—by Melissa Hansen, Washington State Wine Commission
Winter preparation for vineyard sustainability is undeniably important for wine grape growers in Washington and other northern grape-producing regions. Growers use irrigation to prepare vines for winter survival, yet there’s been little scientific research in this area. A new research project supported by the Washington wine industry aims to learn how to best winterize grapevines without compromising wine quality.
Washington State University’s Markus Keller, Chateau Ste. Michelle Distinguished Professor in Viticulture, leads the new four-year project that was awarded a grant this year from the Washington State Grape and Wine Research Program. The program is funded by contributions from the Washington State Wine Commission, Auction of Washington Wines, Washington State University and state taxes collected on all wines sold.
This project fits well with Keller’s ongoing research program that focuses on how environmental factors and vineyard management practices influence crop physiology of wine and juice grapes. Additionally, his research team collects critical temperature information annually to track grapevine cold hardiness to help growers determine when cold protection measures should be initiated (see “Vineyard pruning approach cut cold damage after frigid weather” ).
The idea behind the research is to maximize both winter hardiness and wine quality. Best-practice recommendations based on science can then be developed for the Washington wine industry. While there are strongly held but diverging opinions about “best-practice” approaches among wine industry members, Keller said some of the practices are based on a misunderstanding of grapevine function.
Irrigation management
Irrigation is often used as a tool to encourage cold acclimation. For example, one approach to grapevine winterization is to reduce irrigation before harvest, to avoid new growth and induce hardening off, though without overstressing the vines. Such an approach is based on a misunderstanding of grapevine function: There is no scientific evidence for a connection between harvest date, preharvest water management and vine hardening off, said Keller.
He explained that green shoots begin to lignify right after budbreak, although it happens internally and growers can’t see it. Without lignification, shoots can’t transport water and therefore can’t grow. Shoot browning, by contrast, marks the death of the outer cell layers and usually starts before veraison. It has been thought to be a prerequisite for bud dormancy, but other environmental factors may be involved, such as temperature and day length. For example, in the Pacific Northwest, grapevine buds go dormant when the day length shortens to about 13 hours, which can be several weeks after the shoot base starts to turn brown. Better understanding of the grapevine dormancy cycle and its link, if any, with shoot browning is needed.
In a previous field trial with Cabernet Sauvignon in Washington, Keller found no effect on bud or cane cold hardiness from cluster thinning (35 percent yield reduction) or reduced water supply (50 percent reduction from standard regulated deficit irrigation before or after veraison). Another trial with fourfold differences in water supply to Cabernet Sauvignon grapes in Colorado found that although shoot growth and berry size were impacted by the irrigation treatments, bud and cane cold hardiness in the winter was not influenced. Other trials in Washington and elsewhere have found no link between harvest date and cold acclimation.
Root protection
There is a need to protect grapevine roots in the winter. Unlike the aboveground tissues, roots neither go dormant nor harden off for the winter. Previous research has shown that the lethal temperature (50 percent damage level) for roots is approximately 22 degrees Fahrenheit. Roots in dry soil are more vulnerable to sudden temperature changes, thus, standard practice in Washington is to fill up the root zone to field capacity at the end of the season. But how much water is really needed for root protection? Is it field capacity, or can growers have adequate root protection with less water, which may lead to better vigor management the following spring?
Another unanswered question relating to root protection is the timing. It’s assumed that this extra irrigation should occur after harvest, to avoid “diluting” grape quality, even though Keller showed in previous research there was no preharvest dilution effect from irrigation unless the water was applied from overhead sprinklers. In that study, he found berries did absorb water across the skin when their surface was wet, but drip irrigation had no dilution effect. Nonetheless, some winemakers still encourage growers to withhold preharvest irrigation.
Water is a limiting factor in wine grape production in Eastern Washington. In drought years when irrigation districts must reduce volumes and end deliveries earlier than normal, the option to irrigate the root zone before harvest could be valuable. Also, in years when harvest is delayed and stretches past water delivery dates, growers could fill the root zone while they still have access to water.
Field trial design
The new project, which got underway this summer, is complicated. It involves two own-rooted varieties, three different irrigation treatments and varied practices of late-season irrigation to fill the root zone in every combination:
—Standard regulated deficit irrigation (RDI) of controlled water deficit from fruit set to harvest.
—Standard RDI with increased water after veraison.
—Standard RDI with decreased water after veraison.
—Irrigation to fill top 3 feet of soil to 26 percent soil moisture (field capacity) before or after harvest.
—Irrigation to fill top 3 feet of soil to 20 percent soil moisture before or after harvest.
The varieties selected — Chardonnay and Cabernet Sauvignon — span a spectrum from early to late harvest while having a similar onset of bud dormancy. Keller’s research team will measure soil moisture, soil temperature, leaf water status, trunk sap flow, shoot growth, shoot browning, pruning weight, cold hardiness, budbreak percentage, bud fruitfulness and early-season vigor. Berries will be sampled for wine quality metrics.
“There is so much we don’t know about grapevines and dormancy,” Keller said. He added that grapes grown in Mediterranean climates, like that of California, receive rain in the winter, and growers don’t have to worry about dormancy and cold acclimation nearly as much as Washington growers do. This research will answer some basic grapevine physiology questions in a practical way and clarify the connections, if any, between shoot growth, lignification, browning, bud dormancy transitions and cold hardiness. The new learnings will be translated into best-practice, science-based recommendations for the Washington wine industry.
And while climate change brings a new urgency to this research, with its more frequent frosts in early fall and late spring and more extreme arctic cold snaps in winter, it’s an area overdue for study. We look forward to sharing this valuable information with you in the next few years.
To learn more about the WSU cold hardiness model, visit: wine.wsu.edu/extension/cold-hardiness.
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