Because of their increasing sophistication, ease of use and affordability, computers are used to maintain temperature, relative humidity, and CO2 concentrations within optimal limits even in relatively small greenhouse ranges. Computers can control many mechanical devices within a greenhouse (vents, heaters, fans, evaporative pads, irrigation valves, fertilizer injectors, shadecloths, energy saving curtains) based on preset criteria such as specific outside or inside temperatures, humidity, wind, and carbon dioxide levels. The limitations on such units, however, are that these factors cannot be controlled independently, especially in warm climates. For example, if vents are opened to reduce temperature, humidity and CO2 levels are also reduced. Another problem is that for many of these factors, especially under high light and temperature conditions, and high vapor pressure deficit, it is not clear what conditions are stressful to the plant. Much of the modeling work has been conducted in Northern Europe or Canada and there is little data to indicate, for example, the trade-off between cooling costs, and yield reductions. Transpirational cooling of individual leaves is reduced when shadecloths are used, but again the overall benefits in terms of crop yield are unclear.
Although it is possible to have very sophisticated computer control algorithms, most growers utilize their own experience or empirically-developed ‘expert grower’ models to program computer set-points. They then make adjustments based on observations of crop performance and stage of crop growth. Environments are modified throughout crop growth to ‘nudge’ the plant toward a particular growth pattern. Some of these adjustments can be very sophisticated and would be difficult to implement without computer controls and a rockwool or hydroponic growing system, but all growers can benefit from examining their plants closely to determine how they are growing and making some environmental adjustments. This issue is discussed further in the ‘Temperature’ section below.