Managing vine stress
Once the ovule begins to swell, clusters are less susceptible to stress.
The shoot tips in this Columbia Valley Cabernet Sauvignon vineyard are missing, a sign that canopy growth stopped and the shoot tips were burnt off.
To understand deficit irrigation, growers must also understand stress management and how to "read" how much moisture is in their soil profile.
Managing stress is about timing, intensity, and duration, said Dr. Russell Smithyman of Washington State's Ste. Michelle Wine Estates. The vine reacts differently to high temperatures in May than in August, just as a few hours of 100-degree temperatures will stress vines differently than several weeks of 90°F temperatures.
Smithyman believes that preconditioning the vine or fruit is the most important consideration when it comes to stress management. That's why leaf stripping should be done early in the season, near bloom, though too much exposure at bloom can impact fruit set. Exposing fruit early to sunlight not only enhances development of flavonols in the fruit but also preconditions the vine to better withstand sunlight and stressful temperatures later in the season.
But when is enough enough? Recognizing the progression of stress in vines can help growers from going too far in withholding water.
Before fruit set, tendrils and clusters are the first to show stress, with the tendrils getting limp and brown. If the soil is dry at bloom, clusters can desiccate, he added.
Once the ovule begins to swell, clusters are less susceptible to stress. "That's the key—when you see the ovule swell, we know that we're much safer about imposing stress to slow down canopy growth," he said. Ovule swelling usually occurs around the end of June, or early July.
After fruit set, the shoot tips are the primary stress indicator. "If you know what your goal is for your canopy (average shoot length, number of leaves per shoot), then you won't care if the tips burn off as long as you already have the leaf area developed."
Basal leaves are the next to show stress, with clusters showing stress last. Berries within clusters can collapse if they are stressed too far, he said.
Smithyman added that there are varietal differences in vine response to stress. Syrah reacts differently than Cabernet Sauvignon, with the basal leaves of Syrah yellowing while the shoot tips are still growing. "Stress management is about learning those differences. It goes back to knowing your vineyard and knowing what's happening."
While he pointed out that excess stress conditions can occur, vine response to excess stress doesn't usually happen overnight. By knowing where the weak spots are in a vineyard, growers can use the weak vines as their "stress indicators."
"If you do have a catastrophe with your indicator vines, you won't lose all of your block. And the vines will bounce back the next year."