Growing conditions > Introduction
Greenhouse climate conditions
Originally the reason for growing in a greenhouse was providing shelter from adverse weather conditions. Nowadays the objective is achieving maximum yield and quality by optimising the environmental conditions. The main control factors are light, temperature, humidity, CO2 concentration, root-zone conditions and plant nutrition.
Light
Light induces photosynthesis in plants, also known as CO2 assimilation. This is uptake of CO2 gas and formation of carbohydrates that are transformed to plant tissue. Light is therefore the basis of plant growth. Generally the levels of natural light are high in summer. The light component is not easily too high, but the heat component in the solar radiation is often excessive. Measures are needed to prevent excessive plant temperatures. In winter, the natural light level is insufficient for growing tomatoes and similar crops in some parts of the world (e.g. northern Europe, Canada and South of New Zealand). It can be feasible to use artificial light, but the economics of this depend on cost and price structures.
Temperature
Temperature is an important factor for plant growth and development (development is the formation of new organs, e.g. leaves, flowers). For most plants a certain temperature strategy has been developed, e.g. certain day and night temperatures. This can be adjusted and refined to get the best out of a particular variety. Growers often add further refinements in temperature control to steer the plants in a particular way, or to improve the energy-efficiency. The finer details of optimal temperature control are very well determined for tomatoes and some other main crops.
Air humidity
The humidity in the greenhouse is the result of outdoor air humidity, plant transpiration, ventilation rate and other factors. Too low humidity causes stress on the plants and makes them wilt. Too high humidity stimulates fungal growth and causes mould on the plants. It also hampers transpiration and hence uptake of nutrients. The control computer can be set so that these risky humidity situations are avoided. A good control programme also has options to control air humidity in an energy-efficient way.
CO2 concentration
The concentration of CO2 (carbon dioxide) gas can be lower in the greenhouse than outside because plants consume CO2 at a high rate. Greenhouses make it possible to artificially increase the CO2 concentration by injecting CO2 gas. This boosts the photosynthesis and hence the growth and production by up to 25%. CO2 gas is often obtained by combustion of a suitable fuel. CO2 enrichment during the day can be connected to heating at night, by using a heat storage tank.
Root-zone conditions
Conditions in the root-zone are very important too. Variables are temperature, oxygen concentration (aeration), acidity or alkalinity (pH), total salt concentration (EC or CF), and the concentration of particular nutrients. More details about the root-zone conditions in the next chapter.
Greenhouse climate control
The main gear used to modify the environmental conditions are heating, venting, CO2 injection, and optionally auxiliary gear such as fans, screens (either for energy conservation, shade or black-out), lights (either for assimilation or day length control), misters, foggers, roof sprinklers. They are all connected to a computer that contains specialised software.
Computer
The greenhouse control computer receives information from sensors that monitor the growing conditions in the greenhouse, as well as the weather conditions and solar radiation outside. The computer calculates the conditions required by the grower (‘set points’) at a certain moment, and compares them with the measured conditions of that moment. If necessary, the computer activates the control gear (e.g. heating). Obviously the computer does what the grower has instructed through the choice of the ‘settings’.
Further reading
Temperature control: ‘dif’, pre-night and other tricks for crop steering (2004)
Controlling plant growth in greenhouses - introduction (2006)
Steering plants in generative or vegetative direction (2007)
Tools for plant balance control prioritised (2007)
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