Environmental Horticulture, University of California, Davis, CA 95616-8587
(prepared for CORF News December 1997)
Conventional irrigation practices of greenhouse plants are generally designed around the idea that water is cheap and plentiful and that the rooting medium should become completely saturated at each irrigation. Potting media are designed so that they drain well, allowing excess amounts of water drain away rapidly. Also, some excess irrigation frequently is justified to leach any undesirable salts from the root zone.
Since there is no such thing as a completely uniform irrigation system, there will be a plant within each irrigation circuit which received the least amount of water. The customary practice is to irrigate in such a way that even this plant receives a full irrigation including the leaching treatment. As a result, the other plants will receive more water than they need.
Thus it is common in the industry to apply considerably more water than is actually needed for optimal plant growth. In one irrigation study at UCDavis we found that we could reduce conventional irrigation amounts by 80% and still produce decent-looking plants. In another study in several commercial greenhouses, with a variety of potted crops, we found that growers had the potential to generate savings in applied water and fertilizer ranging from 25% to over 95%. The degree of savings tended to depend on how seriously irrigation was taken. The nurseries that did not have an employee accountable for good irrigation were the ones with the most waste. In these places irrigation was done "on the side" along with other activities (e.g. packing, spraying, etc.). This was not optimal for the crop since irrigations almost always came too late or too early and were always too long.
Development of an optimal irrigation strategy is based on knowledge of the effects of non-optimal irrigation. Irrigating with too little water after letting the root zone run dry may result in a zone in the pot which is too dry for roots to survive, resulting in root loss in these areas. As a consequence, the potential for nutrient and water uptake of the plant is diminished, probably resulting in poor growth and increased disease potential. With fertigation, inadequate amounts of water also mean inadequate amounts of applied fertilizer, unless the injection system is adjusted to compensate for this.
Irrigation amounts applied in excess of what the rooting medium can hold generally has no detrimental effects on the crop if the medium is designed to hold a lot of water while still retaining a significant amount of air space. Most commercial mixes are designed this way. However, if you use a mix that is not formulated with this in mind, then it is possible to over-irrigate. If this occurs, then this may result in water logging which involves inadequate oxygen for the roots.
Even if excessive watering is not a problem for the plants, it does represent wasted resource (purchased water and fertilizer). Furthermore you may be held accountable if the discarded fertilizer ends up in the local drinking water.
The key to optimal irrigation is to know: 1. when the root zone of a typical plant has reached a level of dryness where it is ready to be watered, and 2. the point during an irrigation where the medium has become wet enough so that the irrigation can be stopped
At the University of California we have done research into sensing moisture content of growing media and using this for making irrigation decisions in greenhouse production. We use a tensiometer to monitor the moisture conditions in the root zone. Once some dry threshold is reached, an irrigation is scheduled. During the irrigation the media moisture level is monitored until some wet threshold is reached. At that point the irrigation is stopped. Our studies have shown that this type of control can be automated and that it can be used to produce quality plants. Our experience in these studies leads to several specific recommendations regarding irrigation in greenhouse production: