Taking the guesswork out of irrigating wine grapes

Until now, wine grape growers have essentially irrigated their crops based on the color of the wine they were making, watering one way for white wine grapes, and another for red grapes. Although manageable, the method has been susceptible to trial and error. But WSU scientists have debunked this conventional model with breakthrough new science, allowing growers to conserve resources and customize irrigation practices for specific grape varieties.

Joelle Martinez, a WSU doctoral candidate, has been working with professor Markus Keller on this project for the past four years, trying to find a new solution to outdated irrigation practices.

micro-irrigation system in grapevine vineyard
New research will allow growers to tailor watering strategies to specific grape varieties.

“Most of the existing research was done on various varieties, in different parts of the world, on different root stocks and in different soils,” Martinez said.  “There was no standardized system to quantify and identify varietal behavior.”

Irrigating wine grapes is something of a counter-intuitive process. Instead of watering fully and frequently, growers deprive their vines of water through a practice called “deficit irrigation” to develop the desired composition of the grape’s skin and juice content.

The level of water-stress reached during deficit irrigation is often monitored based on “leaf water potential”—a measurement of the vine’s water status while its soil is drying out. Measuring a vine’s leaf water potential allows growers to assess the overall health of their grapes relative to the stress they are enduring through water deprivation.

Conventional wisdom divides grapes into two polar categories: the first includes grapevines whose leaf water potential remains constant even as its soil dries out; and the second, includes grapevines with leaf water potential that drops as soil moisture is depleted.

This binary classification system has for many years been the standard.

“Most growers started to think you could manage a vineyard in these two separate ways successfully,” Martinez said, which presents another problem:   conventional irrigation methods are based on research that only accounts for four varieties. “It’s easy to classify grapevines into two categories when you are only looking at four varieties.”

But Martinez and Keller, who have studied 18 varieties, suspected that a diversity of grape varieties necessitated a more customized and nuanced approach.

“When you begin to look at that many varieties you will see a completely different picture,” she said.

The picture that has emerged from Martinez’s research is that a two-category system is insufficient. “The 18 varieties we studied fall into a continuum where varieties are positioned relative to one another. You can’t separate them into two categories.” There are extremes on either end, she added, but the continuum suggests that individual varieties require a customized approach to irrigation.

Her research will help growers tailor their watering strategies to conserve resources and improve the quality of their grapes.

Right now, Martinez’s research is based on grapes grown in Washington state, but  Markus Keller’s three-year funding award from the Specialty Crop Block Grant Program will include a collaborative project in Bordeaux, France.