Propagation/Greenhouse ManagementUnit 1.3 | Part 1 – 107
Lecture 2: Managing Environmental Conditions
B. Passive and Active Environmental Management: A Brief Overview
The methods and tools used to manage environmental conditions in greenhouses normally
fall into one of two broad categories: Passive or active. Here we briefly describe these
categories; see Lecture 3, Greenhouse Heating, Cooling, Lighting, Irrigation, and Climate
Control Systems, for a more detailed discussion.
- Passive methods of environmental control are part of the functional design of most
greenhouses and represent a low-tech approach that does not involve the ongoing use of
energy to regulate conditions. They include:
a) Heating by the capture of solar radiation as sunlight passes through the greenhouse
glazing and warms the interior air
b) Cooling via side and end wall vents that draw in cooler air from outside and ridge
vents at the top of the greenhouse that allow the heated air to be exhausted out of the
structure. Shade cloth or whitewash can also be used to help cool the greenhouse.
c) Air circulation via the venting system. As with cooling, exterior air enters the structure
through open side and end wall vents, and the air already in the greenhouse exits
primarily through ridge vents and vents placed high on end walls.
d) Irrigation can be delivered by hand or by overhead spray systems.
e) Lighting comes exclusively from sunlight. Light reduction via whitewashing and the use
of shade cloth is another form of passive management.
f) Additional physical methods to heat and cool the greenhouse include the use of shade
cloth, white washing, and energy curtains
- Active Methods
Active environmental controls use an external energy source to power heating, cooling,
venting, supplemental lighting, irrigation, and climate control systems. Active control
mechanisms are not a substitute for passive methods, but rather are complimentary tools
that allow growers to more precisely and predictably create desired conditions. Active
methods include:
a) Heating via conduction (direct contact with heating source), convection (via warm air
circulation), and radiant heat sources
b) Cooling via evaporative mechanisms (pad and fan systems), swamp coolers, and fog
systems
c) Air circulation via exhaust and horizontal airflow fans
d) Supplemental lighting, including incandescent, fluorescent, high intensity discharge,
and high pressure sodium lights
e) Automated irrigation systems, including overhead sprinklers
f) Thermostats, stage controllers, and computer-directed environmental controls that
monitor and control various heating, cooling, circulation, lighting, and irrigation systems
C. Environmental Control in Different Types of Propagation Structures
- Passive Solar Greenhouse: Good environmental control is possible in relatively low-
tech facilities, especially in milder climates where growers do not face extremely hot or
cold conditions. As described above, these greenhouses rely on passive techniques (see
also Supplement 1, Examples of Daily Warm- and Cool-Season Greenhouse Management
Practices in a Passive Solar Greenhouse).
a) Trap solar radiation to warm the air and thus the crops
b) Cooled through the use of venting systems: Combination of end wall vents, roll up/down
sides, and ridge vents, to draw in cooler external air, and exhaust warmer internal air