A MINIATURE GREENHOUSE WITH artificial sunshine and a controlled atmosphere can be built by the horticulturist for experiments with plants. The experimenter can regulate the color, intensity and duration of the light and the composition, temperature and humidity of the air. Hence he can rear plants under conditions that match those of almost any region of the earth in any season.

The miniature artificial greenhouse is simple and effective and so has found its way not only into the horticultural laboratory but also into the home of the city dweller who likes to raise unusual plants. One such person is Francis C. Hall, a lighting engineer who lives in an apartment at 222 Henry Street, Brooklyn, N.Y. 11201. Hall started 10 years ago after he had unexpected success with a single philodendron; now apartment gardening is an active avocation. During the 10 years he has built several miniature greenhouses, making as he did so some interesting innovations. He writes:

"After many attempts to grow flowering plants in our living room, I finally gave up. They all turned yellow, lost their leaves and died. So, like many of our neighbors, I settled for a wall pot of philodendron. This vine will live for a time almost anywhere, and it is easy to replace when it dies. Ours was replaced about every two months for a year or so until one of them unaccountably thrived, growing so vigorously that the tip of the vine reached the floor. I thought we had somehow acquired an exceptionally hardy specimen.

"At about that time we had a visit from a friend who teaches botany at a nearby high school. I called her attention to the thriving plant and asked for an explanation. After examining the pot and its surroundings she asked: 'How long has this table lamp been close to the vine?' I told her we had bought the lamp at about the time we planted the vigorous philodendron. The lamp is turned on every evening at dusk and stays on all night. 'There's your explanation,' she said. 'In effect you have converted this corner of your living room into a miniature greenhouse-not a very good one, but a greenhouse nonetheless.' She went on to explain the lighting requirements of plants and how I could improve my setup. I have been dabbling in amateur botany ever since.

"The key to a successful indoor greenhouse is the lighting, which should consist of a combination of cool white fluorescent lamps of the quick-start type and incandescent lamps that are operated at about 95 percent of their rated voltage. No single kind of lamp will suffice because none radiates the copious amount of energy plants require in the red and blue portions of the spectrum.

"Fluorescent lamps emit energy mainly in the violet, blue and green portions of the spectrum, and incandescent lamps emit mainly in the red and progressively less toward the violet. Experiments have shown that most plants require only the blue, red and far-red rays-the wavelengths extending from about 4,000 to 5,000 angstrom units in the blue and 6,300 to 7,500 angstroms in the red and far red. The mixture of greens, yellows and oranges that the eye perceives as a single shade of green are largely reflected by the plant pigments. Ultraviolet rays are damaging to plants, but most of the ultraviolet radiation in sunlight is absorbed by the atmosphere. The colors in sunlight that are essential to plant growth can therefore be provided by a properly balanced and filtered combination of fluorescent and incandescent lamps.

"In what proportions should the lamps be combined? The question has not been fully resolved. When I built my first battery of lamps, the optimum proportion was thought to be 10 watts of fluorescent light to each watt of incandescent light. Since then the recommended ratio has gradually changed. Some experiment stations, including the U.S. Department of Agriculture's Agricultural Research Service in Beltsville, Md., have used ratios in which the wattages of fluorescent and incandescent light are equal. The results of my own experiments appear to favor the closer proportions.

"Power consumption is doubtless a poor index of spectral intensity because it neglects the performance characteristics of the lamps. A lamp of one type may radiate far more energy in certain portions of the spectrum than a lamp of another, depending on the design of the lamp and the voltage at which it operates. The variation occurs particularly in incandescent lamps. For fluorescent lamps I now use 30-watt cool white tubes of the rapid-start type equipped with ballasts, or transformers, designed to operate 40-watt tubes. Driving the lamps above their normal rating increases the emission of the desired blue light by 30 percent but does not appreciably shorten the life of the tubes. The trick will work only with 30-watt tubes.

"Ordinary incandescent lamps can be used for radiating the essential red light When they are operated at their rated voltage, however, they do not emit strongly in the far red. For this reason I have switched to lamps that are rated at 130 volts. These lamps are used chiefly by industry in inaccessible places where lighting requirements are not severe and where the cost of replacing bulbs must be minimized. Although the lamps are rated at 130 volts, they are operated on 120 volts. Hence they last several times longer than conventional lamps. The yellowish-red emission is ideal for plants.

"These extended-service lamps are available from dealers in electrical supplies in the same wattages and at the same prices as standard lamps. They should not be confused with the long-life bulbs that are currently advertised. I now use six 30-watt fluorescent tubes in combination with six 15-watt incandescent lamps, a power ratio of 180 watts of fluorescent lighting to 90 watts of incandescent lighting.

"The structural details of mounting the lights and associated fixtures are determined largely by the application. For example, if the experimenter merely wishes to floodlight a shelf of plants, the hardware can be screwed to a simple wood frame suspended from the ceiling. In this case the fluorescent tubes can be mounted in standard twin-tube fixtures attached to the wood frame. The incandescent lamps can be spaced uniformly between the fluorescent fixtures. Normally the lamps will face downward. Heated air will rise from the lamps, so that porcelain sockets should be used. If the initial cost is a secondary consideration, the experimenter may wish to install a battery of the special fixtures that have been developed recently for service in miniature greenhouses. The fixtures provide space for both fluorescent and incandescent lamps in a single unit.

"The optimum quantity of light to be used varies with the requirements of the plant. In nature plants grow under a wide range of light intensities, from as little as 10 footcandles to about 10,000. Philodendron will thrive at intensities as low as 50 to 100 footcandles; African violets do nicely at 600 footcandles, and orchids need 1,000. My experiments indicate that most popular varieties of flowering houseplants grow best at intensities ranging from 1,000 to 2,000 footcandles. Some growth chambers found in horticultural laboratories are equipped for lighting levels as high as 8,500 footcandles.

"Inexpensive light meters calibrated in footcandles are now available for measuring the intensity. Alternatively, simple tables for converting the indication of photographic exposure meters to footcandles can be compiled easily. Intensity can also be estimated. A pair of cool white 30-watt rapid-start fluorescent tubes that operate with a 40-watt ballast in a twin fixture will deliver about 1,100 footcandles at a distance of six inches from the tubes, 650 footcandles at 12 inches and 500 footcandles at 18 inches. Within a 30-degree eone below the tubes the intensity falls off uniformly to about 80 percent at the edge.

"At present I have two small greenhouses. One, a lean-to, is installed outside a rear window of our ground-floor apartment [see illustration above]. During part of the day the unit receives direct sunlight. The natural light is supplemented as desired by a battery of electric lights controlled automatically by a clock timer.

"The second chamber was constructed in the form of an ornamental cabinet, the basic details of which are shown in the accompanying illustration [Figure 1]. It consists of two compartments for growing stacked above a third compartment that houses an air conditioner, electrical controls and miscellaneous supplies. The lamps are suspended from the inner surface of the top of the cabinet and the bottom of the top compartment. They are controlled by the preset clock timer; the control for the air conditioner is a Honeywell Airswitch (type T-631-C) that operates when the temperature changes three degrees from the value at which the switch is set.

"Lamps develop a substantial quantity of heat that must be removed to prevent the temperature of the chamber from rising above 75 degrees Fahrenheit, the maximum to which common houseplants should be exposed. Fluorescent tubes operate at relatively low temperature and present no problem. The associated ballasts radiate a fair amount of heat, however, and must be located outside the growth chamber. I installed mine on the top of the cabinet.

"Incandescent bulbs of the recommended industrial type are much cooler than conventional bulbs; they are warm rather than hot to the touch. Even so, the temperature inside a glass and wood enclosure of 40 cubic feet equipped with 500 watts of mixed lamps will rise as much as 40 degrees F. above the temperature of the room. In order to remove this heat my cabinet was equipped with a small air conditioner of the type designed for window mounting (RCA Whirlpool, 4,700 British thermal units).

"The installation of automatic temperature controls adds considerably to the versatility of a growth chamber because it has recently been learned that plants require a daily rhythm of temperature change. This thermoperiod is analogous to the daily alternation of light and darkness. Although the thermoperiod is still under investigation, experiments indicate that for most houseplants the night temperature should be allowed to fall about eight to 14 degrees F. below the daytime temperature. With geraniums I maintain a temperature of 67 degrees during the day and a temperature of 55 degrees at night.

"The control of relative humidity is difficult, and I have not yet succeeded in improvising an automatic system. My cabinets contain trays of moist gravel, and I water the potted soil periodically. Nonetheless, the relative humidity of the air tends to fall substantially on dry days. For this reason I measure the humidity with a psychrometer and spray the plants with water as necessary by means of a hand atomizer. On days when the relative humidity of the room air is high I depend on the air conditioner to remove the excess from the cabinet. Houseplants appear to do well at a relative humidity of between 50 and 80 percent.

"The length of the simulated day is the easiest variable to control. All it requires is setting a clock timer. The importance of the daily rhythm of light and darkness to the growth of plants was first reported in 1920 by Department of Agriculture botanists who were investigating the flowering of tobacco plants. Subsequently it has been learned that the photoperiod acts as a kind of trigger that determines when a plant will blossom, when seeds will germinate, when bulbs will form and so on. The photoperiod also influences the color and size of leaves and the elongation and branching of stems. Commercial growers of flowers such as chrysanthemums now routinely delay the flowering of plants grown in the field by switching on batteries of incandescent lamps for intervals as short as 10 minutes during the night, thus synchronizing the production of flowers with the demands of the market. I follow the same procedure when growing entries for our local flower show.

"What is the optimum photoperiod? The answer depends on the plant. In general plants can be grouped according to their preference for short, intermediate or long days. The perennial chrysanthemum and the poinsettia do best when the days are short-10 hours of light or less. Such a period is characteristic of plants that flower in the fall. Plants that do well at the opposite extreme-20 hours or more of daylight- include the China aster, the African violet, the tuberous begonia and the philodendron. The third category, which includes the rose and the carnation, consists of plants showing little sensitivity to the photoperiod.

"Most of the popular houseplants with which I have experimented seem to do well on a daily exposure of 16 hours to light from which the ultraviolet rays have been filtered. As indicated by the accompanying graph [Figure 3] this emission in fluorescent tubes occurs between 3,500 and 4,000 angstroms. It can be suppressed by inserting a sheet of Mylar W-2 plastic between the lamps and the plants or, if this arrangement is inconvenient, by wrapping each tube in a single sheet of the material. (Mylar W-2 is a product of E. I. du Pont de Nemours & Co.)

"Experiments indicate that the triggering effects of the photoperiod are confined to the red rays in the vicinity of 6,600 angstroms. A few minutes of exposure to red light initiates biochemical reactions that continue for some time in the dark. Ordinary incandescent lamps emit enough red light to trigger the reaction in the case of houseplants. For example, houseplants that grow reasonably well on a window shelf will usually show dramatic improvement, particularly in winter, if they are grouped under a table lamp every day from dusk until bedtime.

"Like animals, plants must have food and water, each plant according to its needs. Conventional techniques of feeding and watering can be used for plants grown under electric lights. It is also possible to use the greenhouse for controlled experiments on nutrients. For instance, I once read that growth had been accelerated as much as threefold by fertilizing plants with carbon dioxide. The author went on to explain that, according to one hypothesis, the lush growth of plants during the Carboniferous period 300 million years ago resulted from the relatively large amount of carbon dioxide then present in the atmosphere and that the period ended when the carbon became locked up in deposits now represented by the fossil fuels. Why not flood my miniature greenhouse with carbon dioxide and grow giant plants?

"I quickly learned that the cost of the gas is too high for my pocketbook. It occurred to me, however, that something of the same effect might be observed if I sprayed the plants with carbonated water. I tried doing so but had poor results with bottled soda water. Apparently chemicals added to this water for retarding the escape of gas harm the plants.

"Carbonated water can be made at home, however, by means of a special bottle that accepts gas from small metal tubes. The homemade product worked. After trying various methods of applying the water in varying amounts I learned that a light mist applied to the leaves every other day doubles the growth rate and reduces the time required for the plant to reach maturity.

"The relatively small amount of gas liberated from the water does not substantially alter the ratio of carbon dioxide present in the air of the chamber. Why, then, do the plants respond so dramatically? I can only guess that the leaves absorb the gas through their stomata, or pores. The application must be made with an atomizer that develops a fine mist, and the leaves should be moistened only lightly. Moreover, the relative humidity of the chamber should be measured after the treatment and lowered if it rises excessively. Some experimenters who tried the procedure without initial success made the mistake of ignoring the relative humidity.

"Otherwise my plants receive conventional fertilizers applied according to the established requirements of each species. Much annoyance and expense can be avoided by using hygienic methods. Pans of water kept on the shelves for maintaining humidity encourage the growth of algae and fungi. The pans should be removed and cleaned periodically when the plants are washed. Much unwanted growth can be discouraged by adding an algicide to the water in the humidifying pans. I use a solution that consists of one ounce of cupric sulfate in one quart of tap water. One fluid ounce of this stock solution is added to each gallon of water used in the pans. Water so treated must not be allowed to come in direct contact with the potted soil. To prevent such accidental contamination surround the pots with plastic liners.

"Seedlings can be developed easily in the greenhouse. They do best in blue and red light; far-red light retards them. To make seedlings sprout I remove all industrial incandescent lamps and substitute a single standard 15-watt lamp. The result is a 12: 1 ratio of fluorescent to incandescent light; the emission of the far red is minimized.

"Seedlings require a somewhat higher temperature for maximum rate of growth than mature plants do. The temperature should be between 75 and 80 degrees F. The soil can often be warmed to this temperature if the plants are raised to within four or five inches of the lamps. Alternatively, the added heat can be developed electrically by installing heating cables in or under the trays. Cables specially designed for this purpose can be bought from dealers in gardening supplies. It is good practice to plant seeds in sterilized soil in order to discourage the growth of fungi and molds.

"The cost of my greenhouse, including the air conditioner, was about $300. To this amount must be added about 15 cents a day for electric power. I can easily imagine a more elaborate and more costly installation. On the other hand, it is possible to conduct many fascinating experiments with little more than a potted plant and a single incandescent lamp. In my opinion few hobbies return more in terms of satisfaction per dollar and none appears to be attracting enthusiasts more rapidly. Gardening under lights came of age scarcely 20 years ago, yet it already has an extensive and swiftly growing literature."

Bibliography

EXPERIMENTAL CONTROL OF PLANT GROWTH. Frits W. Went. Ronald Press Company, 1957.

GARDENING INDOORS UNDER LIGHTS. Frederick H. Kranz and Jacqueline L. Kranz. The Viking Press, 1957.

GROWING PLANTS UNDER ARTIFICIAL LIGHT. Peggie Schulz. M. Barrows Co., Inc., 1960.

PLANT PROPAGATION WITH ARTIFICIAL LIGHT. U.S. Department of Agriculture Reprint No. CA-34-7ff. U.S. Government Printing Office, October, 1962.

The Society for Amateur Scientists (SAS) is a nonprofit research and educational organization dedicated to helping people enrich their lives by following their passion to take part in scientific adventures of all kinds.