Greenhouse – Classification and Key Advantages

A greenhouse is a generic term referring to the use of a transparent or partially transparent material supported by a structure to enclose an area for the propagation and cultivation of plants. Specifically, where the covering material is glass, the structure may be referred to as a glasshouse.

A green house or polyhouse refers to the use of plastic films or plastic sheeting.

Greenhouses may make the use of supplementary heating to maintain a required internal temperature or rely on air warmed in the day to maintain a minimum temperature at night.

When the enclosing material is woven or otherwise constructed to allow sunlight, moisture and air to pass through the gaps, the structure is known as a shade house.

Advantages of the greenhouse

Reducing or managing risk is a key to business success. All business, suffer business risk. This includes competition, variable input costs and uncertain returns.

In horticulture, an extra problem is environmental risk causing uncertain levels of production, difficulty in forecasting when you will have product to sell and even producing for when prices are likely to be higher.

A better quality product can be achieved by eliminating adverse environmental conditions using a greenhouse to:

• Provide an optimum growing environment
• Create longer growing season
• Grow crops out of season
• Get faster growth and higher yields
• Grow different varieties
• Protect crops from cold weather, hail, damaging wind and rain
• Keep pests and diseases out of the crop.
• Shade houses are used to protect crops and sensitive plants from intense sunlight, heat and wind.

Types of greenhouse structures

Classification of a greenhouse is according to its basic shape. Types include Gable, Flat arch, Raised dome, Sawtooth, Skillion, Tunnel etc.

Other types of structures:

(i) Shade houses

Shade houses are structures which are covered in woven or otherwise constructed materials to allow sunlight, moisture and air to pass through the gaps.

The covering material is used to provide a particular environmental modification, such as reduced light or protection from severe weather conditions.

The height of the structure will vary according to the type of crop being produced and may be as high as 8 metres.

Shade houses are used over outdoor hydroponic systems, particularly in warmer regions.

(ii)Screen houses

Screen houses are structures which are covered in insect screening material instead of plastic or glass. They provide environmental modification and protection from severe weather conditions as well as exclusion of pests.

They are often used to get some of the benefits of greenhouses in hot or tropical climates.

(iii) Crop top structures

A crop top is a structure with a roof but which does not have walls. The roof covering may be a greenhouse covering material such as plastic or glass, or shade cloth or insect screening.

These structures provide some modification of the growing environment such as protection of the crop from rain or reduction of light levels.

Classifying greenhouses

Greenhouses are a technology based investment. The higher the level of technology used the greater potential for achieving tightly controlled growing conditions. This capacity to tightly control the conditions in which the crop is grown is strongly related to the health and productivity of the crop.

The following three categories of greenhouse have been defined to assist people in selecting the most appropriate investment for their needs and budget.

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1) Low technology greenhouses

Low technology greenhouses have significant production and environmental limitations.

A significant proportion of the industry in Australia currently uses low technology structures. These greenhouses are less than 3 metres in total height. Tunnel houses, or “igloos”, are the most common type. They do not have vertical walls. They have poor ventilation. This type of structure is relatively inexpensive and easy to erect. Little or no automation is used.

While this sort of structure provides basic advantages over field production, crop potential is still limited by the growing environment and crop management is relatively difficult. Low level greenhouses generally result in a suboptimal growing environment which restricts yields and does little to reduce the incidence of pests and diseases. Pest and disease control, as a result, is normally structured around a chemical spray program.

Low technology greenhouses have significant production and environmental limitations, but they offer a cost effective entry to the industry.

2) Medium technology greenhouses

Medium technology greenhouses offer a compromise between cost and productivity. Medium level greenhouses are typically characterised by vertical walls more than 2m but less than 4 metres tall and a total height usually less than 5.5 metres. They may have roof or side wall ventilation or both. Medium level greenhouses are usually clad with either single or double skin plastic film or glass and use varying degrees of automation.

Medium level greenhouses offer a compromise between cost and productivity and represent a reasonable economic and environmental basis for the industry. Production in medium level greenhouses can be more efficient than field production. Hydroponic systems increase the efficiency of water use. There is greater opportunity to use non-chemical pest and disease management strategies but overall the full potential of greenhouse horticulture is difficult to attain.

3) High technology greenhouses

High technology greenhouses offer superior crop performance. High level greenhouses have a wall height of at least 4 metres, with the roof peak being up to 8 metres above ground level. These structures offer superior crop and environmental performance. High technology structures will have roof ventilation and may also have side wall vents. Cladding may be plastic film (single or double), polycarbonate sheeting or glass. Environmental controls are almost always automated. These structures offer enormous opportunities for economic and environmental sustainability. Use of pesticides can be significantly reduced. High technology structures provide a generally impressive sight and, internationally, are increasingly being involved in agribusiness opportunities.

Although these greenhouses are capital intensive, they offer a highly productive, environmentally sustainable opportunity for an advanced fresh produce industry. Investment decisions should, wherever possible, look to install high technology greenhouses.

Environmental Systems

Greenhouses provide a shelter in which a suitable environment is maintained for plants. Solar energy from the sun provides sunlight and some heat, but you must provide a system to regulate the environment in your greenhouse. This is done by using heaters, fans, thermostats, and other equipment.

a. Heating

The heating requirements of a greenhouse depend on the desired temperature for the plants grown, the location and construction of the greenhouse, and the total outside exposed area of the structure. As much as 25 percent of the daily heat requirement may come from the sun, but a lightly insulated greenhouse structure will need a great deal of heat on a cold winter night. The heating system must be adequate to maintain the desired day or night temperature.

Usually the home heating system is not adequate to heat an adjacent greenhouse. A 220-volt circuit electric heater, however, is clean, efficient, and works well. Small gas or oil heaters designed to be installed through a masonry wall also work well.

b. Air Circulation

Installing circulating fans in a greenhouse is a good investment. During the winter when the greenhouse is heated, it is necessary to maintain air circulation so that temperatures remain uniform throughout the greenhouse. Without air-mixing fans, the warm air rises to the top and cool air settles around the plants on the floor. The fan in a forced-air heating system can sometimes be used to provide continuous air circulation. The fan must be wired to an on/off switch so it can run continuously, separate from the thermostatically controlled burner.

c. Ventilation

Ventilation is the exchange of inside air for outside air to control temperature, remove moisture, or replenish carbon dioxide (CO₂). Several ventilation systems can be used. Be careful when mixing parts of two systems. Natural ventilation uses roof vents on the ridge line with side inlet vents (louvers). Warm air rises on convective currents to escape through the top, drawing cool air in through the sides. Mechanical ventilation uses an exhaust fan to move air out one end of the greenhouse while outside air enters the other end through motorized inlet louvers. Exhaust fans should be sized to exchange the total volume of air in the greenhouse each minute.

d. Cooling

Air movement by ventilation alone may not be adequate in the middle of the summer; the air temperature may need to be lowered with evaporative cooling. Also, the light intensity may be too great for the plants. During the summer, evaporative cooling, shade cloth, or paint may be necessary. Shade materials include roll-up screens of wood or aluminum, vinyl netting, and paint.

e. Controllers/Automation

Automatic control is essential to maintain a reasonable environment in the greenhouse. On a winter day with varying amounts of sunlight and clouds, the temperature can fluctuate greatly; close supervision would be required if a manual ventilation system were in use. Therefore, unless close monitoring is possible, both hobbyists and commercial operators should have automated systems with thermostats or other sensors.

f. Watering Systems

A water supply is essential. Hand watering is acceptable for most greenhouse crops if someone is available when the task needs to be done; however, many hobbyists work away from home during the day. A variety of automatic watering systems is available to help to do the task over short periods of time. Bear in mind, the small greenhouse is likely to have a variety of plant materials, containers, and soil mixes that need different amounts of water. Time clocks or mechanical evaporation sensors can be used to control automatic watering systems. Mist sprays can be used to create humidity or to moisten seedlings. Watering kits can be obtained to water plants in flats, benches, or pots.

g. CO₂ and Light

Carbon dioxide (CO₂) and light are essential for plant growth. As the sun rises in the morning to provide light, the plants begin to produce food energy (photosynthesis). The level of CO₂ drops in the greenhouse as it is used by the plants. Ventilation replenishes the CO₂ in the greenhouse. Because CO₂ and light complement each other, electric lighting combined with CO₂ injection is used to increase yields of vegetable and flowering crops. Bottled CO₂, dry ice, and combustion of sulphur-free fuels can be used as CO₂ sources. Commercial greenhouses use such methods.

COLD FRAMES

A cold frame is an outdoor growing “area” built without a bottom but with a solid-sided frame of wood, cement or brick, and a removable hinged top, glazed with glass, Fiberglas, or plastic. Cold frames are invaluable. For instance, they take some of the spring bulge from a greenhouse. By using them for growing greenhouse-started annuals and perennials, you make under-glass room for a new crop of salable plants. Then there are plants such as delphiniums, pansies, and Oriental poppies, to be planted in the frame in late summer and kept there over winter. The cold frame makes an excellent “cold-42 conditioning” rooting area for the spring-flowering bulbs you wish to force.

Cold frames are found in home gardens and in vegetable farming. They create microclimates that provide several degrees of air and soil temperature insulation, and shelter from wind. In cold-winter regions, these characteristics allow plants to be started earlier in the spring, and to survive longer into the fall and winter. They are most often used for growing seedlings that are later transplanted into open ground, and can also be a permanent home to cold-hardy vegetables grown for autumn and winter harvest.

Cold frame construction is a common home or farm building project, although kits and commercial systems are available. A traditional plan makes use of old glass windows: a wooden frame is built 30cms or a foot or two high, and the window placed on top. The roof is often sloped towards the winter sun to capture more light, and to improve runoff of water, and hinged for easy access. Clear plastic, rigid or sheeting, can be used in place of glass. An electric heating cable, available for this purpose, can be placed in the soil to provide additional heat.

LATH HOUSE

A lath house is a valuable asset in raising seedlings, rooted cuttings, and young rhododendron plants prior to setting them out in the garden. Properly used, it will modify the environment in which they are growing by offering protection from hot, drying summer winds, reducing the intensity of the sunlight, lowering temperatures, and by maintaining a higher humidity. During December, January and February, the sides may be covered by plastic sheeting, which will furnish additional protection from the winter cold.

The lath house here in Westport Point is simply a framework of treated lumber covered with snow fencing. The corner posts are 4″X4″s, and the remainder of the framework 2″X6″s and 2″X4″s. If you decide to build a lath house, it is suggested that a sketch be made first, taking into consideration the width of the snow fencing, location of a door, etc. Be certain there is enough headroom. The first one I built was only five feet in height, but eventually was replaced by one having a minimum height of six and one-half feet, which is much easier to work in. It was also found that in order to provide enough shade, two layers of snow fencing was necessary on the roof, a layer running in one direction and a second layer at right angles to it.

After using the second lath house for about a year, it was apparent that additional shade was needed. The roof was first covered with black shade cloth, a total disaster because the black cloth absorbed heat from the sun, making it hotter inside the lath house than out in the full sun. Fortunately, I then was able to locate a shade cloth made of woven strips of plastic and treated aluminum, which reflects the sun’s rays and gives the effect of high shade inside the house. This cloth is left on in winter and, with the plastic sheeting on the sides, it gives added protection to the plants. However, a word of caution if the cloth is not removed in winter. In this case, some support must be provided inside the house for the rafters or roofing joists, using vertical 2″X4″s, because a heavy, wet snow fall could pile up on the roof and bring the entire structure down.

In summer, air should be allowed to circulate freely through the lath house in order to keep temperatures down, and therefore, large plants must be kept at some distance. If plastic sheeting is used on the sides of the house during winter, it must be removed as soon as the worst of the cold weather is over. After a couple of years of experience, I decided to install a misting system, which serves a dual purpose of both irrigating and keeping the plants cool on very warm days.

The plants will be in the lath house for at least one year, or possibly two or three, depending on their rate of growth. The goal is to give them a needed helping hand until they are well established, and have larger, healthy root balls before being moved to the garden. In the garden the plants should continue to be treated as if they were in a nursery until they are able to take care of themselves.

Use a design of any shape or size for your lath house, keeping in mind the finished product must furnish enough shade and good air circulation. Many uses will be found for it, such as raising young nursery plants as well as those in containers, starting seeds and cuttings, and as a location for house plants which normally are moved outside during the summer.

THE HOTBED

A hotbed, obvious as it may sound, is basically a cold frame with heat. While cold frames receive all of their heat directly from the sun, hotbeds are heated with electric soil cables, stable manure or steam, or hot water heated with flues. The hotbed can be used earlier in the spring and later in fall and early winter than the cold frame.

Hotbeds are constructed just the same as cold frames, with a slope to the south to admit heat from the sun and to allow water or snow to run off. Plants growing in these frames are protected on cold spring nights with the same kind of mats sug­gested for cold frames.

Hotbeds are usually built to be permanent structures, with the frame of wood, concrete, or brick extending into the ground below the frost line. As with the cold frame, you can build it yourself, purchase a kit of materials for building it, buy a ready-built one, or have someone construct the entire thing for you.

A soil-heating cable furnishes the simplest kind of heat for the hotbed and these cables come in a variety of sizes and prices. The type used for hotbeds is insulated and enclosed in lead or plastic sheathing. The cables are made in several lengths but the most useful sizes are 40, 60, or 80 feet, all adapted for use with an ordinary electric service of 110 volts.

A 60-foot cable will heat a 6- by 6-foot hotbed. You should reckon your cable to suit your space. Each 60-foot cable car­ries an electrical load of approximately 400 watts. In our area the cost of operating such a cable on a continuous 24-hour basis is about 1 cent per hour. You should have a thermostat to regu­late air temperature and another to regulate soil temperature. However, you will find that during many hours of the day the sun will heat the hotbed enough so the thermostat shuts off the current. As spring nears, the outdoor temperature rises and the artificial heat will be on for shorter periods of time.

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