Overly aggressive, early walnut irrigation can saturate soils and deprive roots of necessary oxygen to grow. Soil fungal pathogens such as Phytophthora thrive in saturated soils and infect tree roots. The result is declining tree health, productivity, and higher incidence of tree death.
Delaying the start of irrigation too long can result in trees that are stressed from the lack of water. Impacts may include smaller walnuts and modest declines in yield as a result of lower kernel weight. When delays in spring irrigation are followed by inadequate irrigation in the summer and fall, it may lead to higher crop water stress and more impact on kernel weight, color, and bud development for next year’s crop.
Experimenting with Irrigation Start Date
Table 1 summarizes the effect of timing the start of irrigation on Chandler/Paradox walnut yield and nut weight responses in an ongoing irrigation experiment in Tehama County. A randomized and replicated experiment has been underway since 2014. The soils are Columbia silt loam and fine sandy loam. Preliminary results are given for 2015 after two consecutive seasons of evaluation.
More seasons are needed to evaluate longer term effects on tree loss, orchard longevity, and consistency of production.
In this walnut irrigation experiment, a large window of time (more than 90 days after leafout) is being evaluated to observe the effect of irrigation start on Chandler walnut. Lower nut weight was the first statistically significant (large, highly repeatable) response observed as a result of delaying the start of irrigation.
In 2014 (data not shown), nut weight declined significantly by 9 to 12 percent when walnut irrigation was delayed past mid-June or 75 days after leafout. In 2015, nut weight declined even more significantly when the start of irrigation was delayed beyond mid-June or 60 to 75 days after leafout.
In 2014, there was no statistically significant decline in dry in-shell walnut yield (data not shown) across the broad window of irrigation start dates. In 2015, relative yields were only 4 percent lower when the first irrigation was delayed 45 to 60 days after leafout as compared to when the start of irrigation was delayed 25 to 30 days.
Dry in-shell yields were not consistently low enough to be statistically different when the start of walnut irrigation was delayed 60 to 75 days after leafout but the yield reductions averaged 13 percent lower and were economically important. When the irrigation start date was delayed more than 75 days after leafout, dry in-shell yield was consistently lower (averaging 16 to 19 percent less) and statistically and economically significant.
Despite worries by many that delaying the start of irrigation in the spring would result in depleted moisture leading to more severe stress later in the summer when cutoff for harvest occurs, this was not the case. The treatments that started irrigation in early to late June were actually less stressed following cutoff of irrigation for harvest than were the treatments that had initiated irrigation in late April.
This suggests that the trees with delayed spring walnut irrigation may have improved root growth and/or the spring vegetative growth slowed earlier in the season allowing more time to mature and, thus, stressed less at harvest. Delaying the start of irrigation until mid to late May resulted in a combination of minimal effects on yield and opportunity to save water.
More in-depth results from this experiment can be found in the report to the California Walnut Board, (http://ucanr.edu/repositoryfiles/2015-115-160270.pdf).
Review of this irrigation experiment is not intended to suggest that the start of the irrigation season can be delayed for long periods of time for every walnut orchard. It does indicate that there may be a 30 to 60 day window after leafout to begin irrigation. When optimized, it can protect tree root health from overirrigation, avoid too much crop stress from the lack of water, and reduce irrigation water needs and costs.
A variety of walnut irrigation management tools can help make orchard specific decisions about when to begin the irrigation season. Using at least two of these monitoring tools in combination is encouraged because each tool has limitations.
Starting Walnut Irrigation Based on ETc and Rainfall
One approach is to track estimates of evapotranspiration (ET) that reflect current weather and compare it to the amount of spring rainfall received since leafout. The first irrigation is not necessary until the cumulative ET exceeds the amount of spring rainfall received since leafout by at least the amount of water that will be applied in a typical irrigation event (usually 18 to 24 hours of irrigation). If there is concern about tree loss from root diseases and saturated soils, the first irrigation can be delayed even longer until the difference between cumulative ET and spring rainfall is equivalent to 2 to 4 irrigation events or 36 to 96 hours of irrigation.
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To acquire real-time ET reports during the walnut irrigation season email email@example.com for Tehama, Butte, and Glenn, firstname.lastname@example.org for Sutter and Colusa Counties, or email@example.com for Sacramento, Solano and Yolo Counties. This approach requires knowing the hourly water application rate of the irrigation system.
Starting Walnut Irrigation Based on Soil Moisture
There are numerous manufacturers and providers of soil moisture sensing equipment. Some detect volumetric soil moisture content and some measure soil moisture tension. Soil moisture levels can be measured manually or automatically with dataloggers and delivered on demand via cellular and internet services. An important aspect of monitoring soil moisture depletion is placement of the soil sensors to achieve good representation of the root zone and soil variability. The decision to begin the irrigation season can be determined by comparing the amount of soil moisture depletion to the amount of irrigation that will be applied and balancing them. Irrigation should begin before 50 percent of the plant available soil moisture is depleted in the root zone.
Starting Walnut Irrigation Based upon Orchard Water Status
The pressure chamber and midday stem water has been the state of the art for monitoring tree water stress for some time. Sustained levels approaching 2 bars below the fully irrigated baseline are a reasonable threshold to begin irrigation. A free on-line UC ANR Publication 8503 describes in detail how to use the pressure chamber to guide water management decisions in walnut.