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Dry Creek Road Vineyard

A vineyard along Dry Creek Road in north Napa on Wednesday morning

As an extended heat wave roils the state, grape growers in Napa are taking steps to keep vineyard workers cool on the hottest days. Meanwhile, the abrupt surge in temperatures has marked a significant shift in the growth cycle of the vines and their fruit.

Working outside for up to 10 hours a day, vineyard workers have been directly exposed to the week’s high temperatures, which in Napa were expected to hit 98 degrees on Wednesday, before peaking at 101 degrees on Thursday. Upvalley locations will be hotter.

The Napa Valley Grapegrowers group on Wednesday said its members were adhering to the state’s standards for Heat Illness Prevention, which are set by regulatory agencies like Cal/OSHA and AgSafe.

Those standards require employers to provide workers with shade when temperatures rise to 80 degrees or higher and to provide 10-minute rest periods every two hours at temperatures of 95 degrees or more.

Procedures also include effective monitoring of all workers through methods such as a mandatory buddy system or regular communication with workers who work alone.

Employers must further train workers on the signs and symptoms of heat illness, develop emergency response procedures and train workers on how to execute those procedures when necessary.

The recent surge of extreme heat is the latest episode in a year marked by spates of severe weather in Napa, including bouts of excess rain in the early months of the year and a damaging one-off hail storm that struck some vineyards last week.

“But based on later bud break and the unprecedented amount of rainfall, the vines needed more heat,” said Caleb Mosley, viticulturist with Mike Wolf Vineyard Services. Mosley said he sees the heat “as a positive for the vines,” which needed stress from the heat to slow down their vegetative cycles.

The abrupt change in temperature marks a turning point in the growing season where growth will now be concentrated more in the berries than in the vines themselves. With the string of hotter days, vines are experiencing water stress, which causes them to shift growth from leaves to their fruit.

For growers, having the appropriate amount of canopy size and structure for shade is essential at this point to protect the fruit from heat stress.

The current heat wave, Mosley noted, has been more humid than growers are used to, which has helped to tamper stress from the heat, at least on the vines.

“... The harder it is on humans tends to be the inverse on vines,” Mosley said. “People are having a tough time with humidity. The vines are doing a lot better than we are.”

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