Section 4: Common Irrigation Systems Used in Greenhouses
Water is a very important issue in greenhouse crops production. Concerns over water availability have resulted in water use restrictions in some parts of the U.S. (i.e. California, Florida, and Arizona). In many other locations, concerns over runoff of water containing fertilizers and pesticides have resulted in regulations requiring that runoff from greenhouse operations be contained and not allowed to flow into ground water, streams, or lakes. These types of concerns over water use and runoff have changed the way that water and fertilizer solutions are provided to greenhouse crops.
There is an expression that goes something like this: "The person at the end of the hose controls your profits". Although this is an exaggeration, it demonstrates the importance of proper water management in greenhouse crops production. Proper irrigation is important because it impacts so many aspects of production. Either too much or too little water can be detrimental to the development of a crop. Over watering may result in elevated pH, increased disease incidence, poor root and plant growth and leaching of mineral nutrients from the root substrate. Under watering can result in poor root and plant growth, flower and leaf abscission, increased disease incidence and high E.C. levels. In large production situations, it is difficult to provide water to plants only when needed. Natural variations in the rate of water loss due to evaporation and transpiration (evapotranspiration) require irrigation of some plants and not others. However, it is not practical or economical to hand water large numbers of plants so maintaining uniform and optimal moisture levels across a crop can be difficult.
There are many ways in which water (and thus mineral nutrients when a liquid fertilization program is being used) may be applied to a crop. Each of these methods or systems has advantages and disadvantages. It is common for greenhouse operations to utilize more than one type of irrigation system depending on the crops being grown. Various types of irrigation systems are discussed below.
Hand watering
Irrigating crops by hand using a hose with some type of water breaker is a time-consuming and costly process. However, irrigating by hand might be required at times when uneven drying of substrates occurs and "spot" watering needs to be conducted.
Overhead emitters
Overhead emitters spray water or fertilizer solution into the air as droplets. Some of the droplets fall on the substrate surface and plants and move down into the substrate. Depending on volume applied, some of the water or fertilizer solution in the substrate my move out of the substrate through the holes in the bottom of the container (leaching). Some of the droplets land on foliage or flowers or on walkways or evaporate in the air. Much of the water applied using overhead emitters will be lost to evaporation or runoff. In fact, overhead emitters are one of the most wasteful methods of irrigation and can cause significant runoff. However, these systems are relatively cheap and can be readily adapted to many types of crops. There are numerous types of overhead emitters. Different emitter heads can be used that adjust the area over which water is sprayed (spray pattern), the volume of water emitted (listed as gallons per minute or gpm) and the droplet size.
Irrigation booms
Irrigation booms are similar to overhead emitters in that they supply water through the air and over top of the crop. However, rather than being stationary as in the case of overhead emitters, irrigation booms have arms with emitters along the length of the arms. The boom arms are attached to a motor that is mounted on a track above the benches or floor. A retractable hose connects the boom with the water or liquid fertilizer solution source. The emitters spray water or fertilizer solution as the motor moves the boom arms down the length of the track. The emitters may be changed to provide the desired droplet size (and booms may have emitters with different droplet sizes mounted on the arms at the same time), and the speed of the boom may be changed to adjust the amount of solution applied with each pass.
Boom irrigation systems can be controlled using various types of clocks, timers or computer controls. Many booms also have a programmable control unit attached to the boom that may be used to control when the boom operates and the speed at which it moves. Additionally, many booms are designed so that magnets may be placed on the track to differentiate zones. A sensor can detect the location of the magnets as it moves along the track. The control unit can be programmed to water these zones differently (i.e. different frequencies, speeds, etc.).
As with overhead emitters, irrigation booms spray the water or fertilizer solution into the air before it lands on plants or in the container. Some of the solution lands on plant foliage and is lost to evaporation. However, because the boom arms are typically closer to the plants than most overhead emitters and because, if designed and used correctly, water is typically not sprayed on sidewalks, irrigation booms are more water efficient than overhead emitters. Irrigation booms still apply water and fertilizer solution in such a way that leaching and runoff may occur. Irrigation booms are typically more expensive than simple overhead emitters. However, if the boom arms only cover the area where plants are grown, plants are spaced close together (or emitters adjust to match plant spacing) and the appropriate volume of solution applied during each irrigation pass, the amount of water and fertilizer solution wasted or lost through runoff can be minimized. Reducing water and fertilizer usage and runoff reduces production costs and reduces a greenhouse operation’s potential impact on the surrounding environment.
Drip tubes (Chapin or spaghetti tubes)
Drip tube irrigation systems supply water through individual tubes placed on the substrate surface of each container. The water or fertilizer solution trickles from the tube and spreads through the substrate. Large containers may require more than one drip tube to provide for even distribution of solution over the substrate surface. Water or fertilizer solution is pumped through supply lines and to each individual container. The supply line(s) to each container terminate with some type of emitter. The emitter is designed to keep the supply line in the container and to help spread the water or fertilizer solution over the substrate surface.
Various types of emitters may be used with drip tube irrigation systems. Some are small lead weights that hold the supply line in the container and spread the solution out in two or three directions over the surface of the substrate. Other types of emitters are made of plastic. The bottom end of the emitter has a small spike that is inserted into the substrate. The spike holds the emitter in the container. The top of the emitter forces the solution against a small plastic plate which causes the solution to spray out over the surface of the substrate. The amount of water supplied to the containers is a function of the flow rate of the supply lines (gallons per minute supplied as a function of the size of the supply lines and the water pressure), the delivery rate of the attached emitters (listed on emitters as gallons per minute) and the time that the system is operated
Drip tubes are a relatively low-cost and simple irrigation system. Although this is a top-watering method and leaching can occur, if the volume of water applied is only that which is needed to maintain a moist substrate, leaching can be greatly minimized. Because water or fertilizer is being applied directly to the substrate, this system can save significant amounts of water and fertilizer as compared to overhead emitters. However, since every container must have at least one drip tube, benches can become covered with drip tubes and keeping track of a potentially large numbers of drip lines and emitters can be a challenge. Further, if the number of containers on a bench changes, the number of tubes must change (tubes must be added, removed or plugged if not being used).
Additionally, greenhouse managers must watch for clogged drip tubes. If a tube becomes clogged, the container it serves will not receive water or fertilizer solution. Often greenhouse managers will periodically inject acid through the drip tubes (without plants being present) to dissolve any build up of fertilizer salts that are clogging the drip lines or emitters.
Capillary mat systems
This type of irrigation system uses a lint mat covered with a perforated black plastic. Drip lines (drip tape) are placed on the lint mat and underneath the perforated black plastic. Water or fertilizer solution is supplied to the lint mat through the drip lines. The black plastic is used to exclude light from the lint mat and thus reduce algae growth on the mat. Containers are placed on top of the perforated black plastic. Often when the containers are initially placed on the perforated black plastic, they are top watered (allowing a small amount of leaching from the bottom of the container) to establish a capillary connection with the wet lint mat.
As the substrate in the containers lose water through evapotranspiration, water or fertilizer solution moves from the lint mat into the substrate through the holes in the bottom of the container by capillary action. The mat is kept constantly wet through periodic application of water or fertilizer solution through the drip lines. If the containers are picked up, knocked over, or if the lint mat or substrate is allowed to dry out, the capillary connection between the lint mat and the substrate inside of the container will be broken and water or fertilizer solution will not be able to move from the lint mat into the substrate. If the capillary connection is broken, the containers should be replaced on the wet mat and overhead watered (allowing some leaching from the bottom of the container) once to reestablish the capillary connection.
Capillary mat irrigation systems are typically simple and low cost irrigation systems. They may be placed on many types of benches or on floors. However, if the surface is not level or has low spots, irrigation may be uneven. In some cases greenhouse managers will place capillary mats on a bench with a slight gradient. This helps minimize wet or dry spots because the solution in the wet lint mat tends to flow to the low end of the bench (i.e. low spots drain and potential dry spots have water flowing through). The excess water or fertilizer solution is captured in a PVC trough at the low end of the bench and reused.
Capillary mats provide a low-cost irrigation system that reduces water and fertilizer use (as compared to over head emitters) and minimizes runoff. However, irrigation can be uneven and disease organisms spread easily using capillary mat irrigation. Fertilizer Salts can also build up in the mat over time. Additionally, capillary mat irrigation is an example of a subirrigation system and special concerns regarding fertilization apply (see "Special Considerations When Using Subirrigation and Recirulating Irrigation Systems").
Ebb-and-flow benches
Ebb-and-flow benches (also called ebb-and-flood benches) combine an elevated benching system with a closed recirculating irrigation system. Ebb-and-flow benches may be designed as stationary benches or as rolling benches.
The primary characteristic of an ebb-and flow irrigation system is the tray that makes up the bench surface. The tray may be made of hardened plastic or aluminum. The length and width of the tray varies depending upon desired dimensions. The tray has side walls that are generally 4 to 6 inches tall. On the bottom inside of the tray there are two layers of channels with one being about ½ inch deep and running the length of the tray and the next set of channels being about ¼ inch deep and running perpendicular to the length of the tray.
At one end of the tray there is an inlet and an outlet. The inlet allows fertilizer solution (or water) to be pumped into the tray. As fertilizer solution is pumped into the tray, it first floods the deepest of the channels. When the deepest channels are flooded, the solution floods the next level of channels. After both sets of channels are flooded the solution continues to rise above the bottom surface of the bench. In a production situation, containers with plants would be placed on the tray surface. As the solution floods the tray and rises up around the containers, the solution comes into contact with the root substrate inside of the containers and the fertilizer solution moves up and into the substrate in the container by capillary action. Thus, rather than top watering, the crop is subirrigated (water and/or fertilized from the bottom). When flooding, the depth of the solution is maintained so that it does not extend more than approximately ¼ to ½ of an inch up the height of the container (the container is sitting in a fertilizer solution that is ¼ to ½ inch deep). For a 4 or 4.5-inch containers (or smaller) the flooding depth is typically about ¼ inch while it may be ½ inch for larger containers.
The depth of the solution and the exposure time are controlled through several possible methods. In the first, the fertilizer solution is pumped into the bench through one of two pipes. At the same time, fertilizer solution flows out of the tray through an opening with a metal screen. The screen, however, allows the fertilizer solution to flow out at a slower rate than it flows in and the net effect is that the bench floods. The depth of the flooding is dictated by how long the fertilizer solution flows into the bench (inflow rate and screen size may be adjusted to change the flooding rate and level in the tray). A more well controlled method is one in which the fertilizer solution is pumped into the tray. The tray has an elevated outlet so that solution flows out only when it reaches a certain depth (or height). This prevents the solution from rising above a certain height. The inflow of solution can continue for as long as flooding is desired. When the desired flooding time is reached, the pump is turned off and the solution can flow back to the holding tank through the same plumbing that the solution was pumped into the flood tray. Other systems may have the outlet on a timer. The bench is flooded and after a prescribed period of time, the outlet is electronically opened so the fertilizer solution can flow back to the storage tanks. Typically, ebb-and-flow trays are flooded for 10 minutes (total contact time with the water or fertilizer solution) and then the trays drained. This time may vary depending on the size of the container and the substrate being used.
All ebb-and-flow benches have several critical support components. There must be large storage tanks that hold the water or fertilizer solution(s) used to flood the ebb-and-flow trays. The storage tank (holding tanks) must have at least enough capacity to fill the trays being flooded at a given time. Additional storage tanks that contain concentrated fertilizer solution and plain water may be included. The system requires a pump to force the fertilizer solution from the storage tanks to the ebb-and-flow trays in the greenhouse. The capacity of the pump should be such that the zone being irrigated can be completely flooded in 5 minutes or less. Typically, some type of filter (i.e. sand or screen) is placed between the ebb-and-flood trays and the storage tank in the return flow line so that soil or plant debris is screened out. Timers or computer controls may be used to automate the irrigation process.
Probes (sensors) may be placed in the line flowing from the storage tank to the trays to monitor electrical conductivity (E.C.) and pH. If the E.C. is too low (due to plants removing fertilizer elements from the solution), concentrated fertilizer stock solution may be added to the fertilizer solution to increase the E.C. If the E.C. is too high, plain water may be added to lower the E.C. If the pH is not within an acceptable range, an acid such as phosphoric acid, sulfuric, or nitric acid may be added to decrease the pH or a base such as potassium hydroxide may be added to increase the pH.
There is a risk of spreading soil-borne disease-causing organisms such as Pythium and Phytophthora species in recirculated fertilization solutions. Therefore, after the solution is drained from the bench and filtered, it may be treated with U.V. light or ozone to kill any organisms present. Additionally, chloride, fluoride or copper may be added to the fertilizer solution to kill any disease-causing organisms present. More detailed information regarding disinfestation of water and fertilizer solutions is presented under the “Disinfesting Recirculated Water” in this learning unit.
Although more expensive than overhead emitter, booms or capillary mats, ebb-and-flow irrigation systems provide an efficient and automated irrigation and fertilization system. In addition to preventing runoff of water and fertilizers, ebb-and-flow systems reduce water and fertilizer usage. Additionally, an ebb-and-flow irrigation system is an example of a subirrigation system and special concerns regarding fertilization apply (see “Special Considerations When Using Subirrigation and Recirulating Irrigation Systems”).
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Dutch trays
Dutch trays are very similar to ebb-and-flow benches and they function and are managed in a manner similar to ebb-and-flow benches. However, Dutch trays are usually aluminum. Additionally, where ebb-and-flow systems can be designed as rolling benches, they are usually limited to simply rolling from side-to-side to minimize the number of aisles required and thus increase space usage efficiency. Dutch trays are placed on steel tracks that serve to not only support the trays, but the trays (which have small wheel-like structures underneath) roll on the metal support structure. This allows trays to be rolled together to maximize space usage efficiency, but the track system also serves as a transportation system around the greenhouse facility.
The tray units for a Dutch tray system may be rolled down the length of the greenhouse and spaced tray to tray. When the plant material needs to be moved to another location in the facility, the trays may be rolled on the steel tracks to a major aisle or walkway. When a change in direction is needed, the tray may be offloaded from tracks going in one direction and onto tracks going in a perpendicular direction using pneumatic lifts that raise and lower sections of track.
Because of their degree of mobility, Dutch trays cannot be physically connected to the fertilizer solution supply line or the drain line (otherwise all the tubes and supply and drainage lines would have to move with the trays). This problem is solved by having stand-alone supply lines and a drain line that runs underneath the benches but does not connect directly to the trays.
Although a Dutch tray system provides an automated transportation system, a closed recirculating irrigation system and maximizes space usage efficiency, it does limit access to crop. For example, if crops that need to shipped or handled are in the middle of a greenhouse, many trays might need to be moved to gain access to the trays with the desired plants. One solution to this problem has been to utilize various types of crane systems that can move over top of the trays, reach down, pick up a desired tray and bring the tray to the end of the greenhouse and place it on an open track.
Although more expensive than overhead emitter, booms or capillary mats, Dutch tray irrigation systems provide an efficient and automated irrigation and fertilization system. In addition to preventing runoff of water and fertilizers, Dutch tray irrigation systems reduce water and fertilizer usage. A Dutch tray irrigation system is an example of a subirrigation system and special concerns regarding fertilization apply (see “Special Considerations When Using Subirrigation and Recirulating Irrigation Systems”).
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Troughs
Troughs are narrow linear structures typically made from aluminum, plastic or PVC. There are several distinct types of troughs but they all are generally designed to hold a single row of containers or plants. Troughs may be open faced and designed to have containers placed on them or they may be a closed faced (or partially closed faced) and designed to have plants placed directly into the trough without a container. In all cases, the trough is mounted on some type of support structure with a slight grade (2% to 3%) from one end to the other to facilitate the flow of water or fertilizer solution.
One type of trough system is designed to support plants in containers. In this case, the trough is open faced and often has a shallow channel in the trough where the container is placed. The trough is positioned on a slight grade to facilitate the flow of the water or fertilizer solution. At the high end of the trough, a supply tube discharges water or fertilizer solution into the trough. The solution flows down the length of the trough in a shallow stream. As the solution comes into contact with the containers, and thus the substrate inside of the containers, solution is taken up into the substrate through capillary action. Excess solution is recaptured at the opposite end in a collection tray or manifold. The excess solution is typically filtered and returned to storage tanks. The recollected solution may be disinfested and monitored in the same manner as described for ebb-and-flow trays.
Another type of trough system is commonly used for the hydroponic production of greenhouse-grown vegetables. These are typically closed-faced troughs or partially closed-faced troughs made from plastic or food-grade PVC. The trough is designed so that seedlings or young plants may be pushed or slid into the top of the trough. The seedlings may have been germinated in Oasis® foam, an Ellipot® or in some other type of substrate. The root ball and substrate are held in place by the trough and thus the trough provides physical support to the plant. The shoots of the plants grow above the trough while the roots grow inside of the trough. As with the previous type of trough system, a continuous stream of water or fertilizer solution flows inside the trough from one end to the opposite end since the trough has 2% - 3% slope from the upper to the lower end. The flowing solution is typically a thin film or layer of solution a few millimeters deep that baths the roots (but typically does not totally cover them) as it flows down the length of the trough. In this way, the plants have access to water and mineral nutrients, but are not totally submerged in water. This method of supplying water and nutrients is usually referred to as the nutrient film technique (NFT). The trough used in this type of system may have varying designs and dimensions, may be supported on some type of elevated benching structure or may be close to the ground.
Another type of trough system is typically used for propagation. This system uses PVC pipes with a notch removed (partially open faced) along the length of the top of the pipe. Oasis® foam or trays with some type of solid substrate may be slid into the trough. Vegetative cuttings are stuck into the foam or substrate. The cuttings are misted using an overhead mist system until roots begin to develop. As with other trough systems, water or fertilizer solution flows down the length of the inside of the trough. While roots are developing (and plants are being misted), water is used in the trough to keep the substrate moist. After roots begin to develop, a fertilizer solution may be used to provide both water and mineral nutrients to the developing cutting (and mist may be terminated). When the cuttings are ready for shipping, the foam cubes or cutting trays are slid out of the trough and packaged for shipping.
When troughs are used, the same types of support components such as storage tanks, pumps, supply lines, recollection lines, filters, solution disinfesting systems and monitoring probes may be used as for ebb-and flow bences. The primary difference in a trough system and an ebb-and-flow system is that in the ebb-and-flow system, the fertilizer solution is gradually and evenly raised up around the containers and then drained away. In a trough, plant roots are continuously exposed to a thin flowing stream of water or nutrients.
Flood floors
A flood floor is essentially a hybrid of a concrete floor and an ebb-and-flow irrigation system. The greenhouse floor is poured concrete with raised edges or curbs that allow the floor to be flooded and drained. As with ebb-and-flow benches, water or fertilizer solution moves into the substrate through capillary action.
There are two basic flood floor designs. The first is known as a traditional flood floor. In this type of flood floor, the concrete is poured and a laser is used to level the floor with a slight grade from the sides towards the center (creating a very shallow “V”). The center is usually 1 to 1 ½ inches lower than the high points along the curbs or edges. Water or fertilizer solution is pumped into the flood floor. In most cases, the inlets are located in the low center portion of the floor. The pump capacity is usually designed so that the flood floor can be filled to a depth of ¼ to ½ inch in approximately 5 minutes. The floor remains flooded for up to 10 minutes and is then drained back through the same supply line used to pump the solution into the flood floor. As with ebb-and-flow benches, the solution may be pumped through a filter before it is returned to the storage tank. The solution may also be treated with ultraviolet light or ozone to kill any potential plant pathogens.
Another basic type of flood floor is referred to as a cascading flood floor. This type of production surface combines the concept of food floors with troughs, and they are best suited for small containers (less than 6-inch containers). Cascading flood floors are concrete flood floors that are sloped from one side to the other with a drop of approximately ¼ to ⅜ inch from the high side to the low side. The water or fertilizer solution is pumped into the flood floor on the high side, flows down the slope and is recollected on the low side of the floor. The solution is pumped into the flood floor at a rate such that 30 to 45 seconds are required for the solution to cross the flood floor and a solution depth of approximately ¼ inch is maintained. The flooding process continues for approximately 10 minutes and is then terminated. The recollected water is pumped back to storage tanks. As with traditional flood floors and ebb-and-flood benches, the recollected solution is typically filtered and may be treated for disease-causing pathogens.
Regardless of the exact design, the advantages of flood floors include reduced water use, elimination of runoff, and high space usage efficiency. The primary disadvantage is the initial cost since they can be expensive to install. Flood floors are an example of a subirrigation system and special concerns regarding fertilization apply (see “Special Considerations When Using Subirrigation and Recirulating Irrigation Systems”).
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Floating Beds (Deepflow technique)
Floating beds are most commonly used in hydroponic production of vegetables such as lettuce (see "Hydroponics"). Plants are floated on a fertilizer solution using Styrofoam sheets or plug trays. Initially, fertilizer solution moves into the substrate in the trays through capillary action. Later as the plants develop, roots may grow out of the tray and into the fertilizer solution. The plant roots grow into the fertilizer solution. Since the plant is taking nutrients form the solution and excreting other components, the solution needs to be monitored and adjusted as required.
Root misting (aeroponics)
This is another method used in hydroponic systems. The plants are suspended in the air (typically under a bench and in the dark or under very low light) and the roots are periodically misted with water or fertilizer solution (see "Hydroponics").
Hanging basket systems
Hanging baskets may be irrigated using several methods. Hanging baskets may be placed on traditional benches where they are watered by hand or with drip tubes. Hanging baskets may also be placed on flood floors (with the drainage trays temporarily removed). They may be hung in the air and irrigated using one of several types of elevated systems. The simplest elevated system for hanging baskets is one in which baskets are hung using simple hooks. A drip irrigation line is often placed in the hanging basket to provide water or fertilizer solution. Another system is referred to as an ECHO system. In this system, baskets are hung on hooks that are part of a rotating line. The line may be rotated when irrigation is required. As the containers pass one end of the loop, they pass under an irrigation nozzle. Finally, the cage system is one in which baskets are hung from a suspended cage or metal structure. This structure is usually automated so that it can be moved from one end of the greenhouse to the other. This allows material to be moved so that plant material under the baskets can receive sunlight for at least part of the day.
