Citrus, Measured: What 697 California Blocks Are Telling Us Right Now

Research by Phytech’s Agronomic Research Department, led by Ido Gardi, PhD, Head of Agronomic Research.

Across California citrus, the difference between productive water stress and simply leaving trees thirsty can be surprisingly narrow.

For this analysis, we looked at 697 citrus blocks across 41 growers, combining direct trunk and fruit measurements with metered irrigation data from August 15 through September 26.

The result is a real-time view of how the trees themselves are responding — from regulated deficit irrigation and fruit sizing to autumn flush, water use, and pumping efficiency.

This Month at a Glance

• 697 measured citrus blocks

• 41 growers

• 1.20 in ETo over the last 7 days

• 0.69 water ÷ ETo on unstressed blocks

• 48% of blocks on RDI target139 blocks showing stress in the last two weeks

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Is Your Grove on RDI — or Just Thirsty?

Fewer than half of the monitored groves are currently hitting the RDI target window.

Regulated deficit irrigation works when trunk growth slows or stops while fruit growth continues. The goal is controlled stress — not simply giving the tree less water.

Across the monitored blocks, mandarins were running wettest, with roughly 4 in 10 still showing trunk growth, while Navels were the thirstiest.

Measured on trunk + fruit gauges · Aug 31–Sep 26 · lemons excluded because they are managed primarily for acid rather than sugar.

What to look for

If both trunk and fruit growth are flat, consider adding approximately 10% back.

If trunk growth is still active, there may be room to trim irrigation by approximately 15%.

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The Fruit-Size Race

Grapefruit and lemons are currently ahead. Valencias are falling behind.

Comparing the same blocks during the same week one year apart, Valencia and blood orange fruit is 16% smaller, and most of those blocks are currently behind their target growth curve. That matters because deficit irrigation only works if fruit development is protected.

Measured on fruit gauges · same blocks, same week, this year vs. last year · crops with fewer than 10 blocks not shown.

What to look for

If fruit size is already behind target, protect fruit growth before pushing RDI further.

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Autumn Flush Watch

One in six monitored blocks in Tulare and Kings Counties showed a jump in trunk growth during the last two weeks.

At this point in the season, trunks should generally remain quiet until November.

A late vegetative flush can redirect both water and nitrogen away from the fruit, potentially reducing the benefit of irrigation and fertilization intended for crop development.

Measured on trunk gauges · Sep 13–Sep 26 compared with the previous two weeks · regions with 15+ monitored blocks.

What to look for

If the trunk-growth line starts bending upward, the soil may be staying wet for too long. Consider stretching the interval between irrigation events.

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Water and Pumping Hours Left on the Table

The data also points to a significant operational opportunity.

Approximately one in three monitored groves appears to be receiving more water than the trees currently need.

In these blocks, trunks continued growing throughout the month — an indication that water availability may be exceeding the crop’s current demand.

At the same time, one pumping hour in six occurred during peak power-rate periods.

Across the relevant blocks, the opportunity represents approximately:

0.6 in/ac of potential water savings per block this month

1,940 hours of potential pumping-time savings across 140 blocks

Measured from irrigation events · 695 blocks · Aug 31–Sep 26.

What to look for

Where trunk growth indicates excess water, consider trimming irrigation by approximately 15%. Where operationally possible, move peak-hour irrigation sets to early morning to reduce energy costs.

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Is 0.65 Really Your Grove’s Kc?

Crop coefficients are essential planning tools — but the tree can tell us whether the assumed coefficient is actually matching conditions in the field.

Across the monitored citrus blocks that remained unstressed, actual water received landed between approximately 0.66 and 0.73× ETo, depending on crop, with grapefruit at the upper end.

Interestingly, roughly three in ten unstressed blocks maintained healthy tree response while receiving less water than the planning coefficient would suggest.

Valencias stood out for another reason: stressed Valencia blocks received less water than their unstressed counterparts.

Measured as metered irrigation water ÷ reference ET · Aug 15–Sep 26 · crops with 10+ unstressed blocks. Applied water includes system losses; blocks with meter gaps were excluded.

What to look for

If trees remain healthy while receiving less than 0.6× ETo, that may represent a genuine water-saving opportunity. If a block requires more than 1.0× ETo to avoid stress, check irrigation uniformity before simply adding more water.

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Let the Crop Have the Final Word

RDI, Kc, weather data and irrigation schedules all provide valuable guidance.

But ultimately, the most important question is whether the tree is responding as intended.

Direct measurements of trunk growth, fruit growth and applied water make it possible to distinguish between productive stress, unnecessary irrigation, and stress that is already beginning to compromise the crop.

That is the difference between managing from an assumption and managing from what is actually happening in the grove.