Irrigating Greenhouse Crops
Irrigation Scheduling
Irrigation scheduling is generally explained as applying the right amount of water at the right time. Most plants use more water on hot, sunny days than cool, overcast days. The moisture needs of crops vary with their stage of growth with larger plants needing more water than smaller ones. The season of the year also affects the water requirements of the crop. Other factors affecting irrigation requirements include greenhouse humidity, the type of irrigation system, the type of growing media used, the type of containers used, and the crop itself. Some plants require much less water than others to produce the same amount of growth. Not to mention scheduling irrigation in greenhouses is especially challenging because different crops may be grown together in a single irrigation zone. In such cases, the best approach may be to estimate the water needs of the crop that requires the most water and over-irrigating the other crops.
Irrigation Scheduling Methods
To schedule irrigation for most efficient use of water and to optimize production, it is desirable to frequently determine the soil water conditions throughout the root zone of the crop being grown. A number of methods for doing this have been developed and used with varying degrees of success. In comparison to investment in greenhouse irrigation systems, these scheduling methods are relatively inexpensive. When properly used and coupled with grower experience, a scheduling method can improve the irrigators chances of success.
Visual and Tactile Assessment of Plants and Medium
Most greenhouses successfully monitor irrigation efficacy based on the feel and appearance of the plants and the growing medium. The best technique is to observe the relative ease with which water can be squeezed from the medium and attempt to correlate this moisture condition with plant appearance and container weight. This process requires a lot of experience and is very subjective but can be very effective when used by a knowledgeable, experienced nursery manager.
Monitoring Container Weights
Monitoring container weight is one of the few objective, nondestructive, and repeatable techniques for monitoring irrigation in container greehouses. Container weight is also the best way to determine irrigation needs early in the growing season before plants are large enough to show moisture stress or test. Container weight decreases between irrigations because the water in the growing medium is lost through evaporation and transpiration.
Monitoring Evapotranspiration Rates
Estimates of evapotranspiration for greenhouse-grown plants can be made using evaporation pans, automated weather stations, or atmometers, all of which can provide estimates of the amount of water loss in a given surface area. Evapotranspiration values are expressed in units of inches per day or centimeters per day. Evaporation pans measure water evaporated from a standard sized pan of water and then a correction factor must be applied to estimate evapotranspiration.
Substrate Moisture Sensor Technologies
The basic idea behind using sensor technologies to control irrigation is simple: when plants use water, they take it up from the substrate, so the water content of the substrate decreases. Sustrate water sensors detect these changes and can be used to open an irrigation valve when the substrate water content drops below a user-determined set-point. This results in frequent applications of small amounts of water, and the frequency of irrigation is adjusted automatically based on the rate of substrate water depletion. This irrigation approach automatically replaces water that is used by plants or lost through evaporation and assures that plants are never exposed to drought stress.
Monitoring Leaching Fraction
Scheduling irrigation in greenhouse crops can be difficult since many factors influence the decision of when and how much to irrigate. In making the decision to irrigate, many growers rely on their experience with the crop and the substrate in which it is grown, current weather conditions, or forecasts. In the absence of experience, the irrigation program is usually based on a set schedule and volume to ensure crops are watered thoroughly, which is commonly too much.
Measuring LF and Adjusting Irrigation
The measurement of LF during an irrigation event requires the measurement of two separate components: (1) the amount of leachate and (2) the amount of water applied to the container substrate. The following assumes the same units of measurement for both components:
Using LF in an Irrigation Program
LF testing can be used in several ways to help guide irrigation scheduling. It is important to note that any given LF test is only a “snapshot” of irrigation efficiency and, like most tests, methods used and results obtained are only useful when considered in light of other variables that come into play.
Conditions Affecting the Frequency and Volume of Irrigation
Irrigation scheduling must take into account the entire production system. Factors to consider include the performance of the irrigation system, the weather, water quality, available water holding capacity of substrate (or media), and the crop(s) produced.
Irrigation System
An irrigation system applies water at a given rate (e.g., inches per hour) and at a certain level of uniformity within an irrigation zone based on irrigation design, maintenance, and routine audits. A system with poor uniformity does not apply water evenly over the entire irrigation zone. Growers typically compensate for poor uniformity by increasing the amount of water applied so that plants in drier spots receive a sufficient amount of water.
Weather
Daily weather affects crop water needs through water lost through transpiration and the evaporative losses from the soil surface (together, transpiration and evaporation are termed “evapotranspiration” or ET).
Water Quality
The quality of the water used for irrigation can also affect the volume that should be applied to crops. The total salinity of the water, as measured by its electrical conductivity, and the crop sensitivity to salinity will directly affect the leaching fraction necessary to maintain salt levels within tolerable levels.
Available Water Holding Capacity of Substrate
This is the maximum amount of water that substrate can store to be extracted by the plants. It is the water held between field capacity and permanent wilting point. The total available water in the substrate root zone for a specific crop is equal to the crop’s rooting depth multiplied by the available water-holding capacity per unit depth of the substrate. Total available water is also constrained by the total rooting volume of the container.
Crop Type
Water use and salinity tolerance differ among nursery crops. The volume of water needed is also influenced by the time of year because of crop growth patterns, daily changes in the weather, and ET rates. Furthermore, the size or location of the container, plant spacing, substrate used, plant canopy diameter and height, cultural practices (e.g., pruning), and use of plant growth regulators and fertilizers will affect water capture and use.
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