Biological Over Calendar: 4 Key Signals to Time Your First Irrigation Right by Ziv Attia, PhD

Western U.S. growers are about to enter one of the season’s first critical decision points: When to start irrigating?

US Head of Agronomy Ziv Attia, PhD has clear, practical guidance on grounding that decision in plant and soil signals - not the calendar.

Well said Ziv!

"Irrigation should be a biological decision not a social one", says US Head of Agronomy Ziv Attia, PhD, as temperatures begin to rise across California and ET projections trend upward, irrigation season feels like it’s right around the corner. That early-season warmth can create pressure to start watering, especially when neighboring orchards begin irrigating."

But starting irrigation or fertigation too early is often ineffective and can actually limit crop potential. When roots and soil biology are not yet fully active, applied water and nutrients are underutilized, leading to inefficiencies that are expensive to correct later in the season.

At Phytech, we encourage growers to ground early irrigation decisions in plant and soil signals rather than calendar dates or neighboring orchards irrigation habits.

From my experience these four simple data points can help determine true readiness:

1. Soil Temperature

Cool soils suppress root metabolic activity and slow the soil microbiome. Until temperatures rise into an active range, the tree cannot effectively use water or nutrients.

2. Soil Moisture Dynamics

Once roots become active, moisture sensors begin to show a characteristic stair-step uptake pattern, typically within the top 6–18 inches of the profile. Without this pattern, the tree is not yet extracting water meaningfully.

3. Maximum Daily Shrinkage (MDS)

Low MDS indicates low water demand. As atmospheric demand and tree activity increase, MDS rises gradually, signaling that irrigation will soon be needed.

4. Trunk Growth Trends

As assimilation increases, measurable trunk diameter expansion follows. Growth is one of the clearest indicators that the trees are physiologically “on.”

If at least 2 of these 4 indicators are not yet present, irrigation can likely wait.