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A water deficit of as little as 0.1 or 0.2 of an inch can cause shriveled tendrils and poor fruit set. These vines were part of Ste. Michelle Wine Estates’s irrigation trials at Canoe Ridge Vineyard.

A water deficit of as little as 0.1 or 0.2 of an inch can cause shriveled tendrils and poor fruit set. These vines were part of Ste. Michelle Wine Estates’s irrigation trials at Canoe Ridge Vineyard.

April flowers bring September grapes. You can count on it. But while those tiny spring blossoms may promise an abundant harvest, it’s not a straight line from bloom to profits. To maximize tonnage and productivity, vintners need to carefully manage water and nutrients from bud break all the way through bloom, a panel of researchers told members of the Washington Association of Wine Grape Growers, during a workshop in May.

Russell Smithyman, Chateau Ste. Michelle’s director of viticulture, calls bloom one of the most important cropping events of the season. But often, he said, what look like abundant blooms can produce a less than perfect crop, with shattered clusters or uneven berry size, sometimes known as “hens and chickens,” not to mention problems like botrytis or mildew resulting from crowded clusters.

Smithyman joined Washington State University researchers Dr. Markus Keller, Dr. Joan Davenport, and Dr. Michelle Moyer to present information about good management practices surrounding bloom. Bloom is the one stage of a vine’s annual life cycle that can’t be controlled, they agreed, but there are steps growers can take surrounding bloom to improve yield and quality in their vineyards. Finding the right balance between soil moisture and nutrition is paramount, but other interventions like pruning and hedging also carry big impacts.

Smithyman described the effects over several years of various approaches to irrigation, as well as bloom hedging. While it’s important to maintain soil moisture during a vine’s growing phases, in his trials, bloom hedging had an even greater effect on the number of berries.

Chateau Ste. Michelle’s studies focused on Cabernet Sauvignon grapes in two different blocks at Canoe Ridge Vineyard, a location marked by warm, windy conditions that can stress grapevines. In Block 19, Smithyman looked at the effects of three different approaches to ­irrigation.

The standard irrigation philosophy calls for a single round of irrigation prior to bloom, he said, with no more water applied until the canopy reaches optimal size (three- to four-foot shoots) and the rate of growth slows. Irrigation is then applied to replace the vineyard’s moisture consumption.  Smithyman compared the effects of two alternatives—deficit irrigation at the beginning of bloom or at veraison.

Besides the timing of irrigation, he also analyzed the effect of different amounts of water.  He found that irrigating to maintain more moisture in the top three feet of soil, even as little as one- or two-tenths of an inch, yielded up to 50 percent more berries per cluster.

Hedging

A second study of fruit set at Canoe Ridge’s Block 16 showed bloom hedging is just as important, and maybe more so. In 2003, Smithyman compared the effects of bloom hedging to divert more sugars in the plant to fruit, July hedging, and use of Vapor Guard to control vine evapotranspiration, in areas showing both low- and high-moisture soils. The results showed that hedging the vines during bloom had an equally dramatic impact on both berries/cluster and tonnage, no matter the soil moisture level.  All the vines subjected to bloom hedging out­performed the rest of the test site, with 72 to 78 berries per cluster, compared to those in low-moisture soil treated with Vapor Guard, and the control group in the high-moisture soil, which each produced fewer than 60 berries per cluster.

But berry count isn’t the whole story. Balancing adequate water with proper nutrients is vital to a vineyard’s yield. “It’s not just the fruit you have to grow. You also have to grow a canopy,” cautioned Davenport. “Plants can’t take up nutrients if they can’t take up water,” and that requires an energetic canopy, with plenty of green vegetative growth to fuel the blossoms.

However, that doesn’t mean growers should pour on the fertilizer. Commonly cited standards for petiole nitrate, for example, are based on conditions in California. Davenport recommends testing both the soil and the plant’s leaves to determine nutrient levels, as well as soil moisture. Higher-moisture silt loam might already contain all the nitrogen vines need, but excessive moisture can cause problems. Nitrogen tends to dissipate as a gas when too much water is present, she said. And remaining nitrogen often leaches outward, stranding it beyond the reach of the vine’s roots. Sandy soil will probably show no nitrogen, but is likely to have plenty of phosphorus.

Davenport’s and Smithyman’s presentations both illustrated how a vine’s demands for water and nutrients are intertwined, but both emphasized that measurements are important. “Every vineyard is different, and every block is different,” Smithyman said. “There are ­various things you can do, and irrigation is just one of them, to influence the number of berries per cluster, and eventual yield.”