Propagation/Greenhouse ManagementUnit 1.3 | Part 1 – 113
Lecture 3: Greenhouse Climate Control Systems
Lecture 3: Greenhouse Heating, Cooling,
Lighting, Irrigation, & Climate Control Systems
A. Passive and Active Environmental Management
As discussed briefly in Lecture 2, passive and active methods are the two general categories of
techniques used to manage environmental conditions in greenhouses
- Passive methods
Passive methods are part of the functional design of most greenhouse structures and
represent a low-tech approach that does not involve the ongoing use of energy to regulate
conditions
a) Heating is achieved by the natural capture or trapping of solar radiation as sunlight
passes through the greenhouse glazing and warms the air within the structure. The
extent to which you can heat or even overheat a greenhouse solely through trapped
solar radiation depends on your regional climate, how the greenhouse is situated
relative to other buildings, trees, etc., and the aspect or slope orientation of the site.
b) Double Wall Glazing: Double wall polycarbonate roofing and double layers of
polyethylene film held aloft by fans can provide a measure of insulation and a buffer
against rapid temperature swings
c) Internal Curtains: Retractable by day to maximize light infiltration and deployed at
night, modern curtains reflect heat back into the greenhouse and further buffer crops
against nighttime low temperatures
d) Cooling occurs principally through the use of side and end wall vents that draw in
cooler air from outside of the greenhouse, and by vents located along the ridgeline
that allow the heated air to escape. The capacity to cool greenhouses solely by passive
means is partly a function of structural design, but is largely determined by local climate
conditions, exposure to prevailing winds, and the intensity of sunlight heating the
house. When it is 90ºF outside, an unvented greenhouse can easily rise to 130ºF. Even
with early, preventive venting, it can be difficult to keep interior temperatures below
100ºF.
e) Some cooling can be achieved by covering structures with shade cloth or whitewashing
to reflect solar radiation, but the efficacy of these methods is again dictated by
local climate. This also reduces light transmission, which can negatively impact crop
performance, slowing growth rates, creating weaker, leggy plants and softer, more
tender tissue.
f) Air circulation occurs exclusively via the design, functionality, and deployment of the
venting system. As with cooling, exterior air enters the structure through side and end
wall vents; the air already in the greenhouse exits primarily via ridge vents and vents
placed high on end walls. Despite a lack of active mechanisms (fans, blowers, etc.) to
exchange air, the side, end wall, and ridge vents sized appropriately for the structure,
can effectively promote air circulation and exchange. This can be a vital tool in limiting
the presence of disease pathogens, as discussed in Lecture 2.
g) Irrigation in passive structures can be delivered by hand or by overhead spray systems.
The greenhouse manager must make ongoing, real time decisions to determine when
and how much water to apply to what crops.
h) Lighting in passive structures comes exclusively from the sun and is dictated by your
regional climate, how the greenhouse is situated relative to other buildings, trees, etc.,
and the aspect or slope orientation of the site. Light reduction via whitewashing and
shade cloth is another form of passive management.