Propagation/Greenhouse Management
Part 1 – 114 | Unit 1.3
- 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.
b) 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
c) Design considerations are based on how hot or cold your climate gets, combined
with the desired temperature ranges for the crops you grow. These will determine
the importance of and type of heating and cooling infrastructure to incorporate into
greenhouse design.
B. Principle Heating and Cooling Mechanisms
- Active/Supplemental Heating can be delivered to the greenhouse environment several
ways:
a) Conduction: Conductive heating occurs when growing containers are in direct contact
with the heat source. Heat is transferred from the source to the soil media and then to
the plant roots and canopy. Electric heat mats, benchtop hot water piping, and radiant
floors are all examples of conductive heating.
b) Convection: Convective heating occurs when warmed air is moved around plants via
fans or other means of air circulation, transferring warmth to the soil and crop. Unit
heaters and perimeter fin/pipe systems, combined with fans, are examples of convective
heating.
c) Radiation: Radiant heating occurs by way of infrared waves transferring heat energy to
the crops. This takes place when crops are placed close to the heat source, such as when
growers install hot water piping under benches, and for the crops closest to perimeter
fin/pipe systems. Most mechanical heat sources actually deliver a combination of
radiant and convective heat.
- Heating mechanisms
a) Unit heaters, functioning by convection, are normally suspended from the upper
structure of the greenhouse, and can be gas or electric powered, depending on
available energy sources and costs. A unit heater consists of a heating element/
fuel combustion chamber and fans to move the heat from the source through the
greenhouse. In larger greenhouses and in colder climates, growers use multiple unit
heaters and/or perforated duct systems to more uniformly distribute heat.
b) Hot water systems, such as the perimeter fin/pipe, and under-bench hot water piping,
heat the air of the greenhouse, which then radiates and is moved by convection to the
containers/soil and crops. These systems can be powered by natural gas, propane, oil,
wood waste, geothermal, or solar batch collectors, depending on resource availability
and costs.
Lecture 3: Greenhouse Climate Control Systems