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Heat Wave Warms Northwest Wine Grapes

Vine growth accelerates by one to two weeks as warm temperatures accumulate

by Peter Mitham
west coast vineyard heat wave
Temperatures in the Walla Walla Valley soared as high as 106°F this week, sending the number of growing degree days soaring.
Ashland, Ore.—While this vintage’s weather is unfolding largely as anticipated by Southern Oregon University weather watcher Greg Jones, who offered a forecast during remarks to the Oregon Wine Industry Symposium earlier this year, the high pressure system that’s been scorching the west for the past few days has accelerated vine growth by one to three weeks.

The accumulation of growing degree days (GDD) in Washington state is trending above the long-term average, with Washington State University reporting 913 GDD through June 30 versus the average of 833.

Similarly, in Oregon, data collated by Jones indicates an accumulation of 1,044 GDD at stations in both Roseburg and Milton-Freewater through June 30. McMinnville is on par with previous years at 744 GDD, but still slightly above the long-term average of 620.

“Summer’s here,” Jones remarked.

The tally of growing degree days has been boosted by bursts of good weather in April and May, and the heat wave of the past few days has felled temperature records in many locales.

Breaking records
July 1 saw daytime temperatures reach 106°F in both Yakima and Walla Walla, busting the previous daily records in each region (97° and 99°, respectively).

Temperatures also soared into the triple-digits in viticultural areas of Oregon, Idaho and California.

A tropical depression that slipped north from Baja California brought above-average humidity that held temperatures in check in the Willamette Valley and Southern Oregon,

“I think we could have been 2°, 3°, 4° warmer if that humidity hadn’t crept in,” Jones told Wines & Vines.

Still, conditions remained sticky, and night-time declines offered little relief.

“The uncomfortable warm and muggy conditions set records Monday morning across southwest Washington and Northwest Oregon,” the National Weather Service said in a record event report issued Tuesday. “Temperatures in most locations only fell into the 60°s...with urban temperatures barely getting below 70° before the morning sun started to heat things up again.”

Temperatures dropped to a low of 63° in McMinnville, higher than the previous highest recorded low of 61° set in 1942.

Ahead of schedule
The good weather has put many vineyards 10 days to three weeks ahead of schedule, although Jones expects the recent humidity could boost disease pressures in Oregon.

The biggest concern for Jessica Cortell, a viticulture instructor at Chemeketa Community College in Salem, Ore., and a consultant to several Willamette Valley vineyards, is canopy management. Rain preceded this week’s heat, fuelling vine growth.

“The fruit set looks pretty good. It looks like we have medium to larger clusters, depending on the vineyard,” she said. “(But) after the rain and this warm weather, everything’s growing like crazy, so we’re just trying to stay ahead of the canopy.”

So long as temperatures stay below 93°F, she expects the fruit to come through in fine shape and harvest to be early.

Similarly, Erik McLaughlin, vice president of sales and marketing at Seven Hills Winery in Walla Walla, Wash., said tours of the winery’s vineyards in the Horse Heaven Hills, Red Mountain and Walla Walla AVAs have been encouraging during the past week.

Too much of a good thing?
While triple-digit temperatures deliver more heat than the vines can use, most have been responding well to the wave of warm weather. Rain fall during the third week of June delivered moisture, now the heat is encouraging growth.

“We are seeing some significant acceleration from the heat,” he said. “We think we are on average about a week ahead of an average season.”

While there has been some evidence of sun strike or sunburn at some locations, it’s early enough in the season that affected berries will drop and sufficient good quality fruit should remain.

The latest weather report circulated by Jones indicates that warm, dry weather will be the order of the day as the season continues.

“The latest guidance from the Climate Prediction Center suggests a continued trend toward drier and warmer across most of the West Coast and the Southwest,” he wrote. “The 30- to 90-day forecasts through late-summer...are showing the western U.S. to have a continued greater likelihood of being much warmer than normal with drier than normal conditions, especially over the (Pacific Northwest).”

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