An overview of climate and crop yield in closed greenhouses

The closed greenhouse is a recent innovation in the horticulture industry. Cooling by ventilation is replaced partly (in semi-closed greenhouses) or completely (in closed greenhouses) by mechanical cooling. Excess solar energy is collected and stored to be reused to heat the greenhouse. In temperate climates, this concept combines improved crop production with energy savings. This paper presents an overview of climate, crop growth and development, and crop yield in closed and semi-closed greenhouses. The technical principles of a closed greenhouse are described and the macroclimate and microclimate arising from this are studied. The consequences of the typical growth conditions found in closed greenhouses for crop physiology and crop yield are examined. Finally, the experiences of commercial growers are presented. In temperate climates, closed greenhouses can reduce the use of fossil fuel-derived energy by 25 – 35%, compared with open greenhouses. With high global radiation, the climate in closed greenhouses is characterised by high CO₂ concentrations, high air humidity, improved temperature control, and a vertical temperature gradient. An annual increase in production of 10 – 20% is realistic, with reduced amounts of supplied CO₂. The yield increase is primarily obtained through increased rates of photosynthesis due to the higher CO₂ concentrations in closed greenhouses. To introduce this innovation into practice, knowledge transfer was a key factor for its implementation and the realisation of increased production levels. Future trends will require minimising the use of fossil fuels and increasing the level of control of the production process. Closed and semi-closed greenhouses fit seamlessly into this trend as they allow for a more controlled climate and higher levels of production, combined with savings in fossil fuel use.