Advanced SWP Interpretation in Almond

Allan Fulton, UCCE Water Resources Advisor, Tehama, Shasta, Glenn and Colusa Counties; and Luke Milliron, UCCE Orchards Advisor, Butte, Tehama and Glenn Counties.

Advanced guidelines for almond are provided in Table 1 (below). These guidelines consider the actual orchard measurements and normalized measurements after referencing baseline SWP (i.e. bars +/- Table 2 baseline, end of article). Field research suggests the ideal range in plant water status for almond is between -2 and -8 bars below baseline. This corresponds to orchard measurements from about -6 to -18 bars.

SWP readings of -2 to -4 bars below baseline before irrigation from March through mid-June will promote almond shoot growth and nut sizing. At the onset of hull split beginning in late June or early July, SWP levels of -4 to -8 bars below baseline will promote hull split and uniform nut maturity leading to timely harvest. Recovering midday SWP levels back to -2 to -4 bars below baseline after harvest promotes bud development of next year’s crop. When almond orchards are exposed to long periods where SWP levels exceed -8 to -10 bars below baseline, young orchards will grow slowly and take longer to come into full production. Older orchards are likely to display defoliation, smaller nut size, and increased incidence of hull tights.

University experiments and production experience with intensive irrigation management that sustains orchards near baseline (minimal tree stress) throughout the season suggests this management strategy can eventually increase tree loss and shorten the economic life of an orchard. Concerns include higher incidence of root and crown rots, higher incidence of hull rot and Alternaria, lower limb dieback, and shortened orchard life. Orchards where SWP fluctuates more within these optimum ranges may both yield competitively and incur less tree loss.

Left photo: Less defoliation after harvest and larger canopy size of a Nonpareil tree when irrigation was managed to keep SWP levels to a maximum water stress of approximately -20 bars during the season. Middle photo: More defoliation after harvest, but similar canopy size for a Nonpareil tree when irrigation was managed to keep SWP to a maximum water stress of about -35 bars during the growing season. Right photo: Complete defoliation after harvest, and reduced shoot growth of a Nonpareil tree where irrigation was withheld all season and SWP reached a maximum water stress of approximately -60 bars. Photos: Allan Fulton
Figure 1. Left photo: Less defoliation after harvest and larger canopy size of a Nonpareil tree when irrigation was managed to keep SWP levels to a maximum water stress of approximately -20 bars during the season. Middle photo: More defoliation after harvest, but similar canopy size for a Nonpareil tree when irrigation was managed to keep SWP to a maximum water stress of about -35 bars during the growing season. Right photo: Complete defoliation after harvest, and reduced shoot growth of a Nonpareil tree where irrigation was withheld all season and SWP reached a maximum water stress of approximately -60 bars. Photos: Allan Fulton

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