Figure 1
Artificial lighting, if properly designed, allows plants to be grown indoors in nearly any setting.
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).
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.
- Low-light plants
Plants referred to as low light intensity plants generally should receive between 50 and 250 foot-candles. (A foot-candle is a measure of light intensity. A foot-candle is defined as the amount of light received by 1 square foot of a surface that is 1 foot from a point source of light equivalent to one candle of a certain type.) Under artificial light, a few plants in this group can be maintained at as little as 10 foot-candles.
One way you can estimate the amount of light available is to calculate the number of watts available per square foot of plant area. Low-light plants should receive between 10 and 15 watts of fluorescent light per square foot of growing space. A single fluorescent tube, such as a 2-foot 20-watt tube or a 4-foot 40-watt tube without any other light provides only enough light for plants in this category (see Table 1).
- Medium-light plants. These plants prefer 250 to 1,000 foot-candles. Best growth occurs above 750 foot-candles unless plants also receive extended periods of direct sunlight. Give them artificial light in the 500 to 1,000 foot-candle range or 15 or more watts per square foot of growing area.
While plants in this group can be held in the 250 to 500 foot-candle range, growth is best with more light. A fixture containing two fluorescent tubes is sufficient for plants in the low to medium light range. Adjustments in the number of tubes used may be made if you regulate the distance between the tubes and plants.
- High-light plants
These plants generally are less satisfactory for growing under artificial lights in the home. However, if you want to try, use special high-intensity lamps. These plants need at least 1,000 foot-candles, or 20 watts per square foot of growing area, but should have higher intensities for best growth and flowering. Fixtures containing three to four fluorescent tubes are necessary for plants requiring high light.
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 |
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.
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.
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 (see Figures 2 and 3).
Figure 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.
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