The conventional impression of a greenhouse is of a warm, moist place redolent of the tropics. The air is gentle and fragrant. In your mind’s ear you can hear the screeching of jungle birds and the roar of a waterfall. It can be a surprise, then, to visit a friend’s greenhouse in midwinter and, upon being ushered into the glassed-in plant area, to discover that the humidity is quite comfortable and the temperature is markedly lower than you expected— you need a sweater. There is nothing tropical about the place. The feeling is more that of a cool mountainside.
There are two kinds of greenhouses. Which of these, cool or warm, you prefer depends on several considerations. The decision rests partly on economics—a warm greenhouse obviously uses more fuel and thus costs more to operate. Also involved is the amount of work you are willing to do—pests multiply more rapidly in a warm greenhouse—and your choice of what you want to grow. The conditions beneficial to carnations, an import from southern Europe, differ from those that please begonias, which come from Central and South American jungles. Orchids, favored by countless greenhouse gardeners, come from all over the world, and while many are tropical or subtropical, there are many from dry cliff-sides in the temperate zones and a few from the far north.
Although the temperatures are different and the plants that will grow are different, the two kinds of Ultimate Greenhouse Gardening are otherwise much alike. The basic techniques differ from those for growing things outdoors or in the house on a window sill, but they apply across the board in the greenhouse. The care needed in plant selection is the same, regardless of temperature. Soil mixtures are similar, since they depend on the type of plant, not on temperature itself. And the general rules for fertilizing, watering and pest control, while specific for careful Ultimate Greenhouse Gardening, must be followed in both warm and cool types.
Even the relatively chilly air of the cool greenhouse can provide
superb blossoms in midwinter; the hyacinths, anemones, kalanchoes, geraniums
and cyclamens at left thrive in night temperatures of 45°.
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The cool greenhouse will be no warmer than 55° or 60° during winter days and at night will have a minimum reading of 50° or even 45°. The warm greenhouse, on the other hand, is one in which the daytime temperature will be in the seventies or eighties, and the night reading won’t be allowed to drop below 60°. In both cases the critical figure is the night minimum. Although most plants will tolerate abnormal heat for a short period provided they don’t lack moisture, they can be severely hurt if subjected to greater cold than is proper for them, even without being brought down to freezing. Some tropical plants , for example, will turn yellow at 50°.
Other plants can endure a wide range but need a particular level to do their best; they just stop growing if the temperature is too low.
COST OF HIGH TEMPERATURE
High fuel costs have made a warm greenhouse seem a luxury. But some gardeners are willing to sacrifice to grow flowers they treasure. One couple that long favored tropical orchids altered their living habits to suit their plants. Their house is small, heavily insulated and kept at 55° at night and during the day while they are at work. Evenings and weekends the thermostat is set at 60°.
By contrast, their attached greenhouse has a night temperature of 60° and a day temperature in the seventies.
"We can put on sweaters; our plants can't," they explained.
"We cut down on energy everywhere else and our bills are no higher than our neighbors' with this size house. Our greenhouse is a big part of our lives; we would cut almost everywhere else before we would let it go." A neighboring greenhouse owner did not agree. "With heating bills what they are now," he said, "our greenhouse is only for plants that can take cool temperatures. We don't heat it above 50° at night, but we are not lacking for plants." In his greenhouse, foliage plants were everywhere-ferns, edible figs, Norfolk Island pines, cacti, as well as such flowers as camellias, geraniums, even jasmine.
"I had given up on the jasmine, and even put it ,under the bench. And there it is, growing up around the side and in bloom.
The African violets are another matter; it’s too cold for them out here, but they do well in the kitchen."
PART OF BOTH WORLDS
Between these two extremes of choice, fortunately, is a middle ground that enables you to grow-economically-some tropical plants along with others suited to a cool greenhouse. For example, one compromise is the intermediate greenhouse, which has a day time temperature of 65° to 75° and a nighttime minimum of 55 or 50°. It provides a home for the less demanding tropicals on the one hand and for some of the more resilient cool-greenhouse plants on the other. Many greenhouse owners, reluctant to pay the cost of supporting a truly warm greenhouse but wanting to experiment with some of the plants needing higher temperatures, maintain an intermediate greenhouse and bring one section of it up to a warmer level. You can do this by partitioning off that section with a glass or plastic wall-and-door unit, sold by many greenhouse manufacturers, or by finding out what part of your greenhouse is naturally warmer and building up the heat in that area.
FINDING MICROCLIMATES
Almost all greenhouses have some warm spots and some colder ones. To identify them, use a maximum-minimum thermometer, which has a U-shaped column of mercury, one side recording the day’s high temperature and the other the day’s low. Keep track of temperature extremes over 24-hour periods at various locations and under various weather conditions. Check the temperature at various times of the day, too—at one moment you might find that it’s 60 degrees next to the glass on the shady side near the house, while in a sunny place the reading is over 70°. A warm area can be made still warmer with auxiliary heat or with heavy insulation ( -- 19).
To add warmth to one part of the greenhouse, use a special kerosene heater designed for greenhouse use and vented outside, or simply plug in a portable electric heater. More efficient spot control over temperature is possible with thermostatically regulated soil-heating cables or mats, available at garden centers or from greenhouse supply firms. They warm the soil in benches or pots, supplying what is called bottom heat because it originates below the plants. Such a cable is looped back and forth beneath the soil to warm plants in a bench; for plants in pots it’s looped in the gravel drainage bed. This bottom heat is surprisingly effective. Cuttings warmed to 70° will root in one third the usual time, and four to five times as many seeds will germinate in a seed bed that is heated, as will in an unheated bed.
Whatever kind of greenhouse you decide to have, you will find a wide choice of plants that are able to flourish in that environment. The encyclopedia listing of Section 5 classifies them as warm-greenhouse, intermediate-greenhouse or cool-greenhouse plants and specifies special temperature requirements within these broad groupings.
COLOR FROM THE TROPICS
In the equatorial world of the warm greenhouse, the opportunities are almost unlimited. You can grow bougainvillea, gardenias, hibiscus and gesneriads—not only African violets but also gloxinias, the Javanese lipstick plant, cape primrose and many others—as well as bizarre bromeliads like earth stars or pineapples, a wide variety of tropical ferns, such aroids as dieffenbachia and calla lilies, and the magnificent orchids of Central America and the South Pacific. Other orchids are suited to the intermediate green house, where nighttime temperatures are kept above 50°—the cattleyas so popular for corsages, the dancing lady orchids whose blossoms flutter in a slight breeze, and a multitude of remarkable hybrids whose flowers are unlike anything found growing naturally.
Many plants from these same families can be grown in the cool greenhouse if the genera are chosen for their tolerance of lower temperatures. Orchids such as lady’s-slippers, cymbidiums and dendrobiums are popular choices. Other suitable plants include cacti, many ferns, tulips, hyacinths, small citrus trees and an assortment of spectacular pot plants—pocket book flowers, camellias and fuchsias. But many owners of cool greenhouses use much of their space to raise flowers for cutting and arranging—calendulas, carnations, chrysanthemums and marigolds grow under glass to a size and perfection unattainable outdoors or on a window sill.
In any greenhouse you can grow the same varieties that you set outdoors or cultivate indoors as house plants, but in doing so you may miss much of the enjoyment. Better results are attainable with varieties that flourish only in the carefully controlled environment that a greenhouse makes possible—with greenhouse varieties of snapdragons , for example, you will get plants larger than the varieties that grow outdoors in the garden, with more spectacular blooms, and with blossoms at specially desirable times. These types of plants have long been used by professional florists and nursery men; many of them are identified as such in catalogs, and some special greenhouse catalogs, once circulated only among commercial buyers, are now also offered to amateurs in advertisements in gardening magazines.
VARIETIES FOR BOUQUETS
For cut flowers particularly, the greenhouse varieties enable you to produce very large blossoms at specific dates— For example, rooted cuttings for the winter-blooming carnations known as perpetual-flowering or florists’ carnations are available in some 25 different colors. In some cases, specialized breeding has produced so many varieties that they are classified according to characteristics of particular interest, such as color, size or blooming period. For three popular cut flowers—chrysanthemums, gladioluses and snapdragons—numbered classification systems have been established, and some catalogues list seeds or corms by the numbers.
Seeds for cultivating snapdragons in a greenhouse are divided into four groups identified by Roman numerals according to the season in which they bloom. In Group I are the white Sierra, pink Spanish Lady, lavender Señorita and other varieties that bloom under glass in early winter in the North, that is, north of the 38th parallel, which runs near San Francisco, Wichita and Richmond. (The latitude makes a difference because it determines the day length, which governs blooming.) If Group I seeds are planted around August 16, the snapdragons flower between December 16 and January 1; planted about August 28 they flower between January 20 and February 10. Varieties in Group TI—Treasure Chest or Debutante , for example—blossom in late fall or early spring north of the 38th parallel and in winter south of that line, the exact dates again depending on planting time. In a similar way, Group III snapdragons bloom in spring and fall in the North, in late fall or spring in the South; Group IV snapdragons bloom in summer in the North, summer or early fall in the South. Data specified in the seed catalogues for each group enable you to have snapdragon flowers at almost any time you like.
SCHEDULING MUMS
A different scheme applies to chrysanthemums. Their rooted cuttings are classified in two ways in some catalogs. First, a variety may carry a numeral between 7 and 15, indicating the number of weeks that generally elapses between the start of 14-hour nights and bloom. Those in the 7-and-8-week group are garden varieties that ordinarily bloom outdoors in the fall; but the 10-or-11-week kinds bloom in the greenhouse in November, and the 12-to-iS- week kinds bloom under glass in December.
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HOW TO HANG AIR PLANTS
A plant that takes its nourishment from the air may be mounted on a slab of tree fern bark. Use wire (any kind but copper, which is injurious to plants) to attach the plant to the bark until the roots are established.
To display several bark-mounted air plants, cut hardware cloth, a wire mesh, to the size you want. Fasten it to the wall where it will block the least light from other plants, and wire the bark- mounted plants onto the mesh.
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In a further refinement, catalogs relate certain mum varieties to the latitude that influences time of bloom. If you want pompom mums in spring , for example, you can plant a 10-to-11-week variety (the white Polaris, the pink Dark Delight, the bronze Tuneful or the yellow Iceberg , for example) in Chicago or Boston, but you would use a 9-to- 10-week variety (the white Icecap, the pink Delmonico) in Dallas, Atlanta or Fort Myers, Florida. Similar lists of choices related to geography are given for other kinds of mums.
Still a different system classifies the hybrid gladioluses. They are grown from bulblike corms classified in five series: the 100s, with flowers less than 2 inches wide; the 200s, with small flowers about 2 inches wide; the 300s, with medium flowers about 3 inches wide; the 400s, with large flowers about 4 inches wide; and the 500s, with giant flowers more than 5 inches wide. In addition to the series number, the catalog may code the variety’s color by number, following a system devised by the North American Gladiolus Council for classifying exhibition blooms. For example, in the number 500 the 5 indicates that it’s of the giant series, but the 00 means that it’s pure white; the same huge white gladiolus with a color spot would be numbered 501. A 410 is a large (4) yellow (10) glad; 414 is the same size but light yellow; 416 is medium yellow, 418 is dark yellow, 422 is light orange, 426 is deep orange. The entire system covers 28 basic colors, but so many distinct shades are included that the classification numbers begin with 00 (white) and run through 96 and 97 (tan and brown).
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EASY-TO-BUILD VINE SUPPORTS
For a row of vines, drop lengths of cord from hooks inserted in holes in the rafters. Fasten the bottom of each string to a dowel wedged in the planting box and anchored with U-shaped lengths of coat-hanger wire.
For a cylindrical support, cut a piece of plastic-coated-wire garden fencing with a 1-by-2 -inch mesh. Its width will be the cylinder’s height. Shape the fencing to fit inside the pot and twist the cut ends together.
For a conical support, space stakes three feet long at equal intervals around the inside rim of a flowerpot. Brace the stakes against the side of the pot and join them at the top with wire or twine to form a tripod.
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Each of these plants, as well as many others popular among greenhouse gardeners, calls for its own cultivation methods, and the controlled conditions of the greenhouse make it easy to adjust cultivation to specific needs. These adjustments, however, are mostly minor modifications to the standard conditions that suit a great many greenhouse plants. In general, most of them have similar basic requirements for soil, fertilizer and maintenance.
A STANDARD SOIL MIX
Although no soil at all is needed for some greenhouse favorites—the epiphytic orchids draw their nourishment from moisture in the air and from pieces of fern fiber or fir bark—proper soil is essential to most to serve as a reservoir of water and nutrients, and as a place suitable for roots to grow and function. Each plant has only a limited amount of soil, and frequent waterings tend to compact it and leach out the nutrients. Most greenhouse plants do well in a soil that contains a good proportion of both organic material, for nourishment and moisture retention, and inorganic material such as sand, for good drainage. Many commercial growers mix their own soil of one part garden loam, one part peat moss and one part sand. But homemade soil mix is almost certain to contain insects, insect eggs, infectious organisms and weed seeds, and it must be sterilized, a troublesome process. A better choice is one of the ready-made potting soils available at garden centers; it should be sterile and contain the necessary ingredients in proper proportion. An alternative is the so-called soilless mix—peat moss, ground bark, sawdust and perlite or vermiculite enriched with chemical nutrients. Although such mixes require more maintenance than potting soils—the nutrients must be replenished more frequently, and they are more difficult to moisten if they get dry— they are light in weight, and uniform bag to bag.
For plants in pots, replenish the soil only as needed when you repot. But if you raise cut flowers in soil-filled benches rather than in pots, replace the soil completely at least every other year, and preferably once a year; otherwise, fertilizer residues may build up or fungus diseases may develop.
Close control over soil conditions is necessary if you want to get the most from your special greenhouse plants, and the problem is complicated by the fact that the plants are in small amounts of soil in pots or benches isolated from the balancing effects of natural conditions outdoors. For routine feeding, liquid house-plant fertilizer is recommended in most cases, the dilution depending on the plant. However, some gardeners favor capsules of slow-release fertilizers, which automatically spread the application over a long period of time, perhaps even the entire life of a greenhouse plant. Their long-lasting effect also may be a disadvantage—the capsules may still be stimulating growth at a time when the plant is ready for a period of dormancy. For plants that require such a period, you must choose a slow-release fertilizer that will last just long enough to match the active growing period—some are compounded to last two months, others as long as 18 months.
CUSTOM-TAILORED NUTRITION
Such mixes, liquid or solid, contain standardized proportions of the three most important nutrient elements—nitrogen, which makes foliage lush and green; phosphorus, which stimulates root growth and flowering; and potassium (usually called potash), which helps a plant to resist disease. The percentage of each is indicated by a sequence of numbers: a common liquid fertilizer is labeled 10-15-10, meaning that, undiluted, it contains 10 per cent nitro gen compounds, 15 percent phosphorus and 10 per cent potash. For most greenhouse plants, a mixture containing the three elements in the ratio of 1-2-1—it may be labeled 5-10-5 or 10-20-10—is suitable when diluted as explained in the encyclopedia section of this volume; if other proportions are required for a particular plant, that fact is noted in its encyclopedia entry.
While regular application of a fertilizer mix suffices as a routine, it does not provide the subtle variations in conditions, attainable only in a greenhouse, that make the difference between ordinary plants and spectacular ones. For precise control over nutrient levels—and also of soil acidity— you should test the soil to learn how much of each essential element is present, and then, if the proportions are out of balance, redress the balance.
A number of easy-to-use testing kits are on the market. Some measure acidity only, while others also indicate the levels of the three nutrient elements. The simple kits consist of glass vials in which you mix soil samples with one after the other of several liquids; the mixture turns color, and you match the color to a chart to find the content of the element being tested.
To check for nitrogen with a typical kit, you fill the miniature test tube one-quarter full of soil, add the solution marked “Nitrogen” until the tube is half full, shake, then let the soil settle. If the liquid turns a reddish brown, reference to the kit’s color chart indicates that a low-nitrogen fertilizer, one containing only 2 percent nitrogen compounds, is called for. At the other end of the scale, a lime green color would call for more nitrogen, supplied by a mix containing 8 percent nitrogen compounds. In a similar way, the needs for phosphorus and potash can be measured.
The simplest way to redress the chemical balance of the soil is to select a fertilizer mix that is suitably proportioned; a wide variety, high in one element and low in others, is available. But specific inadequacies can be more precisely remedied by supplying each element separately in exactly the quantity needed, retesting after each application until a satisfactory reading is achieved. Various compounds for this purpose are sold in gardening stores; they must be mixed with soil to provide the strength desired. To add nitrogen, use sodium nitrate or ammonium sulfate; for phosphorus, use superphosphate; for potash, use potassium sulfate (potash of sulfate) or potassium chloride.
Much the same technique determines the acidity of the soil on the pH scale of 1 (acid) to 14 (alkaline), with 7.0 as neutral. Most greenhouse plants do best in neutral or slightly acid soil, pH 6.0 to 7.0, but some, such as camellias and gardenias, require greater acidity, while cacti generally are adapted to greater alkalinity. The pH is adjusted toward alkalinity with ground limestone, toward acidity with peat moss, ground sulfur or iron sulfate. You can simply sprinkle the chemical on the soil surface and then water, but it’s better to mix the additive thoroughly into the soil. How much to use to cause a desired pH change is specified in the test kits.
The beginning period, when soil and plants are new, is the time to start guarding against pests and diseases. Because the greenhouse is a closed environment with a mild climate, trouble can spread fast. But because it’s closed, ailments spotted early can be contained. If you see signs of an outbreak, spray immediately.
AN ALL-PURPOSE SPRAY
Many of the strictures that limit the use of chemical sprays on outdoor gardens or house plants don’t apply so strongly in the greenhouse. Its environment is so circumscribed that ecological damage is remote. The chemicals cannot harm birds or beneficial insects, nor are they likely to be a residual hazard to pets or children. For this reason, general-purpose sprays, effective against an entire spectrum of pests, can be used regularly in the green house although they ought to be avoided elsewhere. Of course, the chemicals are still dangerous to the user while they are being applied, and normal precautions must be observed; read the labels.
An efficient and safe all-purpose spray that will guard most greenhouse plants against both insects and diseases can be made by adding 1½ teaspoons of malathion (50 per cent emulsifiable concentrate), 1 teaspoon of dicofol (18.5 percent emulsifiable concentrate), 2 tablespoons of captan (50 percent wettable powder) and a quarter of a teaspoon of household detergent to a gallon of water. Apply this mixture every two weeks in spring, summer and fall, and every three weeks in winter, avoiding plants that are in flower. Be sure the spray reaches the undersides of the leaves.
Read the labels on the chemical bottles carefully to find out whether any of the plants you are growing should not be sprayed, since no all-purpose spray is truly all-purpose. The one just de scribed would be toxic to many ferns , for example.
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SPACING RANGING PLANTS
An easy way to space hanging plants as they grow larger and bushier is to slide them along a ceiling-mounted metal rod. The inch-thick rod is cradled on metal shower-curtain hooks inserted into holes drilled in the greenhouse rafters. Place the hooks about 2 feet apart, depending on the length of the rod and the number and weight of the baskets that are to be suspended from it.
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HYDROPONICS IN A DISHPAN
If you are curious about hydroponics try this homemade rig, using a plastic dishpan half full of gravel. Cut a ¼-inch hole in the dishpan and insert a 1-foot length of 5 plastic tubing, fastening it with epoxy cement. Rest the dishpan oh two supports and slide a clean 1-gallon gas can underneath. Fit a gasket with a ¼-inch hole into the can’s pouring spout and insert the other end of the tubing. Connect an electric timer to an aquarium aerator and insert its tube into the can’s air hole. Fill the can with a standard solution of house-plant fertilizer and pump it through the dishpan twice a day for 20 or 30 minutes. Replace the solution weekly.
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Once the greenhouse is insect-free, it will stay that way if openings are screened and if you are reasonably careful to spray all new plants before you add them to your collection. Be particularly vigilant in the fall, when you are bringing plants into the green house after they have spent the summer outdoors; all sorts of bugs will be hitching a ride into your pleasant winter resort. Inspect the plants carefully and hose them off thoroughly but gently, then spray them with the all-purpose spray. If despite these measures you detect signs of trouble, move the infected plants back out of the greenhouse at once, and treat them specifically as indicated in the chart.
With these precautions, you will be able to grow a great assortment of healthy plants. Just how great an assortment is up to you to decide, but start on a limited scale; as your skill increases, so will your desire for a greater variety of plants.
With new plants acquired and set out, and with a good fertilizing and spraying program planned, your principal day-to-day concern will be watering. Use water that is roughly the same tempera ture as the air—every greenhouse needs a hot- and cold-water supply with a mixing faucet. And check the pH of the water with your soil-testing kit. The water from many city supplies is alkaline, and therefore harmful to the majority of greenhouse plants, which require an acid or neutral growing medium. If the pH reading is over 8.0, water your plants every two weeks with a solution of 1 ounce of iron sulfate to 2 gallons of water. Don’t use softened water; softeners introduce chemicals that can harm plants.
JUDGING WATER NEEDS
Water only when the soil has started to become dry, then water until the soil is saturated—but be sure that all excess water can quickly drain away. There are several ways to find out whether a plant needs water. First, inspect both plant and soil. Do they look or feel dry? Is the soil light when it normally darkens when moist? Tap the pot if it’s clay; if the sound is dull the soil will be wet, but if you get a ringing sound the plant needs water. Benches can be tested by plunging a trowel into the soil and digging some up to see if it’s dry beneath the surface. Plastic pots can be tested by lifting them; those that need watering will be noticeably lighter than those that are moist. (Plants in clay pots require watering perhaps three times as often as the same plants in plastic pots.)
Plants need more water when they are in active growth or are flowering, much less when they are dormant. Large plants require more water than small ones. During hot, sunny weather, green house plants need more frequent watering; on cloudy, mild days less is needed, often none at all. Any unusual air movement may dry plants, as does continuous heater operation in cold weather.
High humidity is important to most plants, especially orchids. Plants enhance their own atmosphere by transpiration, whereby they release moisture, but often this is insufficient. One way to raise the greenhouse humidity is to wet the floor and bench areas at least once a day during hot, dry weather. To maintain a constantly high moisture level automatically, you may need a humidifier. But don’t buy one until you have operated your greenhouse for some time; most greenhouse gardeners find that normal watering routines provide adequate humidity.
Such attention to details pays off. Carefully regulating the amount and kind of moisture, adjusting temperatures, providing adequate light, choosing the right kind of soil, watching out for pests, and all the many other steps of patient, vigilant care make the difference between success and failure in Ultimate Greenhouse Gardening. By following them with common-sense attention you can make plants grow the way you want them to, when you want them to.
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The shapes of greenhouses to come
Some three billion years ago, when the earth was young, the very first plants that came into being lived in shallow pools of water warmed by the rays of the sun; later they grew not in water but in soil. For millions of years after that nothing much changed. Then in modern times man developed the greenhouse, which added artificial heat to the equation.
Today a curious turnabout is occurring. Shortages of fertile acreage outdoors have rekindled interest in water as a growing medium for plants. And increasing shortages of fuels have made greenhouse gardeners turn back to the sun not only for growth- assisting radiation but for heat. In the process, the greenhouse has become a hotbed of technological activity that is changing both its appearance and its role.
The plants in the greenhouse at right , for example, are growing hydroponically, entirely without soil. They are anchored in gravel, which contains no nutrients, and regularly flooded with water to which have been added all the substances they need for ideal growth. The results, reports the greenhouse owner, William Hazelett, are harvests of astonishing quality and quantity; they supply the Hazelett household with tomatoes, lettuce, cucumbers, zucchini, chard and Persian melon during all the months when field-grown produce is out-of-season.
Hazelett’s soilless greenhouse gets the supplementary heat it needs from electric heaters. But at research stations around the country, greenhouses are being developed that rely entirely on radiation from the sun and dispense with energy from conventional sources ( -- 38-39). All have devices for trapping and storing solar heat in air or water, then for releasing the warmth to nurture the plants. Most of these experiments were undertaken in attempts to reduce the operating costs of commercial growers. But the de vices and techniques they are perfecting foreshadow those that amateur gardeners may have to adopt in a time when scarcities of the world’s fuel resources make conservation essential.
Hydroponically grown tomato plants 8 feet tall (left) and cucumber vines 25 feet long (right) rise from gravel-filled beds in William Hazelett’s greenhouse.
=== A kitchen garden in wet gravel ===
In an 8-by-12-foot prefabricated fiberglass green house behind his Arizona home, William Hazelett reaps a six-month bounty of vegetables from two strips of gravel on the greenhouse floor. Automatic controls flood the beds with a nutrient solution three times a day. The solution remains around the roots of the plants for 18 minutes, then drains back into the nutrient tank, which is under the center walkway. According to Hazelett, the plants start growing upward earlier, grow taller and bear fruit sooner than they would in soil.
Every two weeks during the growing season, Hazelett empties the 50-gallon tank and refills it with fresh water and nutrients. He shuts down the greenhouse for two months each summer and sterilizes the gravel with chlorine bleach.
In a hydroponic greenhouse, a pump drives water from the holding tank, set below the level of the planting beds, into the beds themselves. When the water level reaches the top of the bed, the pump cuts off, gravity takes over and the water gradually recedes back into the tank. Some beds are slanted slightly to facilitate drainage.
Tomato plants, visible through the open door of the soilless greenhouse, reach almost to the roof, in front of them in a hanging basket is a conventionally grown begonia.
William Hazelett admires a pair of hydroponically grown cucumbers, two of 90 produced in two months by just three plants.
Ripe arid ripening tomatoes flourish on one of 10 hydroponic plants, which yield steadily from December through June.
=== Schemes for trapping the sun ===
Designed for a fuel-scarce world, the experimental solar heating systems shown here in schematic drawings were built at three research centers with the aid of government grants in a federally sponsored program that began in 1975. Each system uses a solar collector separate from the green house itself, in order to interfere as little as possible with the internal space of the greenhouse and its natural light. Each uses water as the heat- collecting medium. In the one below, the collector is a salt-water pool; in the one opposite it’s a heat-absorbing black plastic panel over which water trickles; in a third it’s a three-dimensional aluminum panel with a core of tubing filled with water. All three were intended to be used with heavily insulated greenhouses.
At the Agricultural Research and Development Center in Wooster, Ohio, two identical greenhouses are built side by side for comparison. One is heated by the solar pond at right, the other by natural gas.
[FRESHWATER LOOP; SALTWATER LOOP]
The solar pond for the Ohio greenhouse is lined with heat-absorbing black plastic, it collects and stores the sun’s heat in brine, which is always saltier—and heavier—at the bottom than the top. Heat reaches the bottom of the pool and is trapped there; the hot brine is too dense to circulate to the surface and cool, and the lighter brine at the top acts as an insulation blanket. Brine from the hottest point in the pond is piped to a heat exchanger in a storage tank of fresh water. The heated fresh water is then pumped through the greenhouse.
At Rutgers University in New Jersey, the collector is a black plastic panel sandwiched between two layers of clear plastic. A perforated tube along the top edge of the frame trickles water over the black panel, where it absorbs the sun’s heat. A gutter at the base collects the heated water and pipes it to gravel under the concrete floor of the greenhouse. Heat rises through the greenhouse at night; during the day the water is pumped back to the collector for reheating.
[ HEAT COLLECTOR TUBES]
The collector designed by Lockheed-Huntsville Research and Engineering Center and built in Alabama, consists of a wooden frame covered with vertical and horizontal tubing. When the sun shines, water is pumped up through the vertical tubes and becomes warm. At the top it passes through a horizontal tube into a storage tank (left). When heat is needed in the greenhouse, the hot water is pumped from the storage tank to coils in the greenhouse, then back to the storage tank.
=== A window wall to harness heat ===
The research greenhouse built by the University of Arizona uses an ingeniously designed double glass wall to collect heat. Black-painted venetian blinds, sandwiched between glass, heat trapped air, which is then blown underground for storage (diagrams, below). At night, a pump sprays fire- extinguisher foam into the air space between the two layers of the inflated plastic roof, blocking heat loss. During the day, when the pump is off, the foam liquefies and drains out, and the sun’s heat rays can get in.
When the sun is shining on the glass walls of this greenhouse, a stream of hot air is generated by the black-painted venetian-blind heat collector. The air travels through ducts and warms a subterranean bed of rocks (top). At night and on cloudy days, the air inside the greenhouse is blown across the hot rocks, picks up heat and is circulated through the greenhouse (bottom). HEAT COLLECTOR; GREENHOUSE; ROCK BED
Tomatoes, peppers, cucumbers, lettuce, eggplants and herbs grow in pots, beds and hanging baskets in the Arizona greenhouse.
Articles in this Guide are based on now-classic Time-Life Encyclopedia of Gardening Series from the 1970s ... a timeless series, some titles of which are still available in libraries and bookstores... see our Amazon Store for purchasing options.