What Is Controlled Environment Agriculture (CEA)?

Controlled environment agriculture (CEA) refers to crop production conducted in an enclosed structure that allows for complete or partial control of growing conditions. CEA is designed to provide near-optimal growing conditions by manipulating growth parameters including light, nutrients, carbon dioxide, temperature, and humidity.

Depending on the structure and the level of technology, CEA allows for the extension of the growing season or year-round crop production.

Controlled environment agriculture encompasses various types of structures and production systems, including soilless cultivation in hydroponics and aquaponics and soil-based cultivation in high tunnels, screen houses, or shade houses (Figure 1).

The types of structures and level of technology and automation that support CEA can range from manually controlled shade structures or high tunnels (hoop houses) to fully automated greenhouses and indoor farms.

Almost any type of crop can be produced in CEA, including landscape plants, food crops, and transplants or seedlings. However, economic feasibility must be met. Therefore, CEA is applied to high-value horticultural crops rather than agronomical crops.

Aquaponic systems that integrate fish or aquaculture combined with plant production are also considered CEA.

The most important environmental variable controlled in CEA is air temperature. Other control variables include root-zone temperature, relative humidity, vapor-pressure deficit, carbon dioxide (CO₂) concentration, light intensity and spectrum, photoperiod, and airflow (Table 1).

Climate control can be done manually or automatically using central climate controllers that operate on feedback loops, depending on the structure and available control tools. However, as more control variables are added to the system, the system configuration and control logics become more complex, which requires more sophisticated climate-control systems. Regardless, understanding plant requirements and their responses to various climate conditions is crucial in successful CEA.

Water, nutrient, and sanitizer delivery systems can be integrated into the control systems in addition to aerial environmental conditions.

Types of Structures and Technology

Indoor (vertical) Farms
Indoor farms are designed to maximize space and environmental control, enabling high-density, year-round, crop production. They are usually enclosed structures (shipping containers, repurposed facilities, and warehouse buildings) designed to house multiple tiers of growing systems (Figure 2). Crops are produced using artificial lighting or very little natural light. A high level of technology is required to grow commercial crops indoors.

Temperature, humidity, CO₂, and light are tightly controlled using automated control systems. Lights in indoor farms generate a large amount of heat, and cooling is necessary to maintain optimum temperatures. However, the level of HVAC use varies among different farms. Some indoor farms use mechanical ventilation to introduce outdoor fresh air as a main means to control temperature, while others use optimized HVAC systems inside a well-insulated contained environment so that seasonal influences of outdoor conditions can be minimized. While cooling by ventilation has economic advantages, the introduction of outdoor air can also introduce insect pests and diseases and make it difficult to elevate the CO₂ concentration higher than the outdoor ambient level.

In addition to temperature control, humidity management is also crucial as cooling alone cannot remove enough moisture from the air to maintain the optimum humidity level.

Water filtration and automated nutrient management are also conducted using control systems. Indoor farms typically contain multiple rows of stacked towers or vertical layers of plants, that extend to the ceiling. Short cycle, fast-growing crops (e.g., leafy greens, microgreens, and herbs) are most commonly grown in indoor farms using hydroponic systems such as NFT, Ebb & Flow, and aeroponics.  Indoor farms also produce mushrooms, seedlings, and transplants.

Glass/Rigid-plastic Greenhouse
Greenhouses are designed to harness natural light while enabling a high level of environmental control. They are typically galvanized steel or aluminum frames covered with transparent or semitransparent materials like glass, rigid-plastic (acrylic or polycarbonate) sheeting, or high-performance fluoropolymer plastic film, depending on the desired amount and quality of light transmission and diffusion (Figure 3). Supplemental lighting is commonly used to extend the number of growing hours in a day.

Controlling temperature, humidity, gas exchange, and light in greenhouses can be accomplished with minimal to highly technical systems. Most large-scale commercial operations use advanced technologies to manage these environmental conditions. Artificial intelligence (AI) and machine learning are increasingly being employed to optimize growing environments and maximize yields.

Crops with long growth cycles and tall, vertical growth requirements, such as tomatoes, peppers, and cucumbers, are most commonly grown in greenhouses. However, nearly any crop can be cultivated in a greenhouse.

The choice of production system depends on the crop type and available space. For example, leafy greens are often grown using NFT or DWC, with DWC typically requiring more space for fewer plants. In contrast, crops like cucumbers, tomatoes, and strawberries are predominantly grown using drip-irrigation systems.

High Tunnels
High tunnels, also known as hoop houses, are semipermanent structures used in CEA to extend the growing season and offer partial protection from weather. They are simpler and more affordable than greenhouses but still allow for some automation and environmental control.

High tunnels are typically framed with galvanized steel or aluminum tubing covered with single or double layers of polyethylene film (Figure 4). Double layers can be inflated with air to improve thermal insulation. Temperature and humidity can be moderately controlled by rolling the sides up or down or installing large doors or vent panels to adjust the cross-ventilation. The opening and closing of the sides can be manual or motorized. Low-input heaters can also be used to increase the temperature in high tunnels. Although crops are generally grown in soil within a high tunnel, soilless media and hydroponic production systems can be used. Similar to greenhouses, nearly any crop can be grown in a high tunnel.

Screen Houses
Screen houses are protective structures used to exclude insect pests and birds, reduce wind speed, and filter direct sunlight. They are framed with galvanized steel or aluminum and covered with UV-stabilized polyethylene, polypropylene, or polyester mesh (Figure 5). The size of the mesh varies and is selected based on excluding the primary pest. The mesh is usually white or transparent to maximize light transmission, although green or black mesh can be used for shading (see Shade Houses section below).  Screen houses are fully enclosed but allow air exchange through the mesh.

Screen houses are used to protect high-value crops such as small fruit (blueberries, strawberries, raspberries, etc.) and vegetables. Crops are usually grown in soil or in containers. The nursery industry also uses screen houses, especially in subtropical or tropical regions. Drip-irrigation systems are used for food crops, and automated sprinklers or misters are used to cool nursery plants.

Shade Houses
Shade houses are simple, cost-effective structures used to moderate sunlight exposure, protect plants from heat stress, and reduce evapotranspiration, especially in hot climates. Shade houses rely on passive cooling and natural airflow.  Shade houses are framed with galvanized steel or aluminum tubing, wood, or PVC pipes covered with UV-stabilized polyethylene mesh or woven fabric (Figure 6). The amount of light and heat penetration can be minimally controlled by adjusting the density and/or color of the covering material. Crops are usually grown in soil and irrigated using drip tape. Shade houses are common in the nursery industry for plants grown in containers. Automated sprinkler and misting systems can be used for cooling or to increase humidity in high-heat environments.

Types of Media

In CEA, the growing medium (or substrate) is used to support plant roots, retain moisture, aerate roots, provide nutrients, and/or provide beneficial microorganisms. The choice of the growing medium depends on the crop, production-system type, cost, and availability. While CEA is often associated with soilless systems, soil can still play a role in certain types of CEA, particularly in high tunnels, shade houses, and screen houses, but is less common in greenhouses. Soilless substrates are typically inert or have minimal biological activity, which gives growers more control over nutrient and pH management since the substrate is not contributing to nutrient availability. In addition, the properties of soilless substrates are uniform and predictable, which allows for the use of standardized and automated propagation and growing practices. To control aeration, water retention, and modify root support, some soilless substrates can be mixed at varying proportions. In liquid-culture hydroponic systems, soilless substrate is used only for propagation. Common soilless substrates, their composition, and the common production systems they are used for are listed in Table 2.

Production Systems

Production systems refer to the infrastructure used to grow crops. Under the umbrella of CEA, these systems can be soil-based, hydroponic, or aquaponics systems.

Soil-based Systems

In soil-based systems, crops are grown directly in the soil, either on flat ground, in raised beds, or in containers (Figure 7). Some crops, such as tomatoes, peppers, cucumbers, and string beans, may be trellised for support, while others are grown without trellising.

Hydroponic Systems
In hydroponic systems, crops are grown in an oxygenated, nutrient-rich water solution, with or without a substrate to support the roots. The type of systems used depends on the crop, space availability, and cost.

Aeroponics
In aeroponic systems, plant roots are suspended in air and periodically misted with a nutrient-rich solution. Plants are held in support structures or collars with their roots hanging in an enclosed, dark chamber (Figure 8).

Excess mist condenses and drains into a reservoir, where it can be reused or become drain-to-waste. Leafy greens, herbs, strawberries, and edible flowers are common crops grown using aeroponics.

Deep Water Culture (DWC)
In DWC, plant roots are submerged in a reservoir, which is the core component of the system that holds nutrient-rich, oxygenated water. Reservoirs can vary in size and material, depending on the scale and type of DWC system. They can be made of cement, high-density polyethylene (HDPE), fiberglass, or food-grade plastic, such as polypropylene (Figure 9).

Depending on the material of the reservoir, rubber- or plastic-based liners may be used to create a waterproof barrier that prevents leaks and contamination. Plants in a soilless medium are placed in rafts made of expanded or extruded polystyrene foam or food-grade plastics. Leafy greens and herbs are the most common crops grown in DWC, although almost any crop can be produced in DWC systems.

Drip Systems
Drip systems deliver precise amounts of a nutrient solution directly to the plant’s root zone using emitters or drip tape (Figure 10).

In drip systems, excess nutrient solution can be collected and reused (closed recirculating system) or not reused or recirculated (drain-to-waste). Plants are grown in plastic or aluminum troughs/gutters or buckets containing a soilless medium.

Drip systems are one of the most common systems for fruiting crops like tomatoes, peppers, cucumbers, and strawberries.

Ebb & Flow
Ebb & Flow systems work by periodically flooding the roots with nutrients. After a set amount of time, the nutrient solution is drained back into a reservoir (Figure 11). This process repeats several times per day, keeping the roots fed and oxygenated. Plants are grown in trays or containers containing the growing medium. Flooding is typically done on benches but may be done on the greenhouse floor, depending on the crop and the scale of the operation.

A wide variety of crops can be grown using Ebb & Flow systems.

Nutrient Flow Technology (NFT)
In NFT systems, a shallow, continuous stream (or film) of nutrient-rich water flows through narrow channels where plants are suspended in a soilless medium (Figure 12). The film of water ensures that the lower part of the roots is continually bathed in nutrients. The nutrient solution is circulated from a reservoir to the channels. Channels are supported by galvanized metal, aluminum, or wood frames.

Leafy greens and herbs are the most common crops grown in NFT systems.

Although NFT systems are not ideal for large, heavy, or fruiting crops, if support structures are modified to sustain the crop’s weight and larger channels are used, crops such as tomatoes, peppers, and cucumbers can be grown in NFT systems.

Aquaponics Systems

Aquaponics combines aquaculture (raising fish) with hydroponics (Figure 13).

Aquaponics leverages the natural relationship between fish, beneficial microbes, and plants. Mechanical filters remove fish waste solids, while biological filters (biofilters) contain beneficial bacteria that convert ammonia and nitrite to nitrate. This filtering system provides nutrients to the plants and creates a balanced, self-sustaining ecosystem. Dissolved fish waste serves as the main nutrient source for the plants.

Most aquaponics systems are closed loops or coupled, which means the aquaculture is directly connected to hydroponic systems. They can also be decoupled with the aquaculture separated from hydroponic systems.

In a coupled system, the nutrient solution from the fish tanks flows through filters into the hydroponic portion of the system and then flows back to the fish tanks. In a decoupled system, nutrient solution from the fish tanks is used to top off the hydroponic reservoir and does not recirculate back to the fish tanks.

Most types of hydroponic systems are compatible with aquaculture, including NFT, DWC, and Ebb & Flow.

Leafy greens and herbs are commonly grown using aquaponics, although tomatoes, peppers, and eggplants can also be produced using aquaponics.

Additional Resources

Controlled environment plant production engineering/technology education modules
(ohceac.osu.edu/CEPP-Engineering-Technology-Ed-Modules)

High/mid tunnel systems, The Ohio State University
(u.osu.edu/vegprolab/research-areas/high-mid-tunnel-systems)

Hydroponic GAPs—good agricultural practices for food safety of hydroponic crops
(cfaesosu.catalog.instructure.com/courses/good-agricultural-practices-for-hydroponic-production-systems)

Hydroponic nutrient solution for optimized greenhouse tomato production
(ohioline.osu.edu/factsheet/hyg-1437)

The Ohio State University fruit and vegetable safety team
(producesafety.osu.edu)