(Decanter) - A massive replanting that will uproot 15% of vineyards in Napa Valley over the next five to six years has begun, according to Napa Valley Grapegrowers.

Jennifer Putnam, the group’s executive director, said nurseries were completely sold out of vines until 2015, and that ‘we know these are not for new acres.’

She added that it has been 20 years since a widely used rootstock from UC Davis proved not resistant to phylloxera, forcing many growers to replant.

‘This is an opportunity for Napa to re-evaluate, to reconsider what is planted where,’ Putnam said. ‘The landscape may look different in 15 years.’

An abundant 2012 crop (compared to 2011, total value was up 55% to US$656.2m while production also increased by 50% to 182,859 tons) has, according to Putnam, given growers the optimism and cash flow they need to begin such an effort.

Michael Monette of Sunridge Nurseries, one of the region’s largest suppliers of plant material, said, ‘In 14 years, I have never seen such strong demand.’

That demand is complicated by the discovery of a virus called red blotch disease that causes lower sugar levels in red grape varieties. Growers waiting for clean plant material have created a bottleneck that has some orders on hold till 2015. ‘Sales for 2012-2013 are strong but delayed,’ Monette said.