Horticulture Principles and Practices

FIGURE 12–15 A no-bench greenhouse production system
showing plants placed directly on the floor. (Source: George Acquaah)
FIGURE 12–16 Raised bench. (Source: George Acquaah)
Raised Benches
Raised benches are suited to potted plant production. Most greenhouses have raised
benches of a wide variety of designs and constructions. Some of them are makeshift and
temporary, consisting of brick legs and movable bench tops. The top may be of wood,
concrete, or wire mesh and may or may not have side boards (Figure 12–16). Metal
benches are most common. Molded plastic is sometimes used to make troughs in which
potted plants are grown. This material is also used in the construction of benches for ebband-flow
irrigation. Notwithstanding the material used, the bench must have a system for
draining properly. If wood is used, cedar, redwood, and cypress make good bench
materials because they resist decay. Wooden benches may be painted with copper
naphthenate preservative to prevent decay. Redwood has natural preservatives that are
corrosive to iron and steel; as such, nails and other construction materials that come into
contact with this wood should be of different materials such as aluminum or zinc.
The height of the bench above the floor should be such that cultural operations
(e.g., pinching, spraying, harvesting, and staking) are facilitated. Width of the bench is
also important. It should be narrow (3 to 6 feet or 0.93 to 1.86 meters) enough to permit
pots located in the middle rows to be easily reached. Air should be able to move freely
around and under the bench, as well as around the pots on the bench.
Ground Benches or Beds
Plants to be grown for several years that will grow tall in the process (e.g., cut flower
plants such as roses) are planted in ground beds (Figure 12–17). Ground beds vary in
design and construction. Using ground beds can be problematic from the standpoint
of disease control. If the ground bed has no real bottom in terms of depth, diseases
such as bacterial wilt are hard to control because of the impracticality of thorough
pasteurization of the soil to a reasonable depth. To correct this problem, concrete bins
may be constructed in the ground to hold the soil and to facilitate periodic pasteurization
to control soilborne diseases. These concrete bins have V-shaped bottoms
and drain holes for good drainage. They should be about 6 to 12 inches (12.2 to
30.5 centimeters) deep, depending on the plant to be grown. When drainage is poor,
drainage tiles may be installed and overlaid with gravel before topping with the root
medium. Walkways should be strategically located between beds to allow gardeners
easy access to the beds to prepare them, plant the crop, care for it, and harvest the produce.
These spaces should be graded so that water flows away from the beds to reduce
402 Chapter 12 Controlled-Environment Horticulture