Fine-tune irrigation volume, frequency to save water - Part 1 of 2

No matter where your nursery is located, water management and availability are quickly becoming topics of major concern. Even in what are considered water-rich states, drought, urban competition, runoff water quality and legislation are increasing the need for nurseries to manage water more effectively.

Laws now limit water use in areas of California, Florida, North Carolina, Oregon and Texas, and laws in Maryland, Delaware and California limit nutrient concentrations in runoff. In Michigan, heavy water users must annually report how much water they apply, current conservation practices and conservation implementation plans.

Future legislation is expected to be more stringent, and container nursery access to and use of water is expected to significantly decrease in the next decade. Fortunately there are several research programs with strong grower cooperation and involvement helping to address this issue.

How much water is too much?

One of the most difficult questions to answer for container nursery operators is how to schedule irrigation for proper plant growth and quality. The biggest challenge for both growers and researchers working with irrigation is the huge diversity of types and sizes of plants being grown by nurseries. Current scheduling methods range from irrigating based on a set volume or period of time to using sophisticated sensors to determine plant water use. The most common method is somewhere in between, relying on a grower’s experience and a simple method of estimating plant water status, such as feeling the substrate moisture or feeling the weight of several pots.

Container substrates are designed to be well-drained, and container volume limits the amount of water that can be stored. This results in frequent irrigation applications, often daily, and large amounts of water used. In Florida, container nurseries annually apply 56-120 inches per year in addition to the 40-50 inches of average annual rainfall. Container nurseries in Alabama were estimated to have used 30,000-40,000 acre-feet of water in 1985.

Large irrigation amounts usually lead to large runoff volume, which can carry agricultural chemicals offsite. These practices have worked in the past because in most areas water has been readily available and inexpensive for nurseries to extract. However, emerging constraints on water use and quality mean growers need to find ways to manage water without detracting from production schedules and crop quality.

Recent research has focused on irrigating container plants according to plant water use. This involves applying only the amount of irrigation used by the plant and lost from the container through evaporation (evapotranspiration) since the previous irrigation. Plant demand can be measured using moisture sensors or by estimating daily evapotranspiration. Frequency of application also has to be determined for proper scheduling of irrigation.

Here are highlights of some recent research on water-conserving irrigation practices.

How much should you irrigate?

Scheduling irrigation based on reference evapotranspiration (ET0) and crop coefficients (KC) is the classic method for many field-grown crops.

ET0 is typically determined from meteorological data and is readily available in most states. KC is a value specific to a particular crop that relates ET0 to crop evapotranspiration (ETC), which can be used to schedule irrigation. KC need to be developed experimentally and have been developed for several container plants.

A good relation between these estimates and measured plant water use has been found in some studies. However, other studies have found that KC varied for the same species by location and over the growing season. Estimating KC from easily measured variables, such as plant growth, has been done and would allow growers and researchers to more easily determine ETC for irrigation scheduling. However, since KC is not only specific to a species but can also be affected by spacing, irrigation method, season, geographic location and ratio of plant size to container volume, it has remained an elusive technique for scheduling container irrigation.

Researchers are still trying to develop models that will be easy for growers to use and be applicable for the diversity of conditions encountered at container nurseries. Tom Yeager at University of Florida is working on such a model. His model will ask growers questions about production practices to help account for the many variables that have previously made this approach difficult for container operations.

Over the past two years we have been looking at scheduling irrigation according to daily water use (DWU) for 10 different species per year grown in 3-gallon containers. DWU was obtained by using a substrate moisture probe to measure the difference in moisture content of the container one hour after irrigation (container capacity) and prior to irrigation 24 hours later and taking the difference. A moisture probe is a handheld device that gives readings of soil or substrate moisture within a few seconds.

Average irrigation applied according to DWU was 27-75 percent less, depending on species, than water applied to the control (0.75 inches per day). Runoff volume was reduced by 66 percent to 79 percent, nitrates by 38 percent to 59 percent, and phosphates by 44 percent to 74 percent in runoff water when irrigating according to DWU compared to the control.

Growth index at the end of the growing season was higher for 11 species, not different for nine species and lower for none of the 20 species irrigated at DWU versus the control.

Results from the study indicate that irrigating according to DWU can conserve substantial amounts of water without negatively affecting plant growth compared to an irrigation rate of 0.75 inches per day. Additionally, we now have data on water-use requirements for these 20 species that can be used to help growers group them into high-, medium- or low-water-use categories.

DWU, like KC, will be different for different species, container sizes and other variables, but it can be easily and quickly determined with relatively inexpensive probes. DWU can even be determined by measuring the weight of the container at capacity and 24 hours later.

John Lea-Cox at University of Maryland is investigating a wireless system of sensors to provide data for growers to either automatically or manually schedule irrigation.

How often should you irrigate?

Better growth occurs for many species the closer a container substrate is kept to container capacity, but how far below container capacity can you go before growth is affected? This amount is called the management-allowed deficit (MAD).

Richard Beeson at University of Florida looked at sweet viburnum, Japanese ligustrum and Indian hawthorn and four MAD levels (20, 40, 60 and 80 percent below container capacity) compared to a control irrigated at 0.70 inch per day. MAD was determined by weighing container plants at container capacity and weighing again when plants wilted to give the plant available water. MAD levels were percentages of how much plant-available water was lost before irrigating (the higher the percent, the closer a plant is to wilting).

MAD levels of 20 percent  for sweet viburnum, 30 percent for Japanese ligustrum and 40 percent for Indian hawthorn showed no difference in plant growth or quality compared to controls. Plants at higher MAD take longer to reach the same size because of slower growth rates. Attempting to conserve water by extending the period between irrigation (using higher MAD) has been suggested by some, but Beeson’s results indicate it is more likely to lengthen the production cycle, requiring a longer irrigation period that may add up to more water applied overall.

The difference in plant performance at different MAD levels is thought to be related to the ability of the plant to tolerate drought, so MAD levels will be plant-specific but not affected by other variables.

Once again, the diversity of plants grown at a nursery would make it difficult to use exact MAD levels for each plant. However, either using a conservative MAD -- for example 20 percent -- for all plants or grouping plants by low, medium and high MAD would be manageable, optimize growth and conserve water.

In July NMPRO: How to put this information, and other techniques to improve water management, into practice.

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- Tom Fernandez and Aaron Warsaw

Tom Fernandez is associate professor, and Aaron Warsaw is graduate research assistant, Department of Horticulture, Michigan State University.

June 2008