Growing Plants Indoor Under Artificial Light


Plants can be grown to maturity entirely by artificial light. Whether or not such a cultural method is worthwhile is another story. However, the problem is not simply one of turning on enough light to make up for the lack of sunshine. Light has three “dimensions,” and is profoundly affected by its relationship to another factor of plant growth – temperature. An understanding of all four factors is essential to grow plants by artificial light.

Because it is the least understood, the light-temperature relationship should be explained first. Light and heat must be in balance. When light is decreased, the heat needs to he lowered in order to slow down growth to a point where the amount of illumination falling on the plant is adequate for the level being maintained. Thus, as the Winter days grow shorter, we cannot make up for the lack of the sun’s warmth by moving the plant to a warmer room. Instead we should move it to a cooler spot, or make up for the lack of light by supplying artificial illumination.

Understanding Plant Needs

Oddly enough, warm-weather plants can stand lower light intensities at higher temperature than cool-weather plants can. Plants like cinerarias and calendulas that need relatively low temperatures will suffer more in a dark room at high temperatures than will the poinsettia, unless they get extra light.

aerogarden with grow lights

One of the weaknesses of the use of artificial illumination in growing plants has been the high heat generated by incandescent lights. With the coming of the fluorescents, much of this handicap has been removed. Fluorescent tubes are capable of giving off more light with less heat.

None the less, the three “dimensions” of light – intensity, duration and spectrum present a problem. Intensity refers to the strength of the light, usually measured in foot candles. Photographic light meters are frequently rated in foot candles, so this term is not as esoteric as it once was.

Duration and intensity are often confused. The duration refers to the number of hours the light source sheds its radiation, but duration and intensity arc not interchangeable. You cannot make up for a short day by making the light brighter for fewer hours. There is some evidence that African violets can be grown as well with 600 foot candles for 18 hours as they can with 1.000 foot candles for 12 hours, but this is about the only case where the two qualities are not interchangeable.

Fluorescent tubes can, of course, be burned as long as is necessary. Intensity is another problem. Because the radiation from these tubes does not “project” well, plants must be placed within 12 inches of them to get enough light. For this reason, two 40-watt tubes will only light a space about 12 inches wide and four feet long. Incandescent bulbs, on the other hand, can be suspended three feet above the plants. When used in this way, one 60-watt bulb will substitute for the sun on an area four feet by four feet.

However, this statement should not be interpreted to mean that either fluorescents or incandescents can be used to substitute for the sun on a full-time basis. For many species, such illumination can be used as a supplemental light source. If the plant is one which flowers during the shorter (lays of the year, such extra light can help to hold it back. If it is one that needs long days for normal bloom, the extra light may make the difference between flowers and no flowers.

african violet

In the case of the poinsettia, for example, the usual failure of this plant when grown in the ordinary living room can be traced to the fact that the room is used at night. A light equal to two or three foot candles is enough to interrupt flowering. When father reads his paper under the living-room lamp, a poinsettia cannot be expected to flower in that room. It needs complete darkness for at least 13 to 14 hours a night before it will flower:

The chrysanthemum, also a short-day plant, is often controlled by supplying it with extra light until late in Winter. When flowers are wanted, the lights are turned off, and the plant flowers. Thus chrysanthemums can, by shading in Summer and lighting in Winter, be made to flower at the convenience of the grower. Several florists throughout the United States are now delivering at given number of bunches of cut chrysanthemums to their wholesalers every business day of the year.

Effect of Light on Begonias

The amateur can use artificial light for several purposes. One of the most interesting is to force tuberous begonias into bloom during the dark days of Winter. Being a long-day plant, the tuberous begonia needs extra light. In the open, it stops forming flower buds when the length of the day drops below 12 hours. About mid-September, over the entire United States, tuberous begonias quit flowering and form tubers instead. At Cornell University, it has been found that, if lights are used on the plants just before this condition happens, starting in mid-August, the tubers do not increase in size, and the plants keep on flowering all Winter long. Either fluorescents or incandescents can be used, turning them on at four in the afternoon and off at nine at night.

A great many house plants such as geraniums, browullias, Heavenly Blue morning glories and other plants can be kept in bloom throughout the Winter by this same supplemental illumination. Foliage plants like coleus will keep short and sturdy. English ivy will stop reaching for the sky, and remain short-jointed.

This practice can hardly be considered as growing plants under completely artificial light. How far we can go in this direction, depends on how much money we can afford to invest. I have seen artificial lights that would exceed daylight by 50%, with air-conditioning equipment to pull down temperatures brought about by the heat thrown off by the lights. Yet, such layouts are beyond the ability and the purse of most amateur gardeners.

Plants for the Basement

What does fall into the realm of the possible is the growing of certain plants in a basement, entirely by artificial light. All of these are what we call “forest floor” species. They are plants which have been conditioned to survive under the trees of a forest, where light intensities are low. This group includes plants like African violets, all begonias, browallia, impatiens and many others. If a species will grow and flower in the shade, it can probably be grown entirely by artificial light.

This is where the third “dimension” of light comes in. Until now, the actual spectrum emitted by the source of light was not too important. When all the light needed by the plant comes from an artificial source, the quality is vital. For vegetative growth and flowering, the shorter blue waves (which are close to the invisible ultra-violet spectrum) seem to be important. So, too, are the longer, red waves, close to infra-red or invisible heat waves, but these are not needed in as large amounts. Fortunately, a very close approximation of natural daylight can be had by the use of daylight fluorescent tubes, suspended about 10 inches to 12 inches above the plants. The more light you can use on a given area, within reason, the better. The use of a completely-enclosed case solves many of the problems involved in growing plants by artificial lights. Inside such a case, light, heat, humidity and plant food can be controlled so closely that graphs representing each factor show as practically straight lines.

In the open basement, it is difficult to maintain humidity high enough for good growth. On the other hand, humidity can be kept at close to 100% without much difficulty in the enclosed plant case.

What can be grown in such a case, or on an open bench with high humidity? One of the most satisfactory subjects is the tuberous begonia.: light-sensitive plant, this species thrives on the long days that are possible under artificial light. If lights are kept on 24 hours a day, or are kept on 16 hours and off eight hours, very little difference results. A friend of mine, Harold J. Blulim of LaGrange, Illinois, has grown tuberous begonias from seed to flowering, and kept the plants flowering continuously without a break. He has eight separate enclosed cases of the Startaplant type, in which lie is growing a wide range of plant materials. African violets are particularly successful and impatiens also.

fluorescent grow lights

One of the most important uses for artificial light is in the propagation of plants; here light quality enters the picture again. Instead of the daylight tube, the so-called soft white tube is best for rooting cuttings. These need more of the red end of the spectrum. which can be supplied by the use of this type of tube. Results in the closed case are sensational. I have propagated dozens of types of cuttings, with minimum loss and practically without attention. Once the cuttings are “stuck” in the flats, the lid is closed and need not be opened, except for inspection, until the rooted cuttings are removed.


A highly satisfactory use for artificial light has been in the forcing of Dutch bulbs without a sunny window. Hyacinths, tulips and narcissi are potted as soon as received, and kept in a dark cellar at 48 degrees until the shoots are two to three inches out of the pots. Long, cold storage treatment is unnecessary except when bulbs are to be held back for later forcing. Once the shoots are developed to a length of three inches, the pots are placed under fluorescent tubes. Two 40-watt tubes suspended 10 inches above the pots will light an area 12 x 48 inches. Temperature should be kept as close to 55 degrees as possible.

In forcing bulbs, I find that excellent pot plants can be grown with only fluorescent tubes. However, if the flowers are to be used for cutting, the addition of a 60-watt incandescent bulb to the two tubes seems to produce a plant lasting longer in water.

by R Carleton – 61896

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