Horticulture Principles and Practices

FIGURE 12–10 A saw-tooth
greenhouse range.
Greenhouse Range
A collective term for two or
more greenhouses at a
single location belonging
to the same owner.
may be connected to form a saw-tooth greenhouse (Figure 12–10). Saw-tooth greenhouses
are used in places such as Texas, Florida, and California, where the climate is
mild. Large nurseries, for example, may construct a number of greenhouses on one site
to form what is collectively called a greenhouse range.
The construction costs of connected greenhouses are higher than freestanding
designs. The roofs are longer and thus require more structural strength in the framework
to support the building. They are also lower, and hence the volume of air space is less,
decreasing the amount of carbon dioxide available for plants in winter.
Greenhouses may also be categorized based on the material used in their construction.
These materials may further be grouped according to those used in the framework
of the structure and those used for framing or covering the structure. Greenhouses may
be arranged in multiple units to form large complexes. All of these styles, arrangements,
and types of materials have advantages and disadvantages. These characteristics are
discussed further later in this section.
12.2.2 GREENHOUSE CONSTRUCTION
The material used for greenhouse construction must be strong, light, durable, easy to
maintain, and inexpensive. It is important that the frame cast little shadow.
The basic structural components of a greenhouse are the rafter, end walls, sidewall,
sidepost, and purlins. The National Greenhouse Manufacturers Association publishes
standards to guide greenhouse design.
Material for Framework
The frame of a greenhouse may be made of metal or wood.
Metal Metal frames are more durable but relatively more expensive than wooden
frames. Iron frames are prone to rust and need to be painted (usually white) at regular intervals
to prevent rust. Aluminum frames are lightweight and rust resistant. Since the material
is very strong, greenhouse designs incorporate fewer and more widely spaced sash
bars (beams used to support glazing or covering material without sacrificing the overall
sturdiness of the structure. Further, wider spacing of the sash bars means less shading
from obstruction of incoming light.
Wood Wood was used in early greenhouse designs. It is relatively less expensive but
also less durable than metal. Wood decays over time and is susceptible to insect attacks
(e.g., termites). Like iron, wood requires painting (greenhouse paint) to protect it from
decay and insect attack. Durable species of wood are redwood and cedarwood. For
longer life, wood may be treated with preservatives before use. Mercury-based paints are
toxic to plants and must not be used. Similarly, pentachlorophenol and creosote wood
treatments are toxic to plants. Since wood is not as strong as metal, spacing of sashes in
wooden greenhouses is much closer, especially if heavyweight glazing material
(e.g., glass) is to be used. Closer sash spacing means an increased shading from
obstruction of incoming light.
394 Chapter 12 Controlled-Environment Horticulture