Horticulture Principles and Practices

objects in its path (e.g., the floor, soil, plants, and benches). These “collectors” store the
energy for a short period and then release it into the atmosphere. Some modern greenhouses
depend on solar heat collectors for heating. The initial investment for a solar heating
system is high. As such, low-temperature solar systems are often preferred and used
in conjunction with a conventional heating system for additional heating, especially in
the winter season. A complete system consists of a collector, heat storage unit, heat
exchanger, and control panel.
Biotherm Heating The floor of the greenhouse can be heated by laying down rows
of flexible tubes (Figure 12–24). This strategy of heating a greenhouse, in which the soil
is the primary target, favors enterprises in which pots and trays are placed directly on the
floor (as often occurs in bedding plant production). The floor of the bench can also be
heated by a similar method. This method of heating the greenhouse is called biotherm
heating. Propagation benches benefit from heating from below.
Improving Heat Distribution
Notwithstanding the system of heating, it is crucial that heat generated be uniformly distributed
throughout the greenhouse to prevent cold spots from occurring. Heat distribution
is a more significant problem in large greenhouses than in small ones. When central
heaters are used, heating pipes should be strategically located throughout the facility to
effect uniform heating. Where cold spots occur, nearby plants grow slowly. The heating
system should provide adequate heat not only for the aboveground plant parts but also
for the root zone. In the case of unit heaters, attachments such as perforated polyethylene
tubing located above the plants are used to aid in the distribution of warm air
(Figure 12–25). The other important factor is that heaters should supply heat at a rate to
offset what is lost by conduction, infiltration, and radiation.
FIGURE 12–24 Greenhouse floor heating. Hot water
(90–120°F) is circulated through plastic pipes embedded in the
(porous) concrete floor of this greenhouse. This heating system
uniformly distributes heat ensuring uniform crop production. By
providing heat where needed (i.e., close to the plants), the
greenhouse aerial temperature can in some cases be lowered
and, thus, reducing greenhouse heating costs. (Source: Dr. AJ Both,
Bioresource Engineering, Department of Plant Biology and Pathology, Rutgers
University, New Brunswick, NJ 08901)
FIGURE 12–25 Perforated polyethylene tubing used for
heat distribution in a greenhouse. (Source: George Acquaah)
12.3 Internal Environmental Control 421