Uniformity in the greenhouse is a basic requirement for achieving a good, high yielding uniform crop. Uniformity increases productivity, labour and resource efficiency and product quality.
One of the main areas of variation in a greenhouse is the irrigation system. A high distribution uniformity is critical. Temperature and humidity can also vary significantly in the greenhouse and affect crop growth and production. In some situations, carbon dioxide levels become depleted and affect crop growth.
The following diagram is a snapshot showing the level of harvested fruit over a two week period mid crop at different points of a greenhouse. This type of information can be used to give an indication of the uniformity in the greenhouse and also identify problems which might be managed better. The greater yield gained at the northern end is a factor of higher light levels and day time temperatures. There is also a concrete path across the width of the greenhouse. This diagram highlights the impact of site and greenhouse design on crop uniformity.
There are three key areas that need to be addressed are to make sure the greenhouse environment is as uniform as possible.
Adequate ventilation is necessary to avoid the build up of excess heat, humidity and depletion of carbon dioxide. Both passive (using vents) and active (using fans) ventilation should be capable of completely exchanging the air in the greenhouse at least once every minute. It is better to have more venting capacity than you use, rather than not enough.
Roof ventilation is also better than side wall ventilation. Vents in the walls of a greenhouse are limited in their capacity to provide an even environment. While they are very effective at exchanging air in the first couple of rows of the crop, the crop itself acts as a windbreak. This results in very little air movement towards the middle of the crop.
Moving and mixing of air throughout the greenhouse evens out temperature and humidity differences. Without sufficient air circulation, pockets of hot and cold air can occur. Humidity will also build up excessively around the plants. Side vents are limited in their capacity to fully mix the air in a greenhouse (a factor of the span width of the greenhouse). Roof ventilation circulates air well when in use. Fans can be used to improve air circulation and reduce uneven growing conditions.
Heating is relatively expensive and needs to be efficient. A heating system must be able to distribute heat evenly throughout the crop.
Particular parts of the greenhouse will be hotter or colder. The northern end and western side of a structure will tend to be hotter. The southern end will be colder. The northern end will also have less shading. Screens can be used to reduce the extremes and improve uniformity.