General Irrigation Tips for Young Pistachios

Tobias Oker, Soils and Irrigation Advisor Kern County

Pistachio acreage in California has been steadily increasing in recent years. A recent report by the Administrative Committee for Pistachios puts the bearing acreage of pistachio in California at about 461,000 acres, and the non-bearing acreage at about 144,000 acres. The average acreage for new yearly plantings for the last five years is about 29,000 acres. Much of the new plantings are in areas in the southern San Joaquin Valley that have moderate to high salinity. Generally, young pistachio trees are those that of non-bearing age.

In California, pistachio is usually planted at row spacings of 20 to 22 feet and tree spacings of 15 to 20 feet. In ideal circumstances, the tree grows to cover most of this space by the time it reaches maturity. The amount of orchard space that the tree canopies shade at any given time is a critical component that goes into the estimation of how much water the trees need as they mature. For mature pistachio trees, the intermittent estimation of the crop water requirement (evapotranspiration) is relatively easier to determine due to the availability of research-developed water use curves (crop coefficients) for pistachio trees. In the southern San Joaquin Valley, a mature pistachio tree requires about 46 inches of water every season; with a peak daily demand of about 0.34 inches per day during hottest summer days. Research has shown that the evapotranspiration of tree crops generally tends to reach its maximum by the time the orchard floor area shaded by the tree canopies is between 50 and 60%. Young pistachio evapotranspiration is estimated by multiplying that of a mature tree by an adjustment factor. For example, the crop water demand of a first-leaf pistachio tree, on a peak evapotranspiration summer day is about: 0.34 in./day multiplied by an adjustment factor of 0.1, which yields a value of 0.034 inches per day. Adjustment factors for drip irrigated pistachio in the southern San Joaquin are as follows: 0.10 for year 1; 0.20 for year 2; 0.30 for year 3; 0.40 for year 4; 0.52 for year 5; 0.65 for year 6; 0.78 for year 7; 0.90 for year 8; 1.00 for year 9 and older. For more information on these adjustment factors, visit the website of the University of California Cooperative Extension Kern County Office. 

Figure 1 Button dripper installed on an irrigation lateral

Like many orchard crops grown in California today, pistachio trees are mainly irrigated using microirrigation devices; which are designed to ensure irrigation efficiencies of at least 90% and above. Microirrigation devices operate at low pressures (3 to 20 psi), apply water at low rates (as low as 0.5 gph to highs of 10 to 20 gph), and more precisely compared other pressurized irrigation methods. Examples of microirrigation devices include: inline drip, micro-sprinklers and micro-sprays (sometimes called fan-jets), button drippers.

In the first years of its growth, a young pistachio tree spends most of its energy on vegetative growth. Its canopy is very small and only covers a small fraction of its assigned tree area; implying that it has a relatively small evaporative surface. To efficiently irrigate a young pistachio tree, it is important to ensure that water is directly applied to the area close to the base of the tree and is concentrated to the rootzone. This approach mitigates water and nutrient loss through deep percolation and/or evaporation, and is generally recommended for widely spaced crops. Effort should be made to avoid applying water to the spaces where there isn’t significant root growth – such as the midway point between successive young trees – and this can be achieved by selecting the appropriate irrigation device. For young pistachios, button drippers are recommended because they can be installed close to the tree. Button drippers usually have flow rates ranging from 0.5 to 2 gph. To install a button dripper, the irrigation lateral is punctured to create a small hole which is then plugged with the emitter. For newly potted trees, the dripper should be positioned to apply water directly to the root ball for about 30 days. After the roots have grown into the surrounding soil, one or two button drippers can be installed at least one foot off either side of the trunk. As the tree grows each year, additional button drippers are added onto the lateral; much further away from the trunk. It is important to take note of the flow rate of the drippers, peak evapotranspiration of the tree and soil water holding capacity vis-à-vis state of root growth, when deciding the number of emitters to install as the tree grows. If a grower prefers, the button drippers could be replaced by micro-sprinklers or micro-sprays when the trees are mature and/or when most of the orchard canopy is closed.

To promote root growth in young established trees, it is recommended that they are not frequently irrigated as an imposition of mild stress conditions spurs roots to grow in search of water and nutrients. However, newly transplanted trees may require more frequent irrigation during the initial stages while their roots grow into their immediate soil surroundings. In irrigation practice, the amount of water applied, when it is applied, is determined by the water storage capacity of the managed rootzone, and in accordance with an allowable water depletion rate. This is called irrigation scheduling. The rootzone water storage capacity is mainly determined by soil texture. It is therefore important to find out what type of soil is found in the orchard when determining an irrigation schedule. This is especially important when microirrigation methods are managed to apply water only to the rootzone of young trees. Light (sandy) soils tend to have lower lateral water redistribution and retention capacities. In such soils, more frequent but low volume irrigation may be recommended. On the other hand, very heavy clay soils – though having higher water retention abilities – may also require frequent irrigation because a significant bit of much of the water stored in them is not available to the trees. Soils with the highest water storage capacity tend to be silt-loams.

To aid irrigation management, it is recommended that growers utilize soil moisture probes. These are widely available in the market. Stem water potential of young pistachio trees can be monitored using the hand-held pump-up pressure chamber since their stems are usually thin for high-tech plug-in sensors. Although there is no research developed baseline values of SWP for pistachio trees – unlike almonds for example –, it is recommended that the irrigation strategy for young trees should be to avoid prolonged conditions of water stress. To this, the SWP should be maintained between -9 to 10 bars often. It is also advisable to monitor soil moisture close to the base of tree and at multiple points of the rootzone. To do this, for an example, a probe could be placed within one foot of the tree or dripper, and soil moisture monitored at depths of one and two feet. If funds permit, and depending on the type to device chosen, a longer soil probe – 4 to 6 feet – could be installed to monitor moisture at greater depths, in the long run as the tree grows to maturity. In order to get an orchard-wide overview of the soil moisture status at any time, it is recommended to install several sensors at different locations within the field. To determine the number of sensors and installation locations within a field, a soil map such as that provided by as the USDA’s Web Soils Survey, or UC Davis’s SoilWeb application is helpful. Alternatively, a lab test of soil samples taken from different locations in the field, could be used to guide soil probe installation. Most sensors available in the market report soil moisture either as volumetric water content, or as a soil tension force. Irrespective of units, from an irrigator’s perspective, the values of interest are the field capacity and permanent wilting point of soil at the site and the reading returned by the probe at any given time. For orchards on loamy soils, moisture depletion up to the midpoint value between field capacity and permanent wilting point (or 50% moisture depletion), before resumption of irrigation is considered to be acceptable.

Lastly, the performance of an irrigation system can also be limited by the general state of the “health” of the soil of an orchard site. For example, soils which are highly compacted and/or sodic significantly limit soil water infiltration; leading to water loss. Therefore, when establishing new orchards in “tough” soils, it may be beneficial to carry out some practices to improve the physical, chemical and biological properties of the soil. This may involve actions like tilling to break soil crust, applying amendments to reduce salinity or sodicity and adding organic matter.

Many factors are part of the equation of economically sustainable pistachio production. Efficient water use is one of those, which is particularly important in a state like California where production is heavily reliant on irrigation.

For more information about pistachio irrigation, please call the UCCE-Kern Soils and Irrigation Advisor or visit the website of the University of California Kern County Cooperative Extension office.

Tobias E. Oker

Soils & Irrigation Advisor, UC Cooperative Extension, Kern County

University of California, Agriculture and Natural Resources

Office phone: (661) 868-6218 | Email: teoker@ucanr.edu

Further reading

Administrative Committee for Pistachios. (2024). 2023 Pistachio Statistics. https://acpistachios.wpenginepowered.com/wp-content/uploads/2024/01/2023-Pistachio-Statistics.pdf

Culumber, M., Zaccaria, D., 2021. 2021 Pistachio Irrigation: Mitigation Strategies for Drought. https://shorturl.at/iKMSZDoll, D. (2017). Pistachio Irrigation: Determining Water Needs and Managing Drought. UC ANR. https://ucanr.edu/sites/PistachioShortCourse/files/274447.pdf

Ferguson, L., & Haviland, D. R. (2016). Pistachio Production Manual (L. Ferguson & D. R. Haviland, Eds.). University of California Agriculture and Natural Resources. https://anrcatalog.ucanr.edu/Details.aspx?itemNo=3545

Goldhamer, D. A. (n.d.). Tree Water Requirements and Regulated Deficit Irrigation. Retrieved March 20, 2024, from https://ucanr.edu/sites/fruitandnut/files/73693.pdf International Nut & Dried Fruit Council. (2024). Production share of pistachios worldwide in 2022/2023, by country. Statista. https://www.statista.com/statistics/933042/global-pistachio-production-by-country/Niederholzer, F. (2022). Potted Tree Irrigation After Planting: Getting the First Year Right. https://www.sacvalleyorchards.com/almonds/irrigation/potted-tree-irrigation-2022/ Sanden, B. (2018). Pistachio ET by age. University of California Cooperative Extension, Kern County. https://cekern.ucanr.edu/Irrigation_Management/Pistachio_ET_by_age/ United States Department of Agriculture; Natural Resources Conservation Service. (2013). Microirrigation. In NEH, Part 623, Irrigation, Chapter 7.