Research and Commercial Greenhouses Information
Commercial and research greenhouses are high-tech structures dedicated to the horticultural needs of plants, particularly flowers, vegetables, and fruits. Environmental properties such as temperature, light exposure, irrigation, fertilization, humidity, and ventilation can be precisely controlled for optimal crop growth. Commercial greenhouses typically produce plants in large volume for consumers; research greenhouses are better-suited for plant science and medicinal horticulture.
Such greenhouses provide stable, highly-controlled environments for the cultivation of plants, such as flowers, vegetables, fruits, and transplants, typically for consumers. Greenhouses enable reliable growth of plants despite local climate, soil, or topographical challenges.
These structures maintain mild-to-high (often between 45° F and 100° F, depending on plant and season) temperatures by using glass or plastic materials that transmit visible and near-visible ultraviolet (UV) and infrared radiation (IR). Sunlight is essential to photosynthesis, the primary biological mechanism that plants use for energy. A significant portion of this solar energy is reflected by the items in the greenhouse as longer-wavelength IR that cannot escape the materials of construction. Instead the IR is absorbed by naturally occurring and anthropogenic greenhouse gases, such as water vapor, carbon dioxide, methane, nitrous oxide, ozone and chlorofluorocarbons, that maintain the internal temperature. Keeping the greenhouse enclosed eliminates heat escape via convection.
Other systems and equipment are also important to crop vitality.
Greenhouse configurations vary primarily by shape, configuration, or construction technique.
- Gable: characterized by straight vertical walls and a gable-style roof; good spacing and layout design; easy to manage
- Ridge and furrow: multiple gables span the greenhouse roof
- Uneven span: the roof segments are different sizes, which is preferable for slopes or high latitudes
- Flat arch: straight vertical walls with a single-span arched roof; better temperature stabilization than a gable greenhouse; needs active ventilation
- Gothic: walls are bent over the frame to make a pointed roof; similar to gable-style greenhouses but without the need for structural trusses
- Raised dome: straight vertical walls with a high roof; very stable internal environment; typically expensive to build and heat; high wind loads
- Sawtooth: straight vertical walls with roof panels angled towards prevailing winds; facilitates passive ventilation
- Skillion: straight, vertical, different-sized walls with a roof sloped towards prevailing winds; transmits least amount of light during morning and evening; good for passive ventilation
- Tunnel: a single arch with walls and roofing composed of the same span; poor spacing and layout considerations; requires active ventilation; small tunnels can provide basic protection
- Igloo/dome: a small dome without supporting walls; panels are usually diamond-shaped
- Tri-penta: a small enclosure composed of triangular panels
- Lean-to: this style rests on the side of another building; it has limited space but is usually easy to heat and resistant to wind
- Teepee/A-frame: a triangle- or pyramid-shaped enclosure for planting single rows or just a few plants
- Shade house: an open air structure with a fabric roof that is sometimes retractable.
Gable, flat arch, raised dome, sawtooth, and skillion greenhouses can be placed adjacently to increase square growing space. These are known as multispan greenhouses, which sometimes develop ventilation problems or temperature differences.
The greenhouse frame is made of wood, aluminum or steel pipe, polyvinylchlorate (PVC) tubing, plastic, or some combination of these materials. The frame's main job is to support loads, such as panels, wind, and snow, but it also provides some insulation. UV will eventually discolor and weaken PVC and plastic materials. Doors or access points are typically integrated. Frames sometimes need to be mounted on concrete pads or other foundations.
The material used for the walls and roof play an critical role in the temperature and lighting of the greenhouse interior. Walls and roofs are either composed of individuals panels or rolls of material stretched over large portions of the frame. Opaque materials diffuse light better to produce heartier, more stout plants. Clear materials are optimal for germinating plants or growing plants that will soon be transplanted outdoors. UV has a tendency to discolor polyethylene and polycarbonate materials, and they will eventually need to be replaced within several years. However these materials are more resistant to environmental hazards such as hail or falling branches.
Common covering materials are listed below. R-values rate the insulating ability of a material; higher values equate to better insulation.
Glass: single- and double-paned tempered glass is durable and reasonably impact- and scratch-resistant. It easily withstands thermals expansion. It is clear, so it offers no light diffusion. Glass panels often last 30 years or longer and is considered the safest for greenhouse workers. It is heavy, so additional framing may be needed. Typical R-value for a single-pane panel is .95; a two-pane panel has roughly double the R-value.
Fiberglass: translucent panels provide moderate light diffusion and withstand most weather hazards. Provides some insulation (typical R-value of .85). Preferable when warping, thermal expansion, or chemical and corrosives exposure is a concern. However fiberglass panels may yellow after years of UV exposure and require replacement, and corrugated panels can easily accumulate debris.
PE film is often elected for large-sized greenhouses because it is inexpensive and easy to replace. It delivers semi-diffused light and is a mediocre insulator (R-value: .85), but it can be easily doubled to improve this value. Films usually need to be replaced after a few years.
PE sheeting is stiffer than PE films, is lightweight, and has an inner chamber to improve insulation (R-values of 2.1-2.3). It typically transmits diffused light and has UV inhibitors to improve UV resistance.
Polycarbonate: a durable and lightweight glazing solution that regularly lasts 15 years or more. Typically manufactured as two-ply sheets with an internal air cavity to improve insulation (R-values between 1.43-1.89, depending on thickness). Three-ply panels are also common (R-value: 2.0-2.1). It offers good light transmission that degrades over time due to discoloring. Panels can be difficult to install on curved frames.
Ventilation provides necessary fresh air that plants need for respiration and photosynthesis, and also helps control internal temperature and humidity. Certain plant pathogens also prefer still air conditions. Some plants also need to develop stress tissue that is created by enduring breezes. Fresh air is also necessary after applying chemicals. Common ventilation equipment includes:
- Shutters, vents, and louvers; some are solar powered and actuate independently by heat-expanding waxes or other mechanisms
- Wall curtains; textile greenhouse membranes that can be drawn
- Controllers; for programming ventilation schedules or activation criteria
- Motors and gearboxes
- Scrubbers; for eliminating odors
In certain climates additional heating systems are employed to provide a longer growing season. Conductive temperature losses still occur through the structure's building materials. Natural gas or electric furnaces are the most common solution to low greenhouse temperatures, often in conjunction with thermostats and controllers. Keeping livestock within a greenhouse is a common alternative to boost internal temperatures.
- Convective heaters; fueled by propane, natural gas, oil or electricity; come in many sizes, shapes, and configurations
Radiant heaters: a water heater pipes water to surfaces and floors
Insulation and vapor barriers
Carbon dioxide is another required element for cultivation. Combusting heaters supply carbon dioxide, but it can also be supplied by concentrated gas sources. An oxygen concentrator may be used for research purposes, but is more often integrated as part of an irrigation system.
Most outdoor greenhouses rely on the sun as a primary lighting source. However, artificial lighting duration and light spectrum can be precision controlled, which is important for research and indoor applications. Commercial greenhouses may also elect to supplement natural light with artificial light. Lighting represents a significant energy investment, and heat generated by light sources should be factored into lighting applications if significant.
Growers have several lighting options.
Metal halide (MH)
- produces light in the blue spectrum (450-470 nm)
- optimal for green, leafy, compact plants
- suitable as the primary light source
- average effective lifespan of approximately 10,000 hours
- generally produce 70 to 115 lumens per watt
High-pressure sodium (HPS)
- primarily for use as secondary lighting in greenhouses with natural sunlight
- optimal for budding and flowering plants
- produces orange-red (590-650 nm) or full-spectrum light
- effective service life of approximately 18,000 hours
- produce up to 140 lumens per watt
- deficient in the blue light (475 nm) spectrum
- emit significant amounts of heat
- offered as compact fluorescent lights (CFL) and T5 lights
- more energy efficient than HID lamps
- produces less heat than HID and incandescent lights
- 75 to 90 lumens per watt
- effective lifespan of up to 20,000 hours
- should be placed within a few feet of plants
- 2700k to 3000k bulbs emit high red spectrum light
- 5000k bulbs emit full spectrum light
- 6500k bulbs emit more blue light
- inexpensive capital costs
- largely inefficient—produces more heat than light
- typically outfitted with filters to reduce red spectrum emissions
- average lifespan of 750-2,500 hours
- used for supplementary lighting only
- 10-17 lumens per watt
- customizable light spectrum emissions
- suitable as a primary light source
- maintains spectrum while dimmed to simulate dawn/dusk
- highly efficient and cool
- typically higher investment cost
- may have better photosynthetically active radiation (PAR) levels
- effective lifespans of up to 50,000 hours
- produce 27 to 150 lumens per watt
Shade canopies offer plants reprieve from direct sunlight and heat. They are typically polyethylene materials with small holes in 30-80% of the canopy area. They can be suspended from greenhouse supports, erected with collapsible poles, or draped over frames.
Many plants are about 90% water, which provides the internal pressures plants use to grow vertically. It is also essential for transpiration, the process where plants exchange water for carbon dioxide. Most irrigation systems are automated with the use of a timer and solenoid valves; pressure regulators keep line pressures within specified parameters.
Polyethylene tubing is routed amongst crop rows, planters, and racks, with intermittent barb connectors. Drip tape, a type of smaller tube or hose, mates with these connectors and can be laid on the surface or buried to deliver water directly to the plant. Small quantities of water are brought to the plant roots on scheduled intervals. This is ideal for plants that will remain stationary during their life cycle, and it can also deliver biocides and fertilizers.
This system is similar to drip tape, but instead dripping mechanisms are buried or staked at the roots of plants. Dripper systems are also more easily customized, but retain fluid pressure in the supply line the entire time. A slow, steady rate of water is delivered to the plant.
Sprayers and Misters
Irrigation lines connect to nozzles that atomize water spray onto the plant. This method can waste a significant amount of water, but a single nozzle or sprinkler can irrigate multiple plants and rows. It's also useful for semi-automated and zone watering. A large variety of nozzle types are available. It is also easiest to install and relatively inexpensive.
Greenhouse plants can also be irrigated by hand with watering cans and hoses with sprayers. This technique requires the most labor, but also offers the operator direct oversight of plant vitality. Capillary mats are trays that support pots and once filled with water, allow the plant to be watered from the roots up.
Plants grown with hydroponics systems do not need irrigation as the plant itself is grown in water, not soil. Except for the differences in irrigation and growing aggregate, hydroponic plants still benefit from the stable environment and light offered by a greenhouse. Equipment needed depends on the hydroponic technique used.
- Containers: such as buckets, jars, mesh baskets, and mesh pots
- Reservoirs: channels, trays, saucers, and buckets
- Nutrient solutions: for enriching water
- Foggers/misters: for aeroponics, a type of hydroponics that replaces root immersion with continuous fogging or misting.
- Pumps: air and water pumps for aerating and recirculating nutrient-rich water, as well as tubing, piping, and air stones/wands
- Filters: de-chlorinating; carbon; sediment; reverse osmosis; algae; de-ionizing
- Medium: for wicking water, and retaining moisture and air near plants roots; varieties include perlite, vermiculite, clay pebbles, grow stones, coir peat, rice husks, pumice, sand, gravel, wood fiber, rock wool, sheep wool, and polystyrene
Liquid and soluble chemicals are frequently applied by spray bottles, pneumatic sprayers, or by introducing them into the water supply.
- Fertilizers: solid fertilizers are added to soil and soluble fertilizers are added to irrigation systems to provide varying levels of nutrients, including primary macronutrients nutrients nitrogen, phosphorous, and potassium; secondary macronutrients calcium, magnesium, and sulphur; and micronutrients copper, iron, manganese, molybdenum, zinc, boron, and possibly silicon, cobalt, and vanadium, plus rare mineral catalysts.
- Biocides: herbicides, fungicides, and pesticides are applied in greenhouses to curtail weed growth, fungus growth, and bug infestations, respectively
- Water softeners: to treat water sources before application to crops
- Weatherizers: to add resistance to fruits from threats such as sunburns or over-hydration
- Repellants: repels insects or animals
These devices are generally useful for greenhouse applications.
- Material handling: carts, wagons, and wheelbarrows; conveyors, monorails
- Personnel access: ladders and step-stools
- Organization and storage: racks, tables, shelving, benches and stands, including many rolling types
- Hand tools: shovels, spades, hoes, rakes, augers, scythes, clippers, axes, saws and other various implements
- Horticulture supplies:
- planters, pots, trays, and baskets
- growing mediums (e.g. soil; compost; soil-less mixes of perlite, vermiculite, peat moss, lime, and fertilizer; sand; bark and mulch; commercial potting mixes
- plant supports (e.g. tomato cages, trellises, stakes)
- seed propagation equipment (e.g. warming mats, germination trays,
- Nets/screens/fences/barriers: to exclude insects and animals or designate boundaries
- Measuring equipment: thermometer; hygrometer; lux meter; pH tester; electrical conductivity meter; light quantum sensor; soil water sensor
- Programmable logic controller: for automating greenhouse systems
- Hardware: anchors, ground stakes, fasteners, hooks, and clamps
- Biocontrols: lethal and non-lethal means to eliminate or repel pests, such as toxins, traps, scarecrows, faux owls, cats, and ladybugs
- Personal protective equipment: gloves, aprons, gauntlets, glasses/goggles, boots, hats, and respirators
Plants in a pharmaceutical, bioengineering, and plant science greenhouse have growth requirements met by a highly-regulated enclosure, more commonly called a growth chamber. It includes all systems for heating, cooling, ventilation, humidity, light, and irrigation, as well as possible redundancies. Communications features, such as sensors and cameras, enable monitoring and adjustment via a facility's local area network. Various locks can restrict accessibility and improve confidentiality. Growth chambers are often geared towards a specific research application, such as plant growth, tissue culture, or incubation, or for a specific species, like arabidopsis.
Plants raised in growth chambers and grow rooms have the same botanical needs as those raised in greenhouses. The primary difference is research equipment enables precise control over all environmental parameters. For example, lights are often tuned to provide an exact light spectrum and may have extra heat sinks or active heat exhaust.
Cabinet: these single-unit greenhouses are manufactured in various sizes and may include separate growing chambers. Cabinets may be mounted on wheels for mobility.
Walk-in: are similar to cabinet-style research greenhouses in that all systems and components are provided as a single unit, but are larger to cultivate more or larger specimens. It also allows more variations for airflow patterns and lamp-to-plant distances. Workers may need contamination suits or other PPE to prevent contaminants from entering the chamber, Seed storage: Walk-ins are sometimes used in seed storage applications. Seeds are dehydrated to approximately 5% and are then frozen long-term at temperatures of 0° F, or for shorter periods around 40° F. Humidity is kept between 30-50% room humidity by a chemical dehumidifier.
Grow room: large rooms tailored towards botany provide significantly more growth space, and larger-sized systems compensate for the increased plant needs. The main differences are the need for secure and classified rooms, meaning tighter access control measures and perhaps surveillance. Grow rooms are typically assembled in preexisting spaces with equipment that meets research specifications. For this reason lighting, ventilation, heating, air conditioning, and irrigation systems can be more expensive. Remote control and automation are also common.
- Grow area: not equivalent to square or cubic space; also affected by rack and bench organization
- Structure load: the suitable load of the greenhouse framing and glazing, including dead loads, precipitation loads, and wind loads.
- Temperature: the range of internal temperatures determined by heating and cooling systems and insulation
- Relative humidity (RH): water should not accumulate on leaf surfaces as this can promote microbial growth; ventilation or dehumidification should typically occur when 85% RH is reached
- BTU: for calculating heater outputs
- Daily light integral (DLI): the amount of photosynthetically active radiation (PAR), based on intensity and duration
- R-value and U-value: rates of material thermal resistance and transmittance, respectively; important for greenhouse insulation
- Ventilation rate: expressed as cubic feet per minute (CFM), liters per second (L/s), or air changes per hour (ACH)
- Volumetric water content: measures the quantity of water in a soil to determine adequate irrigation
- Access control: locking doors, windows, vents, and panels prevent unauthorized access and operation
- Base optional: the concrete foundation is not required for structure stability
- Confidentiality panels: dark, opaque panels prevent undesired observation
- Easy panel replacement: broken or discolored panels can be easily replaced; some models require partial greenhouse disassembly
- Connectivity: the greenhouse facilitates remote monitoring by a LAN or Internet connection
- Gutters: rain is routed away from the greenhouse sides to prevent erosion
- PV panels: solar arrays are integrated to provide or supplement electrical power
The National Greenhouse Manufacturers Association maintains an array of standards. Primarily, the NGMA adapts current building codes as they relate to greenhouse construction. Examples include:
NGMA Design Loads (.pdf)
NGMA Design Considerations (.pdf)
NGMA Structural Design (.pdf)
NGMA Heat Loss (.pdf)
Furthermore, certain greenhouses are subject to fire safety, International Building Code, and energy efficiency standards.
ASHRAE 90.1 Users Manual - Minimum requirements for energy efficiency
NFPA 5000 - National Fire Protection Association handbook