Stress management is an important component of regulated deficit irrigation, a practice becoming standard in Washington State red wine grape vineyards. Too much stress during critical times can severely impact wine grape production and quality.

Timing, duration, and intensity of the heat or stress will determine the impact it has on the vine, said Dr. Russell Smithyman, Ste. Michelle Wine Estates. He outlined critical berry development stages and the impact of stress:

• Bloom (fruit set):
Too much stress will cause shatter and can negatively impact fruit set in the following year.

• Fruit set to early July: Ideal time to stress the vine to control canopy growth. It’s hard to have too much stress at this time unless the vine doesn’t have adequate canopy.

• Veraison:
Too much stress may result in sour berry, shrivel, or arrested fruit development.

• Preharvest: Too much stress can result in dehydration, which may be desirable in some cases. A small amount of water is needed at this timing to help develop flavors.

Regulated deficit irrigation does not mean "no water," Smithyman said. Usually about 70 percent of full ET (evapotranspiration) is applied from the onset of irrigation (mid-July) to harvest, he added.

"During excessive climate conditions, when it’s hot, you need to think about modifying your irrigation management practices or use the high temperatures to achieve a smaller canopy with smaller berries," he said.

Ste. Michelle has used stress during bloom to purposely shatter the developing fruit to reduce the crop, but he doesn’t recommend it for inexperienced growers. In some years, they have had to irrigate some blocks during bloom to alleviate shatter.

"But you don’t want to add too much water during bloom, because soon you’ll be trying to slow shoot growth down," Smithyman said. Slightly filling the soil profile during bloom can alleviate shatter, according to Ste. Michelle research data.

The beginning of ovule expansion, during fruit set, is an indicator that stress to the vine can begin.