How to Make a Greenhouse



Traditionally defined as any glass building or adjoining structure which contains plants, today’s greenhouse comes in a variety of sizes, shapes and compositions.

There are several advantages that all-year gardening greenhouses afford. Summer and fall crops yields can be stretched one season longer often through the otherwise deadly winter season. Gardeners also use their greenhouses to gain a head start on springtime planting.



Frosts, blizzards, heat waves, and other weather can be virtually ignored behind the greenhouse windows and walls. Tending fresh fruits, vegetables and ornamental flowers throughout the year is considered by many doctors to be a tranquilizer for daily stress and work tension.

Greenhouses today fall into several general attached (units adjoining the house, window box, basement, patio, or sun-porch greenhouses) or freestanding (full-sized units separate from the house, either above or partially below grade).



Attached Greenhouse

The gardener seeking to minimize construction and maintenance, and possibly even capture some heat for the house, may find that an attached greenhouse is more suitable than a freestanding be most effective, a south wall of the house could be chosen for this type of greenhouse. The spot should not be heavily shaded by trees or other buildings, but it should be protected from strong winds that could chill the greenhouse and possibly weaken it structurally. Greenhouse designers and builders agree that the most efficient use of space and solar heat gain can be obtained by making the length of an attached house about twice as great as its width.

Supplemental heating can be minimized—or eliminated entirely—by taking advantage of some features that are being incorporated into new solar and energy-efficient houses, namely, the use of multiple-layer glazing; nighttime insulating shutters, curtains and shades; and the addition of thermal mass, such as concrete, stone, or brick floors and house walls and rock-filled containers to store solar heat during the night and on cloudy days.



A truly efficient attached greenhouse can actually provide heat for the house in winter. Vents near the floor allow cool house air to enter the greenhouse, where it is warmed and then circulated back into the house by means of another vent near the ceiling of the greenhouse. Since warm air naturally rises, no fan is necessary in many instances to move this air. Such venting not only helps to warm the house, but it also permits good air glow throughout the greenhouse, raises the humidity of the house and distributes plant-loving carbon dioxide from the house to the greenhouse. Of course, vents to the house should be closed during the summer months, and vents from the greenhouse to the outdoors should be opened during this time so that neither the house nor the greenhouse gets overheated.

Although space in the attached greenhouse id limited by the upright building wall, every inch can be put to work by the use of plant benches, ground beds, eave shelves, ledges, and hanging baskets. Straight sides accommodate eave shelves better too, and they provide better ventilation and temperature control. The straight-sided houses are somewhat more expensive, however, because there is more glass area. On the other hand, slanting sides capture more sunshine.



Window Greenhouse

This variety is one of several “mini-greenhouses” designed to produce healthy flowers and herbs at a low cost. Window greenhouses are also used for starting seed in winter and spring before transplanting into the outdoor garden.

Almost any window opening into the house can be use for these small conservatories attached to a windowsill and framing. For gardeners planning to use the window greenhouse year-round, a southeast-facing window is recommended. This direction will obtain ample sunlight even in winter. Small heating units which fit into the window extension, or heating cables, may be used to keep the greenhouse sufficiently warm and insulating shutters or shades can be pulled over the glass at night.



Basement Greenhouse

From the outside a basement greenhouse looks like a sloping cold frame built against the foundation. Inside, it is an alcove in the cellar wall, and a concrete floor raised above the basement floor. Like the foundation, it is built of concrete blocks.

The floor should be at least 3.5 feet above the basement floor because of the sharp angle for the midwinter sun. The foundation wall in front should be about two inches higher than the greenhouse floor to prevent water from running out on the cellar floor to prevent water from running out on the cellar floor.

A shelf placed beneath the glass at one end is used for sun-loving plants and can be duplicated at the other end. The greenhouse should face south, southeast or southwest. With only one hot air vent in the basement, the temperature should stay between 55 and 60 °F (12.78 and 15.56 °C).

Freestanding Greenhouse

To many people, the freestanding greenhouses offer distinct advantages. Most of these greenhouses can catch sunlight from every direction, and they are more adaptable for ground beds. The more energy-efficient free-standing greenhouses have north walls built into a hillside.



The pit-type house, except for sever sub-zero (F.) weather, is sun heated. The only additional heat needed under conditions of extreme cold is usually a 200-watt electric light bulb or a small electric heater. Temperatures in the pit-type house make daily watering unnecessary. Usually only the south side is glassed in, and this is set at a 45-degree angle to admit the most sunshine. Ordinary hotbed sashes can be used.

To add warmth to the pit house, the ends and unglazed side should be double walled with about 31/2 inches of insulating material between. Doors and ventilators should also be insulated. After sunset, the glassed areas should be covered with padding or another insulated covering. When pads are used to cover the sashes, tarpaulins are rolled down over them to keep them dry. Wet padding makes poor insulation.

The Dutch door is best for the pit house because the upper half can be opened for ventilation during the winter. The door should be at the east end of the house to be better protected from prevailing cold westerly winds. A ventilating window can be placed at the west end. This is most important in the pit green-house. It should be open during the warmest hours of every day. Some pit houses use sky-light openings on the top of the unglazed side for ventilation.

Greenhouse Location

Choosing the best greenhouse site is an important step requiring several considerations. Convenience, accessibility, yard space, and general land conditions are variables to consider. Attached greenhouses enable the gardener to enter the greenhouse quickly and easily through adjoining, enclosed entrances. They best suit gardeners with little yard space, but with sunlit base—windows and sills suitable for a house “box,” or enclosed porches.



Contrary to popular belief, the precise direction in which a greenhouse faces is not a crucial consideration. Some plants in attached greenhouses grow best in a southern, southeastern or exposure, in that order. Western provides ample sunlight but lack the shade needed in summer.

Greenhouse Construction Materials

Once the style and location of your future greenhouse selected, construction materials need to be chosen. Gardeners can select from plastic, fiberglass and glass materials.

Above Ground Greenhouses

These greenhouses are made with panels that can be put together with a driver, wrench and hammer. All the parts are furnished, cut to fit in place. The glass is cut to size and is not putty glazed. It goes into glass grooves in the sash and is held weather tight with a special caulking rope.

While the prices for the materials for a prefabricated greenhouse are higher than parts such as glazing bars, sills, eaves, ridge and fittings of a conventional-type greenhouse, the time they save in labor greatly offsets difference.



Greenhouses with polyethylene film or plastic instead of glass are becoming popular for reasons of economy. They are light, so require less rigid supports, but they can rip in heavy winds and the constant exposure to strong sunlight causes them to deteriorate in a short as six months. Thin flexible films are best used as inner glazing only under thicker plastics or glass.

Fiberglass is another popular alternative in greenhouse construction. It is sturdy and transparent material, especially when coated with Tedlar. Fiberglass also makes a good heat insulator, retaining up to 70.8 times more hat than polyethylene film and some plastics. Fiberglass houses provide natural shade, even during intense sun exposure. Fiberglass has its shortcomings as well, however. It is highly flammable and often wears down reducing light transmission and increasing dust and don’t break as easily.

Rigid acrylics come closest to resembling glass, but they are easier to work with because they are five times lighter than glass and don’t break easily.

Glass has a tendency to turn brittle and crack, and while it is good-looking and clean, it is a difficult material for do-it-yourselfers to work with.

Greenhouse Foundation

The walls below the sills of the greenhouse are the hardest part to build. Masonry walls are best because they are more permanent than those of wood. They also offer some thermal mass for heat retention. Poured concrete, brick, cut stone or cinder blocks may be used. Cinder blocks provide the easiest means of building a wall. For appearance’s sake, the outside can be coated with stucco and painted. The attractiveness of a greenhouse depends a great deal upon its walls, for this is the largest solid area.

If you live in the northern United States, the walls of a prefabricated greenhouse should extend below the freezing line. This would be about 21/2 feet in most areas but may be less in the southern states and more in the far North. The footings below the ground can be of poured concrete and gravel. A conventional-type greenhouse is built with steel posts set on footings and encased in piers that extend below grade. The side walls need only go down to solid ground, a few inches below the grade.



After the greenhouse has been selected, located and constructed, the continual task of greenhouse management begins. Managing the greenhouse can be divided into two categories: controlling of the greenhouse climate and handling of the plants.

Greenhouse Heating and Cooling

In areas that seldom get colder than 20°F (— 6.67°C), more traditional greenhouses may need only an electric heater. The heater is inexpensive and can come equipped with an automatic thermostat to turn on the heating element and fan.

For colder environments, gas or hot-water heating systems arc necessary in traditional greenhouses. A no-vent gas unit for heating is highly recommended by many gardeners, since it creates no noxious fumes and costs a few hundred dollars. Coal or wood-burning furnaces can also be used to heat greenhouses.

Some suggestions for conserving fuel include keeping the greenhouse as airtight as possible; using two outside doors and having one serve as a storm door; using mulch to insulate and retain heat; installing heavy-gauge aluminum foil between the heat source and the outside wall to reflect and retain heat; and planting a windbreak of trees and shrubs nearby to retain heat and protect against wind turbulence.

The most important companion to any greenhouse heating system is ventilation. With-out fans to circulate air, the greenhouse temperature can vary from 45°F (7.22°C) on the ground, to over 90°F (32.22°C) nearby in winter. Mount the fan so that it is away from the heat source. This way, warmer air will mix with cool air and pick up moisture in the process. Proper air circulation is an important safeguard against plant infections, since it reduces fungus and mildew buildup.

Shade must be provided during summer months in the greenhouse, in order to grow plants which cannot be set outside. Bamboo or slatted matting may be spread over the glass or whitewashed.



Excessive humidity invites plant diseases and decreased fruit and flower production. Insufficient humidity in the greenhouse hastens development of flowers and fruit at the expense of leaf growth. To increase humidity, the gardener can install mist systems, plastic sheets or glass panes over seed flats or benches.

Handling of Greenhouse Plants

In caring for your plants, try to simulate all conditions favorable to the plants growth and development needs. If this demands a period of rest in the garden, a period should be allowed in the greenhouse. Sun, shade and soil requirements outdoors should be duplicated as much as possible in the greenhouse.

Soil

Concocting the proper soil mixtures is another important requirement for the successful greenhouse gardener. Good soil is an investment in the well-being of greenhouse crops, and it should be well fertilized and cultivated for that reason. Rich topsoil with living organisms, dead organic matter is best for hardy plant growth. Medium-texture soils, rather than fine or coarse compositions, are best for holding moisture, air and soil nutrients. Adding organic matter to sandy soils improves water and mineral retention, as well as helping loosen clay soils.

For most greenhouses, a loam soil is recommended, because of its good drainage and aeration. Greenhouse soils today are often mixes, high in organic matter content. A good mix for bench or potted plants is two parts topsoil, one part sphagnum peat moss and one part sand. Your soil mixture should always be kept fairly moist, in order to sustain the living organisms inside.

Many fertilizers and additives offer vital nutrition to organic matter in greenhouse soils. Poultry and rabbit manures are packed with nitrogen, phosphorus and humus. Both are applied at the rate of eight to ten pounds per 100 square feet of bench planting space. Sheep, cow and horse manures are organic fertilizers which add humus and make good soil conditioners.

Bone meal is a slow-releasing plant fertilizer. The steamed variety breaks down quicker for plant nutrition than raw bone meal.

Lime and wood ashes help neutralize highly acidic soils. Sawdust and wood chips complement successful potted plant propagation. The chips repel snails and provide good drainage.

Peat moss, which puts humus into the soil and holds nutrients particularly well, makes a fine soil conditioner, rather than a fertilizer. Moisten peat thoroughly before mixing into the soil. (Dry peat often resists water absorption.)

Gypsum conditions and alkalizes green-house soils. It also offers calcium to plants and indirectly to gardeners who harvest and eat them.

Vermiculite and perlite lighten dense soil and help start plant cuttings or seed. Fertilizers and plant nutrients are added after roots are established in either medium.

The salt concentration and pH level of soils must be watched carefully in the greenhouse, the state agricultural extension service will check field soil for high salt or pH levels, which can damage plant roots, cause wilting, or slow plant cutting or seedling growth. Loosening the soul and thorough watering help dissolve high concentrations.

Watering

Greenhouse gardens should be watered in the mornings of sunny days. Water should be supplied sparingly to minimize the dangers of fungus. Watering should be thorough. Watering should be withheld, if possible, in cloudy weather, since these conditions make evaporation slow and fungus spores cannot be destroyed as well as they can by hot sun rays.

Here are still more tips on proper watering in the greenhouse:

  1. Try to avoid ice-cold water. Room temperature water is preferred by most greenhouse plants.
  2. Water can run freely over the bench or tub, but be sure that roots are not left soaking.
  3. Keep soil loose for good drainage. Organic matter and sandy loam make the soil healthy and properly drained.
  4. Water plants less in winter, especially those that go into dormancy during cool weather. Their need for water decreases at these times.
  5. Avoid water softened with a commercial water softener. This water contains chemicals harmful to plants. Flushing salty or hard water usually prevents salt buildup.

Insect and Disease Control

In handling plant life in the greenhouse, special care and attention must be given to prevention of pest and disease infestation. Insects and diseases which commonly plague greenhouse crops are easy to control through the use of good-quality, clean seed and plants, in addition to the maintenance of an overall sanitary growing environment. Insects, bacteria, viruses, and fungi which thrive in “hothouse climates” can be battled by following these simple sanitation tips:

  1. Remove diseased and dead plants; keep them far from the greenhouse.
  2. Prevent wild weed growth near the greenhouse. Such growth attracts insects and promotes disease.
  3. Keep the greenhouse neat and free of plant clutter.
  4. Be certain that new plants introduced into the greenhouse don’t harbor new germs and pests.
  5. Start seed, roots and cuttings in soilless mediums. Sterile perlite, vermiculite and peat moss are commended for controlling seedling and cutting diseases.
  6. Provide proper greenhouse ventilation.
  7. Finally, avoid soaking foliage when watering. Also avoid over watering or over fertilizing greenhouse plants.

The organic greenhouse gardener can turn to several safe insect controls, dusts and sprays for disease outbreaks, especially those in the beginning stages. Commercially available controls include sabadilla, rotenone, pyrethrum, and nicotine sulfate. Many gardeners develop their own recipes for homemade pest or disease control.

Not all greenhouse or garden insects are enemies to the propagation of healthy plants, however. Ladybugs, praying mantis, lacewings, spiders, and horse hair snakes are among the many winged or crawling “comrades” in the garden who eat harmful insects.



Cool Greenhouse

In cool temperature and organic soil, bulbs such as tulips, Dutch Iris, Lilies, Daffodils, Hyacinths, Ranunculus, and Anemones are easy to grow and give fine blooms. Lettuce, radishes, Swiss chard, kale, and scallions do very well as do carrots, cauliflower, peas, red and green cabbage, and beets, if you have the extra space they require. Many herbs thrive in the cool greenhouse. A few pots or boxes of rue, sage, mint, marjoram, parsley, chives, and the like will provide garnishes for winter meals.

Farm Greenhouse

While it costs almost twice as much to bring the home greenhouse up to moderate or warm temperatures in comparison with a cool house, many exciting plants can be grown that make it worthwhile. Orchids are among them. With a collection of 75 plants of different varieties, it is nice to have something in bloom every part of the year. Cymbidiums will keep as long as three months. Insects are not a serious problem. Orchids can be grown in greenhouse where it is possible to maintain even temperature and keep the atmosphere fresh and healthy.

Other flowers and plants that do well in moderate to warm greenhouse include amaryllis, azaleas, begonias, ferns and tropical foliage plants, bougainvillea, cactus and gardenias. Tomatoes, cucumbers and melons can also be grown in the warm greenhouse where temperatures are at least 60°F at night.