The drier-than-usual weather that many California wine grape growers have experienced since the start of this year highlights the importance of post-harvest irrigation. Entering dormancy when soils are dry can leave vines more vulnerable to injury from cold temperatures. That, in turn, can lead to stunted growth the following season, reports Larry Bettiga, University of California Cooperative Extension viticulture farm advisor for Monterey, San Benito and Santa Cruz counties.
“It’s critical that root systems don’t dry out during dormancy,” he says. “Proper water management in late fall and early winter can prevent that.”
In his part of the Central Coast. winter in the area of southern Monterey County and northern San Luis Obispo County can bring quite cold temperatures at times. “I’ve noticed that when we’ve had unusually cold weather in late fall, followed by a dry winter, sometimes we see a lot of stunting in the spring,” Bettiga says.
In mid-October, yellowing leaves and wood maturation of canes on older vines in his area were signaling the start of dormancy. Vines won’t become completely dormant until after the first killing frost, he notes.
Meanwhile, young vines can often continue to grow strong and green. “They tend not to harden off wood as fully as mature vines,” Bettiga says. “The use of slight to moderate water stress in late summer can help slow growth so they can hardened off properly,” Bettiga says.
At the same, ensuring good soil moisture shortly after harvest helps keep vines active enough to build up energy reserves. This can help them better withstand cold temperatures.
Finding the right balance between providing enough water for these functions to occur but not enough to start new vine growth and slow down the preparation for dormancy, can be a challenge, he says.
Many growers apply some water after harvest and then cross their fingers, hoping for rain to make up for any shortage.
“If the water doesn’t come naturally by late fall or early winter, you need to jump in and supplement it with irrigation,” Bettiga says. “You don’t necessarily need a full soil profile. But, you do need enough to keep root systems moist.”
That requires attention throughout the dormant period. “Don’t give the vines just one irrigation after harvest and walk away until spring,” he cautions. “Consider the consequences if you do and we have a repeat of last year’s dry winter.”
Done properly, post-harvest and dormant season water management will ensure the vineyard is ready to begin a new season, Bettiga adds. That will be evident in late winter when the awakening vines begin to bleed a little moisture from pruning wounds before leaf buds begin to swell.