Greenhouse Benching and Production Surfaces

The choice of a greenhouse benching (or better stated as "production surface") system is affected by many variables. In the past, the choice of benching 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 methods), additional considerations have become important when determining what type of benching system or production surface to install in a greenhouse. Important aspects to consider include:

Bench Height

The height of the bench is important primarily because of how it impacts people who must handle the plant material. If benches are too low, workers must bend to work with plant materials, and this can result in worker fatigue and even potential health problems. If a raised bench system is used, a height of 3 feet is typically recommended. In many cases greenhouses do not have raised benching surfaces. Rather, the floor serves as the production surface. In this case, bench height becomes irrelevant.

Bench Width

The desired width of a raised bench surface is dictated primarily by two factors. The first is that workers must be able to reach plant materials on the bench. If workers have access to both sides of the bench, 5 feet widths are recommended. If workers can only access the bench from one side, the width of the bench 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.

Benching systems must also be adapted to any automation in use. For example, if boom irrigation systems are used, the width of the boom arms will dictate the width of the benches. If ebb-and-flow benches are used, the width of the bench will be dictated by the irrigation trays that are available.

Air or drainage spaces in bench surface

Openings in the bench surface 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 and potentially differences in mineral nutrition. This is in turn can result in the development of a non-uniform crop. Additionally, "wet spots" on a bench can result in increased disease incidence.

If floors are used as the production surface, they are often covered with small gravel that allows for water drainage. If concrete floors are used, the may be grooved or laid with a small grade to promote drainage of water.

Strength

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.

Cost

There are two primary costs associated with a benching system. The first is what the actual materials and labor will cost to purchase, assemble, and maintain the benching system. Obviously, the lower the cost, the more attractive the benching system. However, the true cost must also include the lifespan of the benching system and whether any production cost savings are achieved by using a particular benching system. If a benching system must be replaced often, its initial low cost might not be as attractive as a more expensive material that will last for a much longer period of time. An expensive benching system, such as ebb-and-flow benches, may be expensive of itself, but it 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 benching systems 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 expressed as a percentage of total area under cover 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 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. Scroll down to learn how to calculate space usage efficiency and to see specific examples of how increasing space usage efficiency can reduce costs.

Compatibility with irrigation methods

Many benching systems combine the benching system with the irrigation system to reduce labor costs associated with irrigating and fertilizing plants. Some of these combined bench-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.

Support requirement

The type of support system required (structure underneath the surface of the bench) for a bench will partially depend upon the load the bench must support. However, different types of bench surfaces will 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 will 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, benching systems may need to change. If the benching systems in use cannot be adapted to the new technology, they will either have to be replaced or the new technology not utilized in the operation. Benching systems that allow maximum flexibility will best allow for the use of new technologies.

Greenhouse Benching Materials

The support structure of elevated benching systems may be made of concrete blocks, wood, or plastic. However, galvanized steel or aluminum are most commonly used to make the structural support of elevated benches.

The bench surface may be made of expanded metal, wire, wood, hardened plastic, chain-link fence, snow fencing, or concrete. Each of these materials have advantages and disadvantages. Hardened plastic is resistant to water and chemicals and can support heavy loads, but hardened plastic benches are expensive and typically only used in special situations such as propagation houses and retail displays. Another use for plastics in greenhouse benching is in the production of the bench liners used in ebb-and-flow benching systems. Wood is readily available and can be lower in cost than other materials (or more expensive than some materials). However, wood must be treated to prevent decay and can be difficult to clean. Wire and chain-link fence is low in cost, but because it bends easily, it requires a significant support structure underneath to prevent bending and warping and to maintain a level production surface. The most common benching material is expanded metal. It is preferred because it is relatively low in cost, strong, and rigid. Expanded metal is also low maintenance and has a long lifespan. Concrete is most often used to form flood floors.

Common Greenhouse Benching Systems

There are many types of benching systems used in greenhouses. Some are raised or 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 facility.

Stationary benches

These benches are elevated and mounted in such a way that they cannot be moved while being used to grow a crop. In some cases, they may be mounted into concrete floors are bolted to the floor and never moved. In other cases, they may be set in place and may be repositioned if necessary when a crop is not present. These benches may be constructed of all of the materials listed above.

Rolling benches

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. This allows for a reduction in the number of walkways and thus an increase in the space usage efficiency. However, the use of rolling benches reduces access to plant material.

Dutch Trays

Dutch trays are a name used to describe a benching system in which benching units or trays are placed on a track system that runs throughout the greenhouse facility. The trays may be easily moved on these tracks In the greenhouse, the trays serve as the production surface and may be placed tray-to-tray to conserve space(as with rolling benches). The trays may alse be moved around the greenhouse structure or to other areas of the facility such as shipping. Therefore, the Dutch tray system may be used to transport plant materials around the operation as needed. The trays are also usually designed to serve as part of an ebb-and-flow irrigation system. Thus Dutch trays combine aspects of a rolling bench system, ebb-and-flow irrigation system and a transport system.

Removable benching

Many types of removable benching systems exist. These are simple and temporary benching systems that are not mounted to the greenhouse floor and may easily be placed into the greenhouse structure when needed and removed when not needed. These systems allow for maximum flexibility and are often used when some crops are to be grown on floors while others need to be grown on raised benches. These benches may be constructed of numerous materials, but they must be easy to assemble and light weight.

Ebb-and-flow benches

This system combines the benching and irrigation systems. Ebb-and-Flow benches allow for efficient irrigation and recirculation of water and fertilizer solution. The plastic trays fill with water and then are drained. The water is collected in tanks and reused. Ebb-and-flow benches may be stationary or combined with rolling benches. Ebb-and-flow benching systems will be discussed in greater detail under greenhouse irrigation.

Troughs

These types of benches are similar to ebb-and-flow benches but utilize linear troughs. The water flows from one end to the other where it is collected for reuse. These are not as common as ebb-and-flow benches when containerized crops are being grown. However, troughs are often used in hydroponic vegetable production. Trough benching systems will be discussed in greater detail under greenhouse irrigation.

Concrete or gravel floors

Using the floor as the production surface potentially increases space usage efficiency and maximizes flexibility. The floor may be concrete, gravel, or compacted soil covered with weed mat. The primary disadvantages of using the greenhouse floor as the production surface are reduced air circulation and the increased potential for poorly drained areas.

Flood floors

Flood floors are essentially hybrids of floor production surfaces and ebb-and-flow benches. The greenhouse floor is poured concrete (with special properties which are discussed under greenhouse irrigation) with raised edges that allow the floor to be flooded and drained just as with ebb-and-flow benches. The advantages of flood floors include reduced water use, elimination of runoff, and a high space usage efficiency. The primary disadvantage is the initial cost since they can be expensive to install.

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. New production technologies have made ground beds uncommon.

Calculating Production Surface Area and Space Usage Efficiency

In addition to designing the individual benches or production surfaces, the arrangement of the benches is important. Besides affecting traffic flow within the greenhouse, the benching arrangement affects the amount of space available under cover for production of crops (space usage efficiency). A greenhouse that has 10,000 ft² under glass will not necessarily have 10,000 ft² available for the production of crops. Some of the space will be used for walkways, work areas, and greenhouse systems. The space available for crop production should be maximized because this is the income generating area.

It is important to be able to calculate the square footage in a greenhouse, the square footage of production area under cover and the space usage efficiency. These calculations can be performed using a few geometric equations.

For rectangles, the length of the structure multiplied by the width of the structure provides the total number of square feet within the structure. As an example, we will determine the total square footage of space and the space usage efficiencies for two different bench arrangements in a greenhouse structure that is 38 ft wide and 60 ft long. In the first benching layout, the side benches (running the length of the greenhouse) are 2 ft wide and extend the entire length of the structure. There are 3 benches spaced across the greenhouse that are 6 ft wide. The central walkways between the benches are 4 ft wide and the end walkways are 5 ft wide.

The total area under cover is:

38 ft x 60 ft = 2280 ft²

The area in benching (production surface area) is:

2 x (2 ft x 60 ft) = 240 ft²

3 x (6 ft x 50 ft) = 900 ft²

240 ft² + 900 ft² = 1140 ft²

Therefore, the space usage efficiency for this benching layout is:

(1140 ft² ÷ 2280 ft²) x 100 = 50%

If we put rolling benches into the same greenhouse structure, our production surface area and space usage efficiency changes. In this layout, there are two 2 ft wide benches along the length of the greenhouse. There is one 4 ft aisle. There are six 5 ft wide rolling benches. As before, there is a 5 ft walkway at the end of these benches on each end of the greenhouse.

The overall area under cover remains 2280 ft²

The two side benches still provide 240 ft²

The rolling benches provide 6 x (5 ft x 50 ft) = 1500 ft²

The total area in benching is 240 ft² + 1500 ft² = 1740 ft²

The usage efficiency is 100 x (1740 ft² ÷ 2280 ft²) = 76%

By using rolling benches, the space usage efficiency increased from 50% to 76%, and we gain 600 ft² of bench space that can generate revenue. We also created 600 ft² of space over which we can spread production costs. If the business is paying $5,000 per year on the cost of the structure, approximately $13.70 must be generated per day just to pay for the greenhouse. In the first benching scenario, there were 1140 ft² of space that could generate revenue. Therefore, each ft² of bench space must be charged approximately $0.012 per day just to pay for the greenhouse structure. A crop that occupies 1 ft² of space (and thus each plant) for 6 weeks must be charged $0.50 to pay for that space. In the second scenario, each ft² needs to be charged $0.008 per day. Therefore, the same crop would be charged $0.34 per plant. The business would save $0.16 in production costs per plant. These savings would go directly to the profit line of the business. All other fixed costs and some variable costs (i.e. heating) are accounted for in this way. Therefore, maximizing space usage efficiency reduces costs.

Van-Wingerden Greenhouse Company
Omni Structures International

© 2005, M.R. Evans