Taking the Temp

Growers face unknowns after heat dome

As director of viticulture for McMinnville-based Results Partners, Leigh Bartholomew makes daily decisions about Oregon’s largest vineyard management company. She supervises the health of hundreds of acres of the Willamette Valley’s most valuable grapevines, whatever the weather conditions may be.

But what about the recent historic heatwave? The June weather event left everyone in the industry with more questions than answers.

Under such unprecedented circumstances, judgment calls made by Bartholomew and other Oregon winegrowers were based more on intuition and accumulated knowledge than on any previous course of action.

“None of us really knows for certain how to deal with this because it has never happened before,” she says. “We have to play it as we go.”

At Temperance Hill Vineyard in the Eola-Amity Hills, longtime vineyard manager Dai Crisp sells some of the industry’s most sought-after grapes from 100 acres of predominantly 30-plus-year-old vines to more than two dozen of the area’s top winemakers. The sustained high levels of heat for three consecutive days and nights made his jaw drop.

“It was bad enough that daytime temperatures were well above 110 degrees,” he says. “But it never got below 75 at night. The cumulative heat unit count during that short period put totals on track to be at least a week ahead of our seasonal average.” As a high-elevation vineyard (660 to 860 feet), Temperance Hill’s harvest can start as much as a week later than most other area sites.

With forecasters predicting the “heat dome” a couple of days before it arrived, Bartholomew was able to take some basic precautions, including no leaf pulling and no sulfur applications, which would have negative implications given the extreme heat. Just prior to the warnings, Crisp had sprayed sulfur, the most effective way to prevent powdery mildew. As a consequence, some of his vines sustained leaf burn.

Despite the need for a short horticultural lesson, both Bartholomew and Crisp emphasized the importance of understanding the purposes of stomata (leaf pores) in the gas exchange or “transpiration” in plants. When the leaf pores open, they release water vapor. When closed, they take in carbon dioxide, which uses sunlight to convert (photosynthesize) nutrients from carbon dioxide and water, with the release of oxygen as a byproduct. For the survival of humans and all other animal life, it’s a rather important byproduct.

Absent sufficient water, as with prolonged, extreme heat conditions, the stomata remain closed, and the process shuts down. Individual berries, ranging from pellet to pea-size this early in the growing season, remain hard and undeveloped, so there is no concern about negative effects on flavors. Also, green, unripe berries do not absorb heat as readily as ones that are ripe and fully colored. But the sudden, drastic shutdown of transpiration would reduce berry counts per cluster, in some cases substantially, resulting in lower total tonnage.

If the intense hot spell had persisted for a few more days, Bartholomew postulates that shriveling of leaf canopies would have been more severe and entire clusters might have been lost, rather than just berries.

“Vineyards with drip-irrigation systems are able to reduce that risk,” she said. “We are already feeling the effects of climate change, so more people are making the decision to put in drip irrigation when they plant new vineyards or additional vines. Quite a few have even installed drip lines in existing vineyards. It’s good insurance.”

However, Crisp points to the high cost of installing drip irrigation, currently at $1.50 to $2 per vine, deterring many growers. There is also maintenance and repair once the irrigation lines are placed. “The emitters and hose couplings need to be tested periodically,” he says. “Some always have to be replaced. Then there are leaks in the lengths of hose line between vines.”

Alternatively, Crisp concludes that under stressful conditions, hand-watering each vine would actually be more cost-efficient, just as he has always done for new vines in the year of planting to ensure their root systems begin to develop. He practices dry farming, which forces the vines to work harder with roots reaching deeper, seeking out adequate water while absorbing more diverse and complex below-ground nutrients in the process.

The only issue with that approach, he admits, is being able to quickly round up a large enough labor crew to do the hand-watering work when a high-stress situation arises. If prolonged high heat hits in August and a small drink of water can make a big difference, it needs to happen as soon as possible. But, then again, experienced workers may already be occupied with an earlier crop harvest such as blueberries or are off fighting fires like so many in 2020.