Organic farming

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

1. 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 Active methods

Active methods are also part of the functional design of greenhouse structures, but use

an external energy source to power mechanisms that enhance the greenhouse grower’s

ability to more precisely manage temperatures, air circulation, and water delivery

a) Active environmental controls inherently drive up construction costs because additional

mechanisms must be purchased and installed. In many climates, and for some crops,

these tools are critical to achieve appropriate environmental control. Over time,

increased labor efficiency and improved crop performance can make up for upfront

costs.