Section 1: Environmental Parameters of Interest

Controlled environments used in horticultural production may be as simple as saran-covered shade houses or as complex as growth chambers. Although greenhouses are probably the most common example of a controlled environment used in horticultural production, the type of controlled environment and system's that are needed depends upon the climate, time of year, crops being produced, the environmental parameters that must be controlled and the degree to which these parameters need to be controlled. The environmental parameters that may need to be controlled include:

Temperature
One of the primary reasons for producing crops under controlled environments is so that temperature can be optimized for the crops being produced. By controlling temperature, production may be extended into seasons where outdoor production would not be possible, or temperature may be manipulated to improve production and quality, reduce production time, break dormancy or control flowering. It is important to remember that lowering temperature may be as important as increasing temperature depending upon the climate (i.e. subtropical and tropical locations vs. temperate locations), time of year (i.e. summer vs. winter) and crops being grown.

Light
Light is a critical component for controlled environment production. In cases such as greenhouses, the only light source may be the sun, or supplemental light (light added from an artificial source such as high-intensity lamps) might be used to increase the light levels or increase daylength. In some cases, such as during summer months, reducing light levels through the use of a sun screening materials (i.e. saran shade cloth) may be required. In other cases, such as growth chambers and seed germination chambers, all of the light must be provided from an artificial source. It is important to remember that there are 3 characteristics of light that are important for the production of crops in controlled environments (including greenhouses).

Quantity (irradiance/intensity)- To maximize production, the quantity of light (number of particles of light called photons) received by the plant needs to be optimized. Too low light or too high light levels can be detrimental and reduce production or crop quality. Different plant species also have different light level requirements. For example, the optimal light level for most ferns is much lower than that for chrysanthemum or tomato.

Duration (photoperiod) - Photoperiod (more correctly the length of the night) affects plant growth and development. For example, some plants (i.e. poinsettia and chrysanthemum) are short-day plants (actually long-night plants) and only flower when the night exceeds a critical length (i.e. 12.5 hours for poinsettia). Photoperiod is often manipulated to either maintain plants in a vegetative state or to induce flowering. To accomplish this, blackout shade cloth might be pulled in the late afternoons to essentially create a longer night (short day) or lights may be turned on at night to essentially create a shorter night (long day).

Quality (wavelength/color) - Visible light quality may be expressed in terms of the light's wavelength (in nanometers) or the color that it appears to the human eye (think of the colors of light in a rainbow). Optimal plant growth requires light of specific wavelengths. For photosynthesis, light in the range of 400 - 700 nanometers (nm) is most effective for driving photosynthesis. This wavelength range is referred to as photosynthetically active radiation. While red (i.e. 660 nm) and blue (i.e. 460 nm) are the wavelengths of light used most efficiently in photosynthesis, red, far-red (i.e. 730 nm) and blue light have significant effects on plant photomorphogenesis (changes in plant growth due to exposure to different wavelengths of light). For example, a light source used for night interruption to create long days and prevent flowering in short day plants (i.e. poinsettia) must have some red light supplied from the light source to be effective. Plants grown under high levels of blue light tend to be shorter while plants growing under conditions with high far-red light tend to be taller and elongated. Light supplied only in a narrow wavelength band can be detrimental to plant growth. Understanding how light quality affects plant growth is very important when selecting a light source to be used to supplement the light available for photosynthesis or to manipulate photoperiod.

Water
One of the most essential components for any successful horticulture operation is water. This is particularly true for production in controlled environments such as greenhouses where intense cropping requires not only large volumes of water, but water with suitable chemical characteristics. Characteristics such as the water's alkalinity, electrical conductivity and concentration of potentially phytotoxic ions such as fluoride or sodium are particularly important.

Fertility
Plants require mineral elements such as N, P, K, Ca, Mg and others for proper growth. The goal in controlled environment agriculture (including greenhouse crops production) is to provide the required elements in the appropriate concentrations and ratios to maximize production. Mineral element deficiencies, toxicities or imbalances can be detrimental to plant growth.

Substrate (root medium)
Naturally occurring soils (field soils) are typically not used for the production of crops in controlled environments (including greenhouse). Instead, substrates (root media) composed of materials such as Sphagnum peat, perlite, vermiculite, composted barks, coir or rice hulls are formulated to create a substrate with the physical and chemical characteristics desired for the specific crop and environment. The substrate must not only provide physical support for the plant, but it must also serve as a reservoir for water and nutrients. Additionally, the substrate must allow for gas exchange so that oxygen is available to the roots.

Humidity
Depending upon the objective, increasing or decreasing the relative humidity experienced by the crop may be desired. During propagation, mist or fog systems might be used to increase the relative humidity and reduce water loss through transpiration. During crop production, venting of water-saturated air may be required to lower the relative humidity and reduce the possibility of foliar disease development (i.e. Botrytis).

Atmosphere
Although the atmosphere may include several environmental parameters, of particular interest are such factors as carbon dioxide concentration, ethylene and substances such as phytotoxic volatiles given off by certain wood preservatives and herbicides. Carbon dioxide is an important component in photosynthesis. Often times in controlled environments (including greenhouses) the carbon dioxide level is below optimal and is limiting to plant growth. In such a case, the carbon dioxide level may be increased by injecting carbon dioxide into the atmosphere. Ethylene gas and other volatile compounds can cause undesirable effects such as malformed growth and flower abortion.

Airflow
Airflow may be manipulated in order to provide a more uniform temperature through the environment or to remove hot air from the environment. Airflow may also be manipulated to control the relative humidity experienced by the crop.

Insects/Diseases
Although insects and diseases are not an environmental parameter in the true sense of the word, they do affect the quality of the crops being produced, and they need to be maintained at or below economic threshold levels.

Section 2: Common Types Controlled Environments Used in Horticulture

There are various types of structures that are used in controlled environment horticultural crops production. The structures and types of control systems that are used are 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 costs. 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:

Hoop Houses
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 common greenhouse design is the quonset-type structure. Therefore, the term quonset should be used only to refer to a type of greenhouse design.

High Tunnels
High tunnels have become popular controlled-environment structures for those growing fruits and vegetables. High tunnels are essentially the same as hoop houses but they are almost always covered with a single layer of polyethylene film and often have retractable side walls and ends that can be rolled up or down depending on the inside temperature and humidity. High tunnels are generally used to provide for temperature modification (as well as protection from wind and rain) to increase the season for small fruit and vegetable production. The temperature protection allows crops to either be started earlier in the season or grown later into the season that would normally be possible for a given location.

Cold Frames
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. The side walls of cold frames may be constructed of wood, concrete blocks or bricks. The top is usually made of some type of translucent material so the light can enter the structure. The roof is also usually hinged on one side the that the roof section may be lifted to open the cold frame. 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
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
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% of the incoming light are most common. These structures are generally used in mediterranean, subtropical (i.e. Florida, southern Texas, southern California) and tropical climates where reducing the light level and providing some measure of cooling (by shading) is desired. If the shade fabric is extended down the sides of the structure to the ground, crops inside of the structure are also partially protected from wind. 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.

Coolers
Coolers allow for plant materials (i.e. bulbs, seed, plants) 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 (0° C) may be required (i.e. freezing in of bulbs). 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
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
Germination chambers are similar to walk-in growth chambers except that they 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 with minimal fluctuations in temperature and relative humidity.

Greenhouses
Greenhouses are the most common types of structures used for production of ornamental and vegetable crops under controlled environments. Greenhouses provide the potential to control all environmental parameters, although to varying degrees depending upon the design of the greenhouse and the types of control systems utilized. The remainder of this learning unit and future learning units will focus specifically on greenhouse structures and the production of crops in greenhouses.

Section 3: Common Greenhouse Designs

There are numerous greenhouse designs. However, most of these are derived from two basic designs: the quonset and the A-frame.

The quonset is based on an arched roof. The arched roof allows stresses (i.e. weight on the roof) 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 whereas in the later they are of unequal (or missing entirely) length.

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 where the spans are connected together 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 numerous stand-alone greenhouses. Having a large unobstructed interior improves the ability to automate common tasks such as irrigation and improves 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 when compared to multiple free-standing structures. 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, maintaining uniform conditions throughout the interior space becomes difficult. 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 extent. Finally, it is easier for insect pests to spread through an open gutter-connected design than it would be with multiple individual greenhouses.

Greenhouses, primarily A-frame 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" or "Venlo" design has higher walls, smaller gables, and narrower individual greenhouse sections and reduced roof area, which reduces the roof surface area (an area of major heat loss) and heating costs. The higher sidewalls of the "Dutch" design provide more room (in height) inside the structure for hot air to rise and thus improves cooling. Although the terms "American" and "Dutch" typically referred to A-frame structures, they are now also used to refer to greenhouse structures with quonset roofs (with "Dutch" structures having smaller quonset roofs per section and taller sidewalls than "American" structures).

Greenhouse sidewalls support the roof and transmit stresses (primarily the weight of the structure) to the ground. In older greenhouses, sidewalls may be only 6' to 8' high (ground to eaves or gutter in gutter-connected design). However, in newer greenhouses, sidewalls are 12' to 14' to better accommodate automation (or the production of highwire vine crops). Sidewalls may be covered with a translucent (allows light to pass through) glazing to the ground or the lower portion (typically the lower 3 feet) may be made of some type of insulated none transluscent material (i.e. concrete, brick, concrete blocks, etc.). In the later case, the portion of the sidewall glazed with the nontransluscent material is referred to as the curtain wall. In other cases, external sidewalls may be designed as a polyethylene film curtain that may be raised during low temperatures to retain heat in the greenhouse and lowered during warm weather to provide improved ventilation and to promote passive cooling.

Greenhouses may have stationary roofs with or without roof vents. Roof vents may open at the central ridge line such as in the MX design. Others may open farther down on the roof with the ridge line as the pivot or hinge point. Other greenhouse designs may have a quonset roof that detaches from one side at the top of the wall (at the gutter in the case of gutter-connected structures) and the entire roof opens (referred to as a clam shell greenhouse). Still other greenhouses may have totally retractable roofs. In this case, the roofs may be closed during low temperatures or when light levels are too high and opened to maximize light availability and to allow for passive cooling.

Numerous other greenhouse designs exist but are less common. The gothic arch is a design similar to a quonset, but this type of arch comes to a point at its apex and provides increased support and a larger unobstructed interior. Gothic arch greenhouses may be single-spans or connected as gutter-connected structures. The sawtooth, which is an example of an uneven span, is more common in high temperature locations since the design allows for improved movement of hot air out of the greenhouse roof vents. Sawtooth greenhouses may be single-spans or connected as gutter-connected structures. The lean-to design is most commonly used by homeowners and conservatories and is an uneven span greenhouse that is connected to another structure with the wall of the alternate structure serving as a sidewall to the greenhouse.

Public facilities, conservatories and botanical gardens often utilize unique greenhouse structures to accommodate public displays. These structures may take the form of geodesic domes, cylinders, classic Victorian structures. Still others may be totally unique structures of highly unusual design.

Section 4: Common Greenhouse Structural Components and Materials

Greenhouse frames (support structure) may be constructed of wood, steel, aluminum or concrete. Modern greenhouses are usually constructed of steel and/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 curved aluminum tubes or even bent electrical conduit or pipes for physical support.

Wood is typically only used for hobby or homeowner greenhouses, cold frames and hot beds. Wood is difficult and expensive to maintain as it needs to be treated with a preservative or periodically painted to prevent rotting. When using wood in a greenhouse, cold frame or hot bed, 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 typically recommended for use on wood being used in greenhouses.

Floors may be constructed of porous concrete, standard concrete, gravel or compacted clay covered with a strong polypropylene fabric. Porous concrete is usually strong enough to bear most loads encountered in greenhouse situations (except for heavy equipment) and allows for drainage through the surface, but it is difficult to clean. Standard concrete is stronger, easy to clean, does not allow drainage through the surface and is more expensive than porous concrete. Often, standard concrete is used in areas where heavy traffic (i.e. equipment) must be supported and porous concrete is used on other areas. Unless being used as part of an irrigation system (i.e. flood floors), concrete floors should have a slight grade to promote drainage (preferably towards a drain where it can be recollected) 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 is low in cost but the floor can become uneven over time, which results in uneven irrigation, puddling and algae growth.

Various types of translucent (allows light to pass through) coverings, referred to as glazings, may be used to cover the greenhouse structure. Greenhouse glazings are discussed in more detail under the “Lighting” learning unit. The glazing may cover the entire greenhouse structure including the roof and side walls or the greenhouse may have a curtain wall. Curtain walls are non-translucent (not allowing light to pass through) sections of the greenhouse side wall. Curtain walls are located along the lower 2' - 4' of the greenhouse side walls. Curtain walls are typically constructed of concrete block, cement, brick or some other non-transparent and well-insulated materials. Because the curtain wall only extends up to approximately bench height (if elevated benches are used), 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, 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. However, where plant material is being grown of the floor, the curtain wall may block light from reaching plants that are placed close to the sidewall (typically within a few feet of the curtain wall). On the northern side wall, this loss of light available to the plants might not be significant. However, along the other side walls, the loss of light due to shading from the curtain wall can be significant and may result in reduced crop of plants placed close to the curtain wall.

Section 5: Basic Structural Design Considerations

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 (see the Learning Links for examples). 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 specializing in greenhouse structures. However, with this in mind, it is still valuable to understand some 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 is 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. Greenhouse structures must be designed so as to be able to support the maximum combined dead and live loads that they will experience. Most often permanent 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 or 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 be 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.

Quonset greenhouses using metal tubes for the structure may be anchored into the ground by inserting the tubes into slightly larger tubes driven 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 dead and live loads expected. However, as the support required increases, there is a reduction in light availability to the plants.

Section 6: Greenhouse Operation Site Selection

The first step in establishing a new greenhouse operation is that of site selection. The following are some important considerations when selecting a site to build a greenhouse operation:

Markets
Which crops are to be grown and how they will be marketed should be considered. Is the operation to be a wholesale or a retail operation? Retail operations depend on exposure to customers and can benefit by proximity to population centers and by being located in high traffic areas. These same factors may be detrimental to wholesale operations.

Topography
The cost of grading and land preparation must be considered. Natural windbreaks such as hillsides and trees may help reduce heating costs but can also block light. Flood plains are usually flat land with good soils. However, in addition to the obvious flooding potential, many municipalities may prohibit building in flood plains or may require the building site to be elevated.

Land-Use Predictions
Urban encroachment can be a problem. Growth and future zoning changes can create conflicts with neighbors and local governments. However, growth and new highways can also create new business for retail operations. Additionally, urban growth around an operation can increase land values. While this can increase the value of the operation (at least the value of the land), it can also result in increased property taxes unless the operation is protected under an agricultural zoning.

Room for Expansion
Urban encroachment can limit expansion possibilities. When selecting a site and purchasing land, the ultimate expected size of the operation should be considered and even land purchased for future growth.

Climate
Climate affects heating and cooling costs as well as light levels. Other factors such as snow loads, wind loads, and hurricanes may need to be considered. Climate can also allow a business to produce a product at a time of the year that other operations in other locations cannot. For example, the annual bedding plant market in the U.S. progresses from south to north throughout spring and early summer.

Labor
Labor can be the greatest cost of doing business. Availability, cost and seasonality of labor all need to be considered. Additionally, worker compensation costs may vary from state to state and can affect the cost of labor.

Accessibility
The business' accessibility to highways and airports for shipping of plant materials and other products should be considered. When retail is involved, consider how easy it will be for customers to reach the operation.

Water
The quantity of water available, quality of water available and its cost must be considered. Environmental aspects related to water quality should also be considered. For example, operations located near rivers or wetlands may be required to take extra precautions to prevent runoff from the operation. States differ whether, and if so how much, water is allotted for agricultural use. In some states, water use is restricted. In other states greenhouse operations may be required to build facilities and implement practices that prevent runoff from the operation. These issues should be thoroughly studied and their potential impacts evaluated before deciding to locate a greenhouse operation in a specific location.

Local or State Regulations
Any new business will need to comply with worker safety laws, environmental regulations and state and local taxes. State and local regulations on things such as worker safety, work compensation, water usage and runoff control may exist. These regulations can significantly impact the cost of doing business.

Flow Patterns
The traffic flow inside of the operation (i.e. equipment, customers, large trucks) and the flow outside of the operation (traffic coming and going) should be considered. The flow of  plant materials and people inside of the operation should also be considered. Wholesale operations need to design facilities that allow plant materials to be easily moved through the facility as they mature and are ready for shipping. Production often begins in a potting area. Plants may then be moved to propagation houses or seed germination chambers and then to a greenhouse. Finally, finished plants are moved to a packing and shipping area. The easier the flow, the less time and labor that is required to get the job done. For retail operations, customer movement is important. Customers should be able to move through the operation and have ready access to materials. Aisles should be wide enough for carts. Often flow can be designed to insure that customers are encouraged to move throughout the sales areas to increase impulse purchases.

Orientation
Single-span houses above 40° N latitude should be oriented with the ridge running east-to-west. This allows the sun to pass through the side rather than through the rafters. Single-span houses below 40° N latitude and all ridge-and-furrow (regardless of latitude) greenhouses should be oriented with the ridge running north-to south so that the shadows created by the ridges move over time rather than constantly shading one area.

© M.R. Evans, 2008, 2009, 2011, 2014