Adapted from the article “A new season – When to begin irrigating and how much water to apply?” by Richard P. Buchner & Allan E. Fulton, UCCE Farm Advisors, Tehama County in the April 2011 Sacramento Valley Prune News

Crop load has a major effect on the final fruit size and quality of French prune. Once the crop load is established in the spring, good management of water and plant nutrition throughout the season is important to achieve large fruit size and quality. If crop load is too heavy, careful and intensive management of water and plant nutrition may not be enough to achieve large, high quality fruit.

Water management and tree water status affects all of the physiological processes within the tree that influence tree growth, fruit size and fruit quality. UC researchers evaluated the effect of water stress applied at specific times and durations during the growing season on drip irrigated French prune. Multiple irrigation management strategies were evaluated for their effect on tree growth, fruit yield, and fruit size.

• T1 – Unstressed control. Irrigation began in April
• T2 – Very early stress. Irrigation began May 4
• T3 – Early stress. Irrigation off May 5 to June 6
• T4 – Mid-season stress. Irrigation off June 7 to July 18
• T5 – Early through mid-season stress. Irrigation off May 5 to July 18
• T6 – Late season stress. Irrigation off July 16 to September 5

Figure 1. The effect of irrigation management on fruit diameter. The off arrows indicate when irrigation was withheld and the on arrows indicate when water was reapplied. T1 is the no stress comparison and T2,T3,T4,T5,T6 and T7 represent various water stress time and duration strategies (Goldhamer, et.al. 1990).

Figure 1 shows the effect of these irrigation treatments on fruit size. Notice that even mild water stress at all pre-harvest stages of fruit development between May 4 and September 5 (T2-6) decreased fruit diameter at harvest. Very early spring stress (T2) reduced final fruit diameter by 10%. The greatest final fruit size reduction, 25-35% resulted from stress May through mid-July (T5). The negative effect on fruit size increased as the duration of withholding water was lengthened.

This experiment emphasizes the importance of good water management, even early season water management, to produce more valuable large fruit. So, when should the irrigation season begin? The answer depends on highly variable orchard settings. Different soil types and rooting depths store different amounts of winter rainfall that contribute to orchard water needs. Rooting depth may be influenced by pre-plant tillage practices, past irrigation management, and the water table. Furthermore, there is the concern of how much deep stored soil moisture to deplete in the early season versus preserving it to help supply crop water needs later in the summer, especially on soils with slow water infiltration.

Different irrigation systems and water application rates also influence when to begin irrigation. Cover crops or resident vegetation are depleting moisture at different rates depending on management, trees are leafing and canopies are developing, root systems are growing, weather conditions fluctuate from cool to warm which influences canopy development and increases or decreases evapotranspiration (ET), and spring rainfall may or may not increase soil moisture storage in the rootzone.

Increasingly, orchard producers are using various methods of monitoring and science-based information to more precisely decide when to begin irrigating and how much water to apply in specific orchard settings. This is especially true for those with drip and microsprinkler irrigation systems which enable greater control of the water application rate.

Some growers track real-time, weekly estimates of orchard ET and in-season rainfall. This information can be related to the soil water holding capacity of specific orchard soils and to the specific water application rates of their irrigation systems to help estimate how much soil moisture storage has been depleted before irrigation begins and then determine how long to run their irrigation system to replenish a portion of the depleted soil moisture.

One strategy is to start irrigating when trees have used enough soil water to make room in the “soil water bank” to hold irrigation water. The challenge is to avoid water logging on the wet side and tree stress on the dry side. A fairly accurate estimate of soil moisture depletion can be made by adding up daily water use. Start summing daily water use when you know the soil profile is full.

Another way to approach the answer is to consider how much water can be applied per set time and start when at least that much water has been depleted. A convenient source of weekly, real-time estimates of crop ET for orchards can be found on-line. This information is also published weekly in several local newspapers throughout the northern Sacramento Valley.

Weekly estimates of crop ET and soil moisture depletion are based on real-time, regional weather conditions and other reasonable assumptions about orchard health and development. Actual soil moisture depletion in specific orchards is likely to be different. Many growers recognize this limitation and have employed advances in soil moisture monitoring. A variety of sensors are available to monitor soil moisture in the root zone of an orchard. They measure either soil moisture content or soil moisture tension. Soil moisture can to be monitored continuously and relayed to growers on-line. General information about these can be found at the UCCE Tehama County website. Information about specific devices and services can be found on-line. Suggested search words: AgTelemetry.com, Irrigate.Net, Irrometer.

In some situations, placing soil moisture sensors in areas that accurately represent an entire orchard can be challenging because of variability in soils and water infiltration, pre-plant tillage, partially wetted soils by drip and micro sprinklers, and uncertainty about root development and distribution. Recognizing this, other growers directly monitor tree stress with a pressure chamber. Here is more information on the use of the pressure chamber to monitor midday stem water potential. Direct measurement of crop water stress coupled with estimates of crop ET or soil water depletion can be used to monitor orchard water status and how trees integrate complex orchard environments as well as estimate the soil moisture depletion and the need for irrigation.

Water management is a critical part to consistently producing large, high quality French prunes. A variety of useful tools are available to assist with deciding when to begin irrigating, how often to irrigate, and how much water to apply.