Drought Effects on Walnut and Irrigation Strategies

FloraPulse is currently developing a microtensiometer system for walnuts. It's been a tough nut to crack, but we are making progress and might have 'walnut ready' FloraPulse systems for sale next year. In the meantime, I wanted to share some irrigation guidance on walnut from the excellent UC ANR guide (reference at the end). For now, we recommend using a combination of ET and pressure chamber measurements - soon you might be able to get continuous SWP on your phone!

Walnut trees that consistently produce economical yields with good kernel quality require about 40 to 42 inches (3.3 to 3.5 acre-feet) of water per acre annually

New tree shoots grow in May and June. This growth is important for walnut production because it supports new flower buds for the following year. Water stress and/or leaf defoliation as a result of underirrigation during these summer months has been shown to reduce the number of floral buds and walnut production for the following season.

Irrigation management affects all phases of nut and tree development. Beneficial drought-management strategies must minimize the interruption of plant transpiration and photosynthesis and its effect on tree and nut development.

If orchard water stress is measured using midday stem water potential (SWP) with a pressure chamber (fig. 2), crop stress levels should be maintained in the –4 to –8 bar range throughout the season (Fulton 2014).

Monitoring orchard stress with a pressure chamber and keeping tree stress in the –5 to –9 bar range can save 4 to 8 inches of water while minimizing the impacts to the trees and crop. The most likely effect of this level of irrigation shortfall spread across the summer and fall months would be a higher incidence of darker kernels and slightly lower edible kernel yields.

Monitoring orchard stress with a pressure chamber and keeping tree stress in the –6 to –10 bar range can realize 12 to 14 inches of water savings while minimizing the impacts to tree growth and nut production. Higher incidence of darker kernels and lower edible kernel yields are distinct possibilities with this level of deficit. Bud fruitfulness in next year’s crop may also decline as a result of the water shortage.

At severe reductions in water, impacts on walnut tree growth and crop yield and quality are unavoidable. For example, if the irrigation water availability is 21 inches per acre (1.8 acre-feet per acre, or about 50% of ET), the strategy might be to apply 50% of the monthly ET levels shown in Figure 1. Also, consider monitoring tree water stress and attempt to keep pressure chamber levels between –8 to –12 bars as much as possible.

Guideline Table for using Stem Water Potential in Walnut from:

Fulton, A., J. Grant, R. Buchner, and J. Connell. 2014. Using the pressure chamber for irrigation management in walnut, almond, and prune. Oakland: University of California Division of Agriculture and Natural Resources Publication 8503. UC ANR Communication Services website,