Section 1: Greenhouse Benching and Production Surfaces
The choice of a greenhouse benching system, or better stated as "production surface", is affected by many variables. In the past, the choice of production surfaces was dictated primarily by labor concerns (i.e. bending and reaching), cost, and ease of movement around the structure. These factors are still important, but with changes in production technologies (particularly irrigation and fertilization methods), additional considerations have become important when determining what type of production surface to install in a greenhouse. Production surfaces in greenhouses may be raised or elevated benching systems or they may be located on the greenhouse floor. When deciding upon a production surface for greenhouse crops production, the following are some important aspects to consider:
Production surface height
The height of the production surface is important primarily because of how it impacts people who must handle the plant material. If it is too low, workers must bend to work with plant materials, and this can result in worker fatigue and even potential health problems. If the benching surface is too high, it may be difficult for workers to reach and work with plant materials. If a raised production surface is used, a height of 3-3½ feet is typically utilized. Where floor-based (i.e. gravel floors, flood floors) production systems are used, height is irrelevant (since it is on the ground) although works must still bend to work with plant materials. In some cases crops may be elevated higher than 3-3½ feet above the floor. Hanging baskets may be hung above head height to increase space usage. Gutters used for greenhouse strawberries may be shoulder height to facilitate picking of fruit and plant maintenance.
Production surface width
The desired width of a raised production surface is dictated primarily by two factors. The first is that workers must be able to reach plant materials. If workers have access to both sides of a raised bench, 5 feet widths are recommended. If workers can only access the raised bench from one side, the width should not exceed 2.5 feet. If plant materials are grown on some type of floor, spacing of aisles will depend upon whether plant materials will need to be accessed between the time they are placed on the floor and when they are removed and shipped. If plant materials must be pinched or handled during production, aisles must be placed in such a way as to allow access to the plant materials by the workers. If plants are automatically irrigated and will not be handled until shipping, worker access is not as important and aisles may be eliminated or spaced far apart.
Production surfaces must also be adapted to automation that is in use. For example, if boom irrigation systems are used, the width of the boom arms will dictate the width of the raised benches or the spacing of the aisles on floor-based production surfaces. If ebb-and-flow benches are used, the width of the bench will be dictated by the irrigation trays that are available.
Air and drainage spaces in production surfaces
Openings in the surfaces of raised benches allow water to drain off the surface of the bench and for improved air circulation. If water collects on the bench surface, and especially if the surface is not level, the substrate of some portions of the crop will tend to remain wet while other areas dry out and require irrigation more often. This uneven drying can result in differences in water availability as well as differences in mineral nutrition if a liquid fertilization program is being used. This in turn can result in the development of a non-uniform crop. Additionally, "wet spots" on a bench can result in increased disease incidence.
Ebb-and-flow benches are raised benches with plastic flood trays that do not have openings in the production surface (otherwise the benches could not be flooded). However, ebb-and-flow flood trays have two levels of channels that allow for drainage away from the container as well as for improving air circulation.
If floor-based production surfaces are used, they are often covered with small gravel that allows for water drainage. If concrete floors are used, they may be grooved or laid with a small grade to promote drainage of water off of the production surface.
Strength
Raised benches must support the load of plants, containers, and wet soil. These loads can be high, particularly when close spacing of plant materials is used. Both the bench surface and its underlying support structure must be able to hold up to the required loads.
Floor-based production surfaces must support the weight of the crops, preferably without settling and creating indentations that retain water and cause “wet spots”. Floor-based production surfaces may also need to support the weight of equipment that may be placed upon them or vehicles that may be driven across them.
Cost
There are two primary costs associated with production surfaces. The first is the cost to purchase, assemble and maintain the production surface. Obviously, the lower the cost, the more attractive is the surface. However, the true cost must also include the lifespan and whether any production cost savings are achieved by using a particular production surface. If a production surface must be replaced often, its initial low cost might not be as attractive as a more expensive one that will last for a much longer period of time. Additionally expensive production surfaces, such as ebb-and-flow benches and flood floors, may be expensive in and of themselves, but they may allow a producer to reduce labor, irrigation and fertilizer costs and may in the long-run prove to be economical.
Another important cost aspect of production surfaces is that of space usage efficiency. A 100,000 ft2 greenhouse will not necessarily have 100,000 ft2 of potential revenue generating area since plants are typically not grown in aisles or other auxiliary areas. The objective is to maximize the area under cover that can be used to actually grow plants and generate revenue. This area is expressed as a percentage of the total area under cover and is referred to as the space usage efficiency. Maximizing space usage efficiency can be very important in keeping production costs down and maximizing profits. All fixed costs (i.e. payments on the greenhouse structure) and many variable costs (i.e. heating the structure) of production must be allocated to the crops grown in the structure. This is typically done by assigning a certain cost per ft2 of production area per week or per day. Any crop using that space is assigned that cost. The greater the actual area that can be used to grow plants, the lower the dollar value that must be assigned per ft2 per day or week. At the end of this learning unit, you will learn how to calculate space usage efficiency and see specific examples of how increasing space usage efficiency can reduce costs.
Compatibility with irrigation methods
Numerous systems have been developed that combine the production surface with an irrigation system to reduce labor costs associated with irrigating and fertilizing plants, to prevent run-off and to save water and fertilizer. Some of these combined production surface-irrigation systems include ebb-and-flow benches and flood floors. In other situations such as with irrigation booms, the bench or production surface must be designed to be compatible (i.e. same width) with the irrigation system. When selecting or designing a production surface, compatibility with irrigation systems (both in the present and in the future) should be considered.
Support requirement
The type of support system required (structure underneath the surface of a raised bench) for a production surface will partially depend upon the load the bench must support. However, different types of production surface materials also require different types of structural supports. Expanded metal is very strong and rigid. Therefore, it requires little support underneath. However, wire mesh is flexible and bends easily. This type of surface material will require many more supports underneath the surface to provide and maintain a level production surface. Rolling benches and Dutch tray systems require a more elaborate support system than stationary benches since they must be easily moved even when bearing heavy loads.
Floor-based benching systems (i.e. flood floors) must be strong enough not only to support plants and people, but also any equipment that may be placed on or driven across the floors.
Adaptability to new technology
As technology changes, production surfaces may need to change. If the production surface in use cannot be adapted to the new technology, it will either have to be replaced or the new technology not utilized in the operation. Production surface systems that allow maximum flexibility will best allow for the use of new technologies.
