Keller pointed out that since Washington growers use deficit irrigation - controlled amounts of water - to manage vine vigor, there were also no differences in canopy size.

"Water deficit overrides any vigor-promoting influence a rootstock might exert in wetter climates,” he said. In other words, growers will be able to continue using the vineyard management techniques they’ve already mastered, even if they grow grafted vines.

"It’s the climate, not the rootstock,” Keller said, referring to Washington’s excellent reputation for producing high-quality fruit. "The differences we did see over the course of this experiment had to do with vintage.”

Both scientists said that their multi-year experiment confirms the fact that scion, vineyard location and vintage are the driving factors of grape and wine quality. This is something growers and winemakers already know, they added: "We just need to be reminded once in a while.”

Three varieties tested

The advice to "have no fear” of grafting comes from data collected over three growing seasons with three wine-grape varieties – merlot, syrah and chardonnay – evaluated on six common, commercial rootstocks as well as on their own roots. One rootstock failed the trial because it over-wintered poorly and was deemed unsuitable for use in Washington’s growing environment.

Each year, wine was made from each grape/root combination and tested for multiple indicators of quality, including those critical to red wine.

Results of this complex, long-term experiment are published in a pair of papers, one focused on plant vigor, yield formation and fruit ripening and the other on grape and wine composition.

The viticultural results show that variations are due to "scion cultivar, spatial differences across the vineyard site and climate variation among years.” In other words, the dominating factors affecting grape growth are vintage and vineyard site and soil variability - not rootstock.

Likewise, the enological results "showed that rootstock caused few significant differences in fruit and wine composition and, instead, the dominant variables were scion and, to a lesser extent, vintage.”

Own-rooted vines recover better

So, right now, there is no good reason for Washington growers to switch to grafted rootstocks. Indeed, for economic reasons, it is better to stay with own-rooted plants wherever possible.

"When we have a freak cold snap, and eastern Washington sees these periodically, vines can die to the ground,” Keller said. "Grafted vines would need to be regrafted because the graft point sits above the ground, while own-rooted vines will likely recover on their own.”

Regrafting is expensive and, indeed, grafted vines create higher initial startup costs because they are more expensive for nurseries to produce.

But, Keller cautioned, eventually there likely will be cause for Washington growers to move toward grafted vines.

"Nematodes build up in the soil over time,” he said, "so increasing numbers of second or third generation vineyard plantings will likely need to be on grafted vines.”

"This type of experiment has never been done before on this scale and with this many variables,” Harbertson said. "It was huge and complex. The methods and results truly reflect Washington’s unique conditions.”