Section 3: Common Greenhouse Benching Systems
There are many types of benching systems or production surfaces used in greenhouses. Some are elevated while others utilize the floor of the greenhouse. Some serve only to support the plant material while others are hybrid systems that also allow for irrigation of the crop and even transport of the crop around the greenhouse facility. Listed below are common benching and production surface systems. In some cases, the system is described as a general category while others are distinct types of benching systems or production surfaces.
General Types of Benches
Stationary elevated benches
This term refers to a general category of bench types rather than a specific benching system. This is because stationary elevated benches may take many forms, but, as the name implies, they cannot be (and are not designed to be) easily moved. Stationary elevated benches may be supported my numerous materials including concrete blocks, wood, steel, aluminum or even hardened plastic. In some cases the support structure may simply rest on the greenhouse floor whereas in other cases it may be mounted in or bolted to the floor. The surface of the bench may be made of wood, wire, wire cages, expanded metal, snow fencing, chain link fence, ebb-and-flow irrigation trays, irrigation troughs or other types of materials. Stationary benching systems generally have the lowest space usage efficiency and offer the least flexibility.
Rolling elevated benches
Rolling benches is a term that also refers to a general category of bench types because there are numerous rolling bench designs. The surfaces of these benches are mounted onto the support structure in such a way as to allow the surface to be moved or "rolled" from side to side or even from location to location throughout the greenhouse facility. Rolling benches allow for a reduction in the number of walkways required and thus an increase in the space usage efficiency. Some types of rolling benches (i.e. Dutch trays) also provide a transportation system within the greenhouse structure. However, the use of rolling benches reduces access to plant material. As with stationary benches, rolling benches come in many forms and may be constructed of different materials. Specific examples of rolling benches are discussed below.
Removable benches
Removable benches are generally stationary but are designed to be repositioned or removed from the greenhouse during certain times of the cropping season. For example, a greenhouse manager might want to have a raised bench when growing a poinsettia crop (to improve drainage and air circulation) but might want to grow the following crop of bedding plants on the floor to maximize space usage efficiency. Removable or temporary benches could be assembled in the greenhouse for the poinsettia crop and then removed for the bedding plant crop.
Many types of removable benching systems exist. These are simple and temporary benching systems that are not mounted to the greenhouse floor. The support structure is typically made of concrete block, aluminum, steel or wood. The surface may be wire, wire fencing, wire cages, snow fencing or other low cost and easily movable materials.
Specific Types of Greenhouse Benches
Wire cage benches
There are several designs of wire “cage” benches. These benches may be simple wire or more complex systems. However, they are all designed for containers to be placed into the openings of the wire. The wire then holds the containers in place. These tend to be relatively low cost and simple raised benching systems. However, the dimensions of the openings in the wire must match the container size being used. These benches may be set to be permanent or removable and are most commonly used in growing tall or top-heavy potted plants since the cage holds the containers in place and prevents plants from falling over.
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 bench 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 the desired bench 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 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 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 bottom of the containers, the solution would come into contact with the root substrate inside of the containers and the solution would move 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 fertilizer solution is allowed to maintain contact with the containers (and the root substrate inside) for approximately 10 minutes and then drained through the tray outlet.
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 fast and 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.
All ebb-and-flow benches have several critical support components. There must be large storage tanks that hold the water or fertilizer solution used to flood the benches. The storage tanks must have at least enough capacity to fill the benches being flooded at a given time. The size of the zone being flooded can be reduced to allow for smaller storage tanks. 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 benches. Typically, some type of filter (i.e. sand or screen) is placed between the bench 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 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 for example), concentrated fertilizer stock solution may be added to the fertilizer solution to increase the E.C. to the desired level. 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 “Irrigation” learning unit. Ebb-and-Flow benches provide an efficient and automated irrigation and fertilization system. In addition to preventing runoff of water and fertilizers, ebb-and-flow benches reduce water and fertilizer usage. Therefore, although they are initially more expensive than many other types of benches, water, fertilizer and labor savings may make up for this initial cost over time.
Dutch trays
Dutch trays are very similar to ebb-and-flow benches. However, the flood trays are usually aluminum. Additionally, where ebb-and-flow systems may 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 crops. For example, if crops that need to be 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.
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Troughs
Troughs are narrow linear benching 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 (1% 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 and herbs. These are typically closed-faced troughs or partially closed-faced troughs made from plastic or food-grade PVC or high density polyethylene (HDPE). 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, rockwool, 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 stream of water or fertilizer solution flows inside the trough from one end to the opposite end. The flowing solution is typically a thin film or layer of solution that baths the roots 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 sometimes 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. This system is discussed in more detail in the "hydroponic systems" learning unit.
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 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 were discussed for ebb-and-flow benches. 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 periodically or continuously exposed to a thin flowing stream or film of water or nutrients.
Gravel and weed-mat floors
Rather than growing on raised benches, plants may be grown directly on the greenhouse floor. In this case, the floor itself serves as the production surface. Advantages of growing on floors are that it affords the ability to maximize space usage efficiency and increases flexibility. In some cases, floor production systems may save money because no benches are required, but in other cases (i.e. flood floors) floor production systems may be initially more expensive than many types of raised benches.
One of the simplest floor systems is a gravel floor. In this case plants are grown on a pad of 3/8 inch crushed rock or pea gravel. This is a low-cost floor production system and the gravel allows for excellent drainage. However, weeds may grow in the gravel and sanitation can be difficult on gravel floors. It may also be difficult to have small containers sit upright and level on gravel floors. So, gravel floors work best with larger containers, containers in trays and flats.
Floors may also be covered with a polypropylene woven fabric (weed mat or ground cover). The fabric is typically black, at least 17 mils in thickness and may have lines or patterns to help with plant spacing. This is again a relatively low cost option that allows maximum use of space and provides flexibility. However, weed mat typically allows for slower drainage than gravel, collects substrate that washes from containers, can allow weeds to grown on top of or through openings in the mat and makes sanitation difficult. Floors with weed mat covering may also become uneven over time which can result in wet and dry spots in the crop. Wet areas can serve as locations where soil-borne diseases can begin to develop.
Concrete floors
Greenhouse floors may also be covered with concrete and the concrete floor used as the production surface. There are two basic types of concrete that may be used for greenhouse floors. The first is standard concrete. This material is made from Portland cement, aggregate, sand and water. It is strong and can support up to 2,500 lbs per square inch. However, standard concrete does not allow water to drain through. Porous concrete is made from Portland cement, aggregate and water. Being able to support up to 600 lbs per square inch, porous concrete is not as strong as standard concrete, but it allows water to drain through. Porous concrete should be avoided in locations such as potting or propagation areas. This is because substrate falling on the floors will tend to clog or block the pores of the concrete and hamper drainage. Also, the porous concrete is more difficult to clean than standard concrete.
Standard concrete is best used in areas where heavy weights must be supported (i.e. drives and walkways). Where standard concrete is used, it should be slightly sloped to allow water to drain off. Porous concrete is preferred in most situations where plants will be grown on greenhouse floors (excluding flood floor situations) because it is strong enough to support plants, people and light-weight equipment and because it drains. When using porous concrete for a greenhouse floor production surface, it should be poured over a gravel base so that the water is able to drain through the porous cement (rather than being blocked by an impervious layer).
In some situations, rubber tubing or pipes may be embedded in the concrete floors. This tubing is connected to a supply of hot water, and the hot water is circulated through the tubing in the floor and in this way the greenhouse is heated. In addition to being highly efficient, this type of heating system has the benefit of placing the heat source close the substrate (if the crop is being grown on the floor) and results in an increase in substrate temperature much like the Biotherm® heating system described in the heating unit.
Concrete floors prevent weeds and are generally easy to clean. They are durable and long-lasting and provide the advantage of maximizing space usage efficiency as well as flexibility.
Flood floors
Flood floors are in a sense hybrids of concrete floors and ebb-and-flow benches. 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, ozone, chloride, or copper 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 filters and may be treated for disease-causing pathogens.
Regardless of the exact design, the advantages of flood floors include reduced water use, reduced fertilizer use, elimination of runoff, and high space usage efficiency. The primary disadvantage is the initial cost since they can be expensive to install.
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Ground beds
Ground beds are most often used for cut flower or greenhouse vegetable production. They are filled with soil or artificial substrates in which plants are directly grown. Ground beds may have sidewalls and be elevated, or they may be level with the greenhouse floor. In some cases the ground beds might simply be the native field soil (sometimes amended and adjusted) that the greenhouse was built to cover.
Hanging basket systems
Hanging baskets may be placed on traditional benches or they may be grown using several types of elevated systems. Elevated systems offer the advantage of increasing spacing usage efficiency but hanging baskets may block light from reaching crops below them. The simplest system for hanging baskets is one in which baskets are hung using simple hooks. A drip irrigation tube is often placed in the hanging basket to provide water and mineral nutrition. 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.
