Laboratory Manual

Experiment 11. Growing Plants Indoors Under Lights

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The first attempt to use artificial light was probably in 1893 by Liberty Hyde Bailey. This work showed that plants could use electric lights for their growth. The use of artificial lights for plant growth did not receive much attention until around 1920 when two scientists, Garner and Allard, showed that flowering of plants was controlled by the length of day or photoperiod to which the plants were exposed. Since then many commercial and home applications of artificial illumination of plants have evolved.

Three factors of light must be considered when growing plants indoors: quality (wavelength), intensity, and duration.

Quality. Sunlight and light from reading lamps appears colorless or white because these beams are composed of the complete spectrum of visible light ray ranging from violet to red as well as other rays. The lower limit of visible light is about 400 nanometers (nm), the violet region, and the upper limit is about 700 nm, the red region. Light with wavelengths shorter than 400 nm is in the ultraviolet region and therefore invisible, whereas light with wavelengths longer is 700 nm is in the infrared region and is also invisible. The vital rays for plant growth are blue-light at 450 nm, red light at 650 nm, and far-red light at 730 nm. The 450 and 650 nm rays are required for plant photosynthesis, the production of food from light, water, carbon dioxide through the catalytic action of the plant pigment, chlorophyll. The 650 and 730 nm wavelengths control flowering through light-induced changes in the plant pigment, phytochrome.

Lamps may be used in combination to provide the requisite wavelengths of light for plant growth. Fluorescent tubes produce blue and red rays, while incandescent bulbs emit the red and far-red rays. When combined in the proper ratio of about 3 watts of fluorescent for each watt of incandescent lamps, conditions can be created which make plant growth possible. In totally dark rooms both kinds of lamps are needed to provide balanced lighting. When only one of these lamps is used balanced lighting can be achieved by placing the plants near a window. The amount of light energy needed for affecting the photoperiod is small in comparison to that needed for photosynthesis. Hence, natural sunlight coming through the window will provide the requisite rays, and the lamps will provide the necessary energy for photosynthesis. In indoor gardening, it is important to remember that blue and red light promotes vegetative growth, whereas the red and far-red light control flowering.

Other plant responses besides photosynthesis and flowering which are governed by light quality are germination of seeds, turning of leaves, reddening of apples, cessation of shrub growth in the fall, the bending of plants toward light and others.

Intensity: Light intensity which is important to photosynthesis may be measured in units called foot-candles and is concerned only with visible light. Full sunlight on a clear summer day has an intensity of about 10,000 foot-candles. A classroom will have an intensity of about 100-foot candles. Photosynthesis and hence rate of plant growth will increase linearly with increases in light intensity from about 100 foot-candles to about 2500 foot candles. A large number of plants are more efficient in use of sunlight and will increase in their rates f photosynthesis up to 10,000 foot-candles. Most plants which are adaptable to growing indoors become saturated with light at about 2500 foot-candles. Plants which respond to higher light intensities up to 10,000 foot-candles includes plants such as corn, sugar cane, and tropical grasses as well as pigweed, crabgrass, and several other weeds.

With two new 40-watt fluorescent bulbs and a 25-watt incandescent bulb one might expect to obtain 500 foot-candles of intensity about one foot directly under the lamps. Light intensity diminishes rapidly with increases in distance from the source. Energy output from the lamps also decreases as the bulbs or tubes become older. A lighting system consisting of two 40-watt fluorescent and one 25-watt incandescent bulb will provide enough intensity to grow some low-growing vegetables such as radish, spinach, leaf lettuce, and many flowers.

Duration. When Bailey first reported on the use of carbon-arc lights on plant growth, he observed that some plants went to seed before edible leaves were made with spinach, lettuce, cress and endive. Also radish roots were poorer and cauliflowers too tall with the extra hours of illumination. In 1920, Garner and Allard found that the length of daylight (actually length of night) had a profound effect on the growth of plants in that it determined whether or not they went to seed, formed bulbs, tubers, or fleshy roots, or whether they formed male or female flowers.

The phenomenon by which daylight affects plant growth or flowering responses is called photoperiodism. Plants can be arranged into three general groups according to their response to daylength.

1. Short-day plants -- Flower only if the length is shorter than a critical length (about 14 hr. depending on species and variety): soybean, chrysanthemum, Maryland tobacco, cosmos, poinsettia, sweet potato, Irish potato tuberization.

2. Long-day plants -- Flower only if the daylength is longer than a critical period (about 10 hr. depending on the plant species and variety): Dill, spinach, lettuce, mallow, Chinese cabbage, radish, nasturtiums, dahlias.

3. Day-neutral -- Flowering appears to be unaffected by daylengths over a wide range of photoperiods: Tomato, corn, most varieties of tobacco, peppers, cucumbers (daylength affects kinds of flowers however), snap beans, peas, African violets, geraniums, roses, begonias, carnations, coleus.

Photoperiod tends to interact with other factors to initiate and develop flowering. Temperature is one of the most important factors which interact with photoperiod. High temperatures generally induce flowering and enhance development in lettuce and spinach, respectively. When growing these crops indoors, keep the temperatures below 65 degrees F at night. Radish which responds to long days for flowering is not temperature sensitive. Bulbing of onions varies widely with variety for photoperiodic response. Early varieties require about 12 hours and late varieties about 15 hours of illumination. Bulbing is usually hastened by temperatures above 60 degrees F. Seed stalk formation in onion is strictly temperature dependent, especially on storage temperatures.

Uses of Supplemental Lighting for Plant Growth

Intensity Functions

Daylength extension: With day neutral plants additional light produces

normal or additional vegetative growth.

Dark day lighting: Provides light during overcast or cloudy days

or during winter months when sunlight intensity is low.

Indoor lighting: Provides a source of radiant energy for green


Photoperiod Functions

Daylight extension: Brings about flowering of long-day plants.

Night-period interruption: Brings about flowering of short-day

plants (actually long-night plants).

Selection of Lamps

Lamp Color

Plants respond to lights of varying colors because there are several different plant pigments which are capable of absorbing light and transferring the energy to chlorophyll which catalyzes photosynthesis. Thus plants may grow in green light, but efficiency in the sue of the total light energy will be small because plant absorption of green light is very small. Plants will grow best if given a mixture of blue (450 nm) light and red (650 nm) light. Plants grown in all red light appear to be overly tall and leggy whereas ones grown under all blue light may be low-growing and stocky. Incandescent bulbs emit light rich in red and infrared rays (heat). Fluorescent tubes emit light richer in blue light, but also with considerable green and yellow rays. Light from both types of lamps appears white because it is a mixture of colors. Of the two, fluorescent lamps have the best spectral distribution of rays and give the most illumination for the energy consumed. Plants often grow well under fluorescent tubes alone but become spindly and pale in color under incandescent lights alone. Fluorescent lamps have an average life of about 12,000 hours, but incandescent ones last less than 1000 hours.

Many reports have shown that plants grown under Cool-White or Daylight bulbs are as good or better than those grown under lamps of various colors but that growth is improved by combinations of fluorescent and incandescent lamps. Special fluorescent tubes have been developed to provide radiant energy which closely follows the absorbtion of light by green plants. these lamps are sold under names such as Grow-Lux, Plant-Grow, and Plant-Light tubes. they cost considerably more than Cool-White or Daylight tubes but may save money on operating costs and replacement costs if incandescent bulbs are used in combination with ordinary fluorescent tubes. Grow-Lux and other plant growth bulbs tend to give plants an artificial glow which may be desirable or undesirable depending on personal taste.

Light Intensity

The light intensity of artificial lights for home use is generally low in comparison to that of sunlight. Two Cool-White lamps will provide an intensity of about 400 foot-candles at foliage heights about one foot from the lamps. This intensity is minimal for the growth of most plants adaptable for indoor growth.

Several lamps providing high light intensities are available for indoor use. Some lamps are reflectorized to give more illumination downward. Others are made smaller in diameter so that several bulbs with the same power output can be fitted in the space of the larger ones.

Use of bulbs of higher intensities of course cost more to purchase, equip, and operate than conventional bulbs. Do not use sunlamps; they emit too much ultraviolet light.


1. Plants require balanced lighting consisting of blue, red, and a weak intensity of infrared light.

2. Sufficient light quality and intensity can be provided by two Cool-White or Daylight fluorescent lamps placed 12 inches or less above the plants. Plant growth is improved if incandescent bulbs are added at the ratio of one watt of incandescent power to three watts of fluorescent power, i.e., one 25-watt tungsten lamp to two 40-watt fluorescent tubes (4-foot lengths).

3. The expected life of a 40-watt fluorescent tube is about 12,000 hours while that of an incandescent lamp is about 1000 hours. The lamps should be used to only 70% of their expected life as they tend to darken and lose light intensity.

4. Lamps should be kept clean to avoid losses in light intensity.

5. Some lamps which may be used:

a. Fluorescent

1) 40-watt (4') tubes (light load*) (12,000 hr life)

(*load refers to amount of current carried)

2) Plant Growth tubes (medium load) (7,500 hour life)

3) High Output Fluorescent (medium load)(12,000 hr life)

4) T-10 (heavy load) (12,000 hr life)

5) T-12 (heavy load)(9,000 hr life)

6) Power-Groove (GE Brand name)(9,000 hr life)

b. Incandescent lamps

1) Standard tungsten filament (750 to 1000 hr life)

2) Reflector lamps of various shapes (1000 hr life)

6. Special fluorescent lamps such as T-10, T-12, High Output, and Power-Groove may require special fixtures and not be adaptable to standard industrial or home fluorescent light fixtures.

7. Do not use sunlamps for plant lighting. They may emit too much ultraviolet light.

8. Measuring light intensities. Light intensity is measured in foot-candles (based on what the human eye sees) or in energy units, such as watts per square centimeter of illuminated surface or watts for a certain color of light emitted (band width). Scientists feel that energy measurements in watts per square centimeter or band width of light are more meaningful than those expressed in foot-candles.


1. Bickford, E.D. and S. Dunn. Lighting for Plant Growth. The Kent State University Press. Kent, Ohio.

2. General Electric. Plant Growth Lighting. Bulletin TP-127. GE, 50 Industrial Place, Newton Upper Falls, Mass. 02164.

3. Kranz, F.H. and J.L. Kranz. Gardening Indoors Under Lights. Viking Press, New York. 1971.

4. Sylvania. Applied Lighting. Engineering Bulletins 0-278 and 0-294. Sylvania Lighting Center, Danvers, Mass. 01923.



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Produced and maintained by Allen Barker
University of Massachusetts, Amherst.