Although applying a timely postharvest irrigation to fill up the soil profile before winter can go a long ways in helping ensure there is adequate moisture the following spring, not all growers are doing so, says Dr. Hemant Gohil.
“There are many reasons why vineyard soil moisture is not adequate in spring,” said Gohil, grape and tree fruit extension educator at Rutgers. Some growers forget to irrigate after harvest is done. Others may still be picking when irrigation districts shut down for the season and fear that irrigating while fruit is still hanging will affect fruit quality. “Moderately watering fruit before harvest has not been found to dilute fruit qualities,” said Gohil, former technology transfer specialist with Washington State University.
Filling up the soil profile at the end of the growing season can be particularly important for growers who practice regulated deficit irrigation strategies in red wine grapes because they likely have soil moisture levels that are below field capacity at the end of the season and need recharging. Gohil says that for those growers who implement severe deficit irrigation, it’s even more critical that the vineyard is fully irrigated before winter.
He’s also heard some growers express fear that irrigating early in spring with cold water will delay soil from warming up. According to WSU’s Dr. Joan Davenport, soil temperatures generally change very slowly and are impacted very little by application of cold water. Soil temperatures change more rapidly when soils have air pockets, often found in dry soils.
“I’ve also found some growers that do top off their soil water profile in the fall, but then assume they are good for the next spring. Sometimes, by the end of March and into April, especially in sandy soils, soil moisture starts to run out,” he said.
“In years when we have dry winters, you may need both a fall and early spring irrigation,” he stressed.