Since crops cannot be grown and money cannot be made in space devoted to aisles, greenhouse benches are arranged carefully to provide the greatest area for growing space. The number and width of aisles are determined by the width of the carts needed in the aisles, and whether the crop can be worked from one side or needs to be accessible from both.
Three methods of arranging benches in greenhouses are commonly used (Figures 19-21, 19-22, and 19-23).
To increase the efficient use of space in greenhouses employing raised benches, some growers have placed benches atop rolling pipes, thereby permitting the movement of benches back and forth (Figures 19-24A & B). This system eliminates the need for permanent aisles and reduces their numbers. Essentially the aisle can be shifted to where it is needed.
Characteristics
Features a wide central aisle to accommodate carts and other equipment; narrow aisles between benches allow access by workers and efficient use of space for growing.
figure 19-22. With peninsular benching, each bench is accessible from both sides and the aisle end. (Delmar/ Cengage Learning)
figure 19-23. Length-of-house benching. The central bench can be worked from both sides. (Delmar/Cengage Learning. Photo by Steve Newman.)
SUMMARY______________________________________
Plant growing structures exist to provide consistent, controlled environmental conditions for the growth of plants. There are several different types of structures. Greenhouses exist in many forms depending on their age, location, and use. They can be built of aluminum, iron, steel, or wood and glazed with glass, hard plastic, or soft plastic. Their styles include: (1) detached A-frame truss, (2) ridge-and-furrow A-frame truss, and (3) quonset. Other growing structures include shade or lath houses, cold frames, hotbeds, and high tunnels. Typical structural plans for these structures are included in the chapter.
The material selected to cover the growing structure depends on the amount of light needed, the length of service required, and the time and cost of materials and maintenance that the grower can accept.
Greenhouses require heating systems to supplement natural solar heat by day and at night. The choices include central steam or hot water systems, and unit heaters. Each has advantages and disadvantages.
Greenhouses are ventilated by using roof and side ventilators, exhaust fans, fan-and-convection-tube systems, or a retractable roof. Ventilation is needed to distribute air evenly throughout the greenhouse, maintain consistent temperatures, and reduce the risk of disease.
The need to cool the greenhouse is greatest during the summer months. Three approaches to cooling are used depending on the time of year and geographic location of the greenhouse: shading the glass, ventilation, or heat exchange and water evaporation.
Energy conservation has become increasingly important in recent years as expensive fuels have sent greenhouse production costs climbing. Techniques such as double-layered plastic coverings, foundation insulation, north wall insulation, and thermal blankets are being tried now. The future will see additional techniques developed.
The style of benches, the materials used in their construction, and their arrangement within the greenhouse all depend on the crops being grown. Benches may be raised or at ground level. They may contain the crops or merely support them. They may be commercially prefabricated or improvised. They may be arranged as cross-benching, peninsular benching, or length-of-house benching. Regardless of these differences, all benches must drain quickly, be of a width that allows workers to reach into their center, and allow crops maximum exposure to light.
ACHIEVEMENT REVIEW
A. SHORT ANSWER
Answer each of the following questions as briefly as possible.
1. List the three purposes of a greenhouse. Then list four crops that are grown commercially in your region of the country for each purpose given.
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2. Match the characteristics on the left with
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Match the characteristics on the left with the covering materials on the right.
1. clean glass
2. polyethylene
shade fabric vinyl
polyvinyl fluoride fiberglass
b. a summer covering used over quonset frame greenhouses
c. allows the highest percentage of sunlight to pass through
d. allows the least percentage of sunlight to pass through
e. has the shortest time of service of the soft plastics
4. Label the parts of the following structural plans.
5. Match the characteristics on the left with
the types of heating systems on the right.
a. adaptable to 1. steam
small 2. hot water
greenhouse 3. unit heater
areas and
often used as
emergency
back-up
b. requires large piping, thereby increasing installation costs
c. uneven heat distribution
d. heat can be transported through a large range of greenhouses without cooling
e. permits the most accurate thermostatic control of temperatures
f. requires smaller piping, keeping installation costs lower
g. may be used with a plastic convection tube to permit more even distribution of heat
h. a safer system with less potential danger in the event a line ruptures
i. most adaptive to heat treatment of soil
j. cost of maintenance is greatest
6. Several bench styles are described below.
For each, describe its best use, cost, and
advantages or disadvantages.
a. A ground bed edged with concrete to separate it from the walk contains a field of drainage tile and six inches of crushed stone.
b. A raised bench with redwood sides and wire mesh base; it contains no soil.
c. A raised bench of concrete blocks, wooden supports, and welded wire fabric.
d. A raised bench of wood has sides and contains soil.
e. A raised bench of concrete blocks, wooden supports, and lath.
7. Diagram and label three common methods
of arranging benches in a greenhouse.
B. ESSAY
1. Write a short paragraph explaining how shading, ventilation, and water evaporation each serve to cool a greenhouse differently.
C. TRUE/FALSE
Indicate if the following statements are true or false.
1. Heat can be lost through both the foundation and the glazing material of a greenhouse.
2. Energy conservation methods employed in greenhouses are aimed at retaining as much heat as possible, and plugging all avenues of escape.
3. Present energy conservation technology is highly advanced.
4. We can anticipate no advances in energy conservation technology in the future.
5. The use of a small fan to inflate a double layer of plastic covering a greenhouse is a fairly effective method of conserving heat energy.
6. Industry trends are toward the production of warm-temperature crops in northern greenhouses.
7. Thermal blankets are used to maintain warmer night temperatures near crops while allowing the rest of the greenhouse to cool down.
8. Since maximum heat loss occurs through the west wall of a greenhouse, full ground – to-roof insulation is proving to be effective in cutting heating bills.
9. Since warm air rises, heating the roof area of a greenhouse is an efficient use of energy.
10. Placing heating lines beneath benches is more efficient than running the lines overhead.
