The irrigation of greenhouse crops is one of the most critical of all production practices. And yet it is frequently overlooked and taken for granted. To provide conditions for optimum plant growth it is essential to become familiar with the factors that influence soil moisture.
Irrigation Systems for Commercial Production
In most cases, water is applied to the upper surface of the media. This water may be applied by means of an overhead sprinkler, a drip or trickle irrigation system, by hand using a hose or similar device, or some combination of these delivery systems. Overhead sprinklers and hand watering have a tendency to “waste” water and also wet the foliage, which increases the potential for diseases and injury. Drip or trickle systems are the most efficient and provide greater control over the amount of water applied. Also, since the foliage does not become wet there is a reduced potential for diseases and injury.
Water may also be applied to greenhouse crops using subirrigation or capillary mats. However, in areas where soluble salts are a problem, mats do not provide for leaching, thereby increasing the risk of salt injury.
By far the most commonly used type of irrigation in Texas is the drip or trickle system. Several types of emitters are available commercially which provide a wide range of capacities. The most common are generally in the 1-3 gallon/minute range.
Frequency of Irrigation
Frequency of irrigation is largely determined by existing environmental conditions. During the months of March -September most Texas growers must irrigate their crops at least once a day and often two to three times. This frequency of irrigation means that growers must consider the physical characteristics of their growing media (i.e. water holding capacity and drainage) very carefully. Particularly where soluble salts are a problem. Often nutritional problems, such as magnesium and micronutrient deficiencies, arise as the result of excess leaching. In these cases special attention must be given to media amendments and nutritional regimes to provide for optimum plant growth.
Amount of Irrigation Water to Apply
The amount of irrigation water to apply is perhaps more important than how and when to irrigate. A general rule of thumb to follow in irrigating greenhouse crops is to apply 10- 15% more water than the container will hold. This facilitates leaching at each irrigation and reduces the potential for the accumulation of soluble salts. Of course the rate of irrigation must be low enough to allow the water to percolate through the growing media as opposed to over flowing the top of the container. When using soluble fertilizers in the irrigation water it is especially important to allow at least 10-15% for leaching to avoid salt build ups.
Condition of the Growing Media
The condition of the growing media is very important in determining irrigation efficiency. Most of the peat moss, bark and other organic constituents used in soilless growing media have hydrophobic or water repelling characteristics. When excessively dry, these materials have a tendency to be difficult to “wet” and therefore require careful attention during irrigation. In some cases a “wetting agent” may be required to provide adequate absorption. The key to avoiding problems associated with wetting is not to pot plants in excessively dry media or allow media to dry out between irrigations. Again, these problems may become more acute in the presence of soluble salts.
Porosity and Water Holding Capacity
Porosity and water holding capacity of the growing media is another factor which influences irrigation practices. Optimum combinations of these two characteristics provides enough large pores to allow for adequate leaching and aeration as well as a water holding capacity which minimizes irrigation frequency. Another consideration in this area are root diseases associated with wet, poorly drained media. Therefore it is important to maintain moisture levels which are not conducive to root diseases.
Water Quality
Water quality largely influences irrigation practices. As previously mentioned, the presence of soluble salts requires that growing media be well drained and that at least 10-15% more water than the container will hold be applied at each irrigation. It is also advisable to reduce the wetting and drying of the media between irrigations to avoid increases in relative concentrations of soluble salts. Be sure to have your water quality tested on a regular basis to monitor these conditions.
Summary
Irrigating greenhouse crops is a critical production practice and should not be taken for granted. Most greenhouse crops in Texas are irrigated by means of a drip or trickle system. The frequency of irrigation is largely determined by existing environmental conditions as well as the physical properties of the growing media. In areas where soluble salts are a problem it is important to apply at lest 10-15% more water than the container will hold to provide adequate leaching. Media which is extremely dry may be difficult to “wet” and care must be taken during irrigation to insure thorough water distribution. Water quality should be checked on a regular basis to monitor soluble salts.
