower light levels. These plants should not receive direct sunlight.

Can lights keep plants from blooming?

Some plants, generally known as short-day plants, can be kept from flowering under the light durations normally used for artificial lighting. Best known in this category are the poinsettia and chrysanthemum. To induce flowering indoors, give these plants only about 10 hours of light each day until flowers become visible and color shows.

Will artificial lights start seeds?

Vegetable, annual flower and some perennial flower seeds may be started successfully indoors under lights for later planting in the garden. For stocky growth, place seedlings within a few inches of the tubes as soon as germination begins.

Is a light timer necessary?

A timer is a valuable asset because lights should be turned on and off regularly and consistently. Twenty-four-hour timers available from electrical supply houses are adequate. The electrical cord from the timer should be the three-prong type, or you should use a grounded adapter. The use of water around plants makes grounding electrical fixtures important.

Gold dust dracaena Figure 4
Gold dust dracaena.
 

Plants for indoor lighting

African violet

Aluminum plant

Arrowhead vine

Asparagus ferns

Aucuba-leaf croton

Begonias

Cacti and succulents

Cast iron plant

Chinese evergreen

Cissus species

DieffenbachiaDieffenbachia


Dracaena species

Ferns

Ficus species

Flame violet

Gloxinia

Herbs

Hoya

Ivy

Jade plant

Little-leaf schefflera

Norfolk Island pine

Orange

Orchids

Palms

Oval-leaf peperomia Peperomias


Philodendrons

Moon valley pilea Pilea species


Pleomeles

Pothos, devil's ivy

Sansevieria, snake plant, mother-in-law-tongue

Schefflera, umbrella tree

Spathiphyllum, peace lily

Spider plant, airplane plant

Swedish ivy

G6515, reviewed May 2002

G6515 Lighting Indoor Plants | University of Missouri Extension

Order publications online at http://extension.missouri.edu/explore/shop/ or call toll-free 800-292-0969.

University of Missouri Extension - print indicia