Without sufficient rainfall, post-harvest irrigation is the key to keeping vines healthy during dormancy and supporting new growth in the spring. Once the crop has been produced and the grapes harvested, the vines continue to convert carbon dioxide and water into carbohydrates and oxygen and to take up minerals until they go dormant. At the same time, the vines are translocating carbohydrates and nutrients to permanent wood where they are stored for use to support respiration during dormancy and to fuel new shoot and root growth next season.
Dry soils during the fall and winter can hinder these processes, resulting in stunted growth and delayed or erratic bloom in the spring.
That was the case this season in a number of Monterey County vineyards, reports Larry Bettiga, University of California Cooperative Extension viticulture farm advisor.
“This was due partly to cold weather last December and partly to the dry soils,” he says. “Sometimes it’s hard to separate the impact of the two, because dry plants are more susceptible to cold injury.”
All the vineyards in this area are irrigated with water pumped from the ground. As a result, the continuing drought has led to over-drafting as growers seek to meet the water needs of their vines.
The effect of over-drafting varies, depending on location, Bettiga notes.
“Most of the wells are still OK,” he says. “Most grape growers should have enough water to finish out this season and do post-harvest irrigation, particularly those in the southern part of the county, where the aquifer has been better served by stored water released from the dams. However, the situation is of greater concern farther north in the Salinas Valley, especially for those aquifers that are potentially affected by sea water intrusion. These are on farms near the ocean where vineyards are not planted.”
Meanwhile, the early start to this year’s harvest should mean an early finish. That would add to the value of post-harvest irrigation by giving vines more time to manufacture and build up their reserves.
“Keeping the leaf canopy active, without re-growing, and producing sugars and taking up nutrients can result in a lot of materials being stored in the permanent wood,” Bettiga says. “And, that can go a long way in helping the vine recover from harvest.”
As they peer ahead to next season, growers are pinning their hopes on wet weather following harvest.
“Right now, levels in our reservoirs are relatively low and the barrel is almost dry,” Bettiga says. “There’s a lot of concern what the situation will be next season if we don’t get an adequate refill of the soil profiles this winter.”
As dry as things are, good rains this winter probably wouldn’t solve the county’s problem of dropping groundwater levels. But, it sure wouldn’t hurt, either. It will take more than several inches of rain to raise groundwater levels, he adds.
“We’d probably still end up over-drafting our groundwater, but it would help minimize that over-draft.”