Natural and mechanical greenhouse climate control

Greenhouses provide higher than ambient average temperatures and protect plants from the effects of wind and rain, however in order to grow plants successfully, it is generally necessary to modify the naturally created internal climate. The climate control can range from that required only to avoid high temperatures which cause severe plant stress, to providing close control of temperature, limiting the maximum and minimum humidity and increasing carbon dioxide concentration in order to regulate plant growth, improve production and quality, and reduce the dangers of disease. In commercial horticulture it is necessary to find a compromise between the agronomic performance of the greenhouse production system and its investment and operating costs, including the climate control system, that matches the requirements and financial resources of the grower. The three elements in a greenhouse climate control system are (i) the method of measurement, (ii) the method used to change the climate and (iii) how the climate change system is regulated. The methods used to measure temperature, humidity and CO2 are reviewed and ways to minimise measurement errors identified. The methods used to modify greenhouse temperature, humidity and CO2 concentration are outlined and their limitations discussed particularly in the context of insect exclusion and fungus disease control. The importance of spatial homogeneity of the climate is stressed. The controlled climate variables are maintained at or between prescribed set point values selected by the grower, by automatic control systems using feedback control. Optimal climate control systems have recently been developed that automatically adjust the set points in order to achieve a specific objective such as maximising the financial margin between crop value and the cost of CO2, or to reduce greenhouse heat requirement. Traditionally, control has focussed on the greenhouse climate and only indirectly on plants. Climate controllers are now available that make on-line measurements of plant temperature, fruit growth and sap flow, enabling estimates to be made of transpiration and photosynthesis. This will provide valuable information which, with the correct interpretation, should lead to the successful inclusion of the "speaking plant" in the control system and thereby to more effective control of the greenhouse climate.