Greenhouse Types and Structures
AGS 250 Horticulture Science
The structures and types of control systems that are used will be dictated by which of the environmental factors must be controlled, to what degree they must be controlled, and the cost of controlling them in relation to the value of the crop(s) being produced. Each environmental parameter that must be controlled increases facility and production cost. The objective is to design a controlled environment structure that allows for the control of those parameters that need to be controlled at the level of precision required. Doing more only adds to the cost of production. Common types of controlled environments include:
A true hoop house is generally only an arched structure or frame that provides cover, and thus some degree of light and temperature control, for crops. Hoop houses are generally used for over wintering plant materials or for starting hardy spring crops (broccoli, cabbage, ornamental perennials) early in the season. They typically do not have heating or cooling systems. Hoop houses may be covered with polyethylene film, shade fabric or may have no covering during warm seasons. Sometimes these structures are referred to as quonset houses. However, one type of greenhouse design is the quonset-type structure. Therefore, the term quonset should be used only to refer to a type of greenhouse design.
Cold frames are similar to hoop houses and serve a similar purpose. The difference is that a cold frame may be partially set into the ground, is typically not as tall as a hoop house and may have a flat roof. Cold frames are generally used for over-wintering plant materials or for starting hardy spring crops (i.e. broccoli, cabbage, ornamental perennials) early in the season. Cold frames may also be used to provide the necessary cold treatments to bulb crops. Cold frames have no heating or cooling systems.
Hot beds are similar to cold frames except that hot beds have some type of heat source and thus provide more control over temperature. The heat source may be hot water or steam from a boiler, electrical heating units, incandescent light bulbs or composting manures placed inside of the hot bed. Hot beds are most often used for starting plant materials in the early spring.
Shade houses(Sometimes referred to as Saran Houses) are structures that are covered with a fabric made of polypropylene, cotton, plastic or other material that is designed to partially exclude light. Some shading materials are aluminized so that light is actually reflected away from the structure. The cover material may be selected to block out varying amounts of light, but typically shading materials excluding 20% to 60% are most common. These structures are generally used in subtropical (i.e. Florida) and tropical climates where reducing the light level and providing some measure of cooling (by shading) is desired. Shade Houses typically do not have heating or cooling systems. Shade houses are most often used in the production of cut flowers, foliage plants and nursery stock.
Greenhouses are the most common types of structures used for production of ornamental and vegetable crops under controlled conditions. These structures provide the potential to control all environmental parameters, although to varying degrees depending upon the design of the structure and its components. We will discuss greenhouses in much greater detail.
Coolers allow for plant materials to be held at low temperatures. Typically temperatures in the range of 35 - 50° F (2° - 10° C) are most common. In a few situations, temperatures below 32° F (0o C) may be required. Coolers are most often used for the storage of vegetables, fruits and cut flowers, holding nursery stock and providing a cold treatment (for vernalization or to break dormancy) to bulb crops.
Growth chambers are computer-controlled enclosed units that potentially allow for very precise control of many or all of the environmental parameters previously discussed. Growth chambers are most often used for research purposes although they may be used in some propagation situations such as tissue culture. Growth chambers may be small reach-in chambers or large walk-in chambers.
Germination chambers are similar to walk-in growth chambers except that they are primarily allow for the control of temperature, humidity and possibly light. They are often large walk-in rooms that are well insulated to minimize temperature fluctuations, and they have some type of fog system used to maintain a high relative humidity. They are designed specifically to provide an optimal environment for seed germination.
There are a variety of greenhouse designs. However, most of these are derived from two basic designs: the quonset and the A-frame.
The quonset is based upon an arched roof. The arched roof allows stresses on the structure to be efficiently transferred down to the ground. Quonset greenhouses may come in two basic designs. In the first, the arch extends to the ground with no sidewalls. In the second, the arch essentially forms the roof and gable sections of the greenhouse and is set on straight vertical walls.
The A-frame usually, but not always, has a series of supporting trusses that form the roof and gables. The strength of this structure primarily comes from the trusses set on vertical walls. The weight of the structure and other stresses are borne by the trusses and transferred to the vertical walls that in turn transmit the stresses to the ground. A-frame greenhouses may be even-spans or uneven spans. In the former, both roof sections are of equal length, but are of unequal (or missing entirely) length in the latter.
These two basic designs may be single stand-alone structures or combined side-to-side to form ridge-and furrow or gutter-connected structures. In this case, the interior walls are usually absent. Most commercial greenhouses now utilize some variation of the gutter-connected design. This is primarily because the gutter-connected design allows for a larger unobstructed interior than would be possible with stand-alone houses. Having a large unobstructed interior improves the ability to automate common tasks such as irrigation and improve space usage efficiency. Additionally, by eliminating interior walls (which would be exterior exposed walls in free-standing structures), the cost of construction materials and heating costs are reduced.
Several potential drawbacks exist for gutter-connected facilities. Since the entire production area is a single space, the ability to maintain different environmental conditions (such as would exist with numerous individual structures) is lost. Additionally, as the size of the gutter-connected span increases, uniformity and control of light, temperature, airflow and humidity can be reduced. One way to minimize these issues is to have drop-walls or curtains made of polyethylene film that can be raised or lowered between sections. This allows sections within the structure to be partially isolated so that different temperatures or relative humidity levels can be maintained if only to a limited degree.
Greenhouses, primarily gutter-connected designs, are often referred to as being of "American" or "high-profile" design or of "Dutch" or "Venlo" design. The American or high-profile design is a traditional A-frame greenhouse with a relatively large roof area compared to the wall area. The Dutch of Venlo design has higher walls, smaller gables, narrower individual greenhouse sections and reduced roof area. This reduces the roof surface area (an area of major heat loss) and heating costs.
Greenhouse walls support the roof and transmit stresses to the ground. In older greenhouses walls may be only 6' to 8' high (ground to eaves or gutter in gutter-connected design). However, in newer greenhouses, walls are 12' to 14' to better accommodate automation. I some cases external side walls may be designed as a polyethylene curtain that may be raised during low temperatures and dropped during warm weather to provide improved ventilation and to promote passive cooling. In other cases, greenhouses may have retractable roofs. The roofs may be closed during low temperatures or when light levels are too high. They can be opened to maximize light available to the plants and to allow for passive cooling.
Other greenhouse designs exist but are less common. The gothic arch is a design similar to a quonset, but this type of arch provides increased support and a larger unobstructed interior. The sawtooth, which is an example of an uneven span, is more common in high temperature locations and those places that receive pervailing winds since the design allows for improved movement of hot air out of the greenhouse roof vents. The lean-to design is most commonly used by homeowners and the geodesic dome is most often used by botanical centers. Some botanical center conservatories have elaborate cylindrical, arched or Victorian designs.
Greenhouse frames (support structure) may be constructed of wood, steel, aluminum or concrete. Modern greenhouses are usually constructed of steel or aluminum. Aluminum is the material of choice since it is light-weight, strong and rust-resistant. Low-cost small quonset greenhouses with polyethylene covering may use bent electrical conduit or pipes for physical support.
Wood is typically only used for hobby greenhouses, coldframes and hotbeds. Wood is difficult and expensive to maintain as it needs to be treated with a preservative and periodically painted to prevent rotting. When using wood in a greenhouse, coldframe or hotbed, never use creosote or pentachlorophenol-treated wood because they contain phytotoxic volatiles. Chromated copper arsenate (CCA), ammoniacal copper arsenate (ACA), copper naphthanate and zinc naphthanate are recommended for use on wood being used in greenhouses.
Floors may be constructed of porous concrete, Portland cement, gravel or compacted clay covered with a strong polypropylene fabric. Porous concrete is usually strong enough to bear most loads encountered in greenhouse situations and allows for drainage through the surface. Portland cement is more expensive, does not allow drainage through the surface and is more expensive. However, Portland cement might be desirable in traffic areas where heavy loads will occur. Concrete floors (unless used as part of the irrigation system) should have a slight grade to promote drainage and prevent puddling of water. Gravel is low cost and allows drainage but can allow the growth of weeds and may not accommodate all types of equipment. Polypropylene fabric can be a low cost alternative but the floor can become uneven over time, can cause puddling and algae growth.
Curtain walls are non-translucent sections of the greenhouse wall. They are typically located along the lower 2' - 4' of the of the greenhouse walls. Curtain walls are typically constructed of concrete block, cement, brick or some other non-transparent and well-insulated material. Because the curtain wall only extends up to approximately bench height, it does not significantly reduce the light available to the crop. However, because it is constructed of a well-insulated material, it reduces heat loss from the greenhouse. In northern climates, the entire north wall of the greenhouse may be constructed as a curtain wall to reduce heating costs. In northern climates during the winter months, a relatively small proportion of the light entering the greenhouse does so through the north greenhouse wall. Therefore, the savings in heating costs outweigh the reduction in light levels in the greenhouse.
Many factors must be considered in the greenhouse structural design. It is difficult to give a specific set of requirements, as there are many exceptions to any rule. However, a structure must meet the building codes for the specific locality. Most greenhouses are now designed by engineering firms or are constructed from packages developed by engineering firms. The design and all the materials are provided by the design firm. In many cases, the design firm will also build the structure in place for an additional fee. Larger installations are usually custom designed and built by an engineering firm. However, with this in mind, it is still valuable to understand basic design considerations.
The primary objective in designing a greenhouse structure is to maximize light transmittance (i.e. minimize obstructions to light entry) while providing adequate support. In many cases minimizing heat loss in important, while in others, allowing maximum air exchange for cooling is desired.
Greenhouse engineers often refer to design loads. The design load includes the dead load and the live load. The dead load includes the weight of the structure, framing, glazing, permanent equipment, heating and cooling units, vents, etc. The live load includes the weight of people working on the roof, hanging plants, snow loads and wind loads. Most often greenhouses are required to support an 80-mph wind. The required snow load is based upon the expected accumulation, the roof slope and whether the greenhouse is a gutter-connected structure or a stand-alone greenhouse.
In gutter-connected greenhouses, the gutters should slope slightly to encourage drainage of runoff from the roof. The gutters or eaves should be high enough to allow for automation with 12' to 14' being recommended. At least one entrance into the greenhouse should be large enough for carts, trucks of other equipment.
Greenhouse structures should be designed to allow for automation. This requires that width of walkways and driveways accommodate carts and equipment. Width of greenhouse bays may need to be designed to be compatible with irrigation systems such as irrigation booms.
The foundation must support the structure and transfer loads to the ground. In some cases, the structure may set on an intact concrete foundation or slab. Supports may be bolted onto the foundation. In other cases, whether or not a concrete foundation is present, the structure may be supported by vertical beams placed on concrete footings. Footings should extend below the frost line or at least 24 inches into the ground.
Electrical conduit or pipe may work well for a small polyethylene covered quonset house. However, it is not strong enough if the diameter of the quonset becomes too great or if the loads are too great The gothic arch increases the strength of the standard arch by more effectively directing the load to the ground. This increases the potential span and the strength of the structure and reduces the need for internal structural supports which in turn allows for a larger unobstructed space. In an A-frame greenhouse, the structural support is derived from the supporting trusses and rafters. The strength and number of rafters and trusses required depends upon the weight of the glazing material, wind loads and snow loads. However, as the support required increases, there is a reduction in light availability to the plants.