Rivulis Warns: “High Nighttime VPD” Could Be the Hidden Yield Thief in Almond Orchards

Rivulis is urging almond growers to pay closer attention to a little-known but increasingly damaging factor in orchard performance: high nighttime vapor pressure deficit (VPD). While most growers are familiar with managing daytime heat stress, new data shows that high nighttime VPD can silently erode tree health, reduce nut fill, and limit yields long before visual symptoms appear.

VPD represents the drying power of the air, calculated from temperature and relative humidity. High daytime VPD is a familiar stressor, prompting stomatal closure and reduced photosynthesis. At night, however, almond trees rely on lower VPD levels to rehydrate tissues and restore the water balance lost during the day.

High nighttime VPD (often above 0.6 kPa) disrupts the almond tree’s ability to fully rehydrate after daytime transpiration losses, leaving trees stressed before the next day’s heat even begins. Over time, this incomplete overnight recovery can weaken flower bud development, reduce pollination potential, and cause measurable yield loss.

“Growers are masters at managing water during the day — but if the trees start tomorrow already in deficit, you’re fighting uphill all season,” said Maoz Aviv, at agronomist at Rivulis. “We have seen orchards where correcting nighttime VPD stress preserved nut size and quality that would have otherwise been lost.”

Targeted Irrigation Timing: The New Frontier

Rivulis advises that one of the most effective countermeasures is strategic late-afternoon or early-evening irrigation to restore water balance before nightfall. Combined with precision drip or micro-sprinkler systems, this approach allows tissues to fully rehydrate overnight, safeguarding kernel fill during critical growth stages.

Advanced irrigation scheduling, guided by soil moisture probes and canopy sensors, can help growers identify when VPD risk is highest, often coinciding with warm, dry nights during bloom, nut set, and kernel fill. Rivulis’ fertigation systems also enable simultaneous nutrient delivery, aligning with the tree’s peak demand periods.

Why It Matters Now

Climate data shows that nighttime VPD levels in key almond-growing regions have been trending higher in recent seasons, driven by warmer nights and lower relative humidity. This is amplifying the stress burden on orchards and making irrigation timing as important as irrigation volume.

“This is the kind of hidden stress that doesn’t show up until it’s too late to correct,” added Aviv. “By integrating VPD monitoring into daily decision-making, almond growers can protect yield potential and improve long-term orchard resilience.”