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The following story appeared in the August 1999 issue of Wine Business Monthly. It is presented here with their permission.

Clippings from the Curbside May Reduce Erosion in Hillside Vineyards

By Mitch Tobin

In the hope of tackling two thorny land use issues at once, the state of California is funding dual studies to investigate whether yard trimmings destined for landfills can help reduce soil erosion in hillside vineyards.

For years, growers have used compost made from grape pomace or plant debris to control weeds and enrich their soil with nutrients and beneficial microbes. Now, a group of scientists, vineyard managers and waste haulers will study if clippings picked up from curbsides in Napa and Sonoma counties can stem runoff from rural slopes planted with grapes.

In one study, a partially composted mulch will be spread between vine rows. In the other trial, the material will be applied to the dirt under the vines themselves. Both sets of researchers will then gauge how rates of soil erosion differ from control plots. They’ll also measure how the mulch affects fruit yield, weed growth and soil quality. In both studies, the mulch will be heated and composted long enough to kill weed seeds and pathogens.

The studies come as some activists and politicians are raising questions about the environmental impacts of hillside farming in Napa and Sonoma counties. At the same time, California municipalities are under a state mandate to dramatically reduce their contribution to the state’s landfills before the end of 2000.

If the techniques prove successful—and economical— backers say hillside vineyards could retain more of their soil while becoming a crucial link in the state’s recycling chain.

Boosting Cover Crops

Growers typically use straw to shield cover crops between vine rows, but straw does little to add organic material to the soil. Using a nutrient-rich compost instead might help produce a thicker, healthier cover crop that does a better job of reducing erosion, according to Paul Skinner, president of Terra Spase, a vineyard technology firm based in St. Helena, Calif. Terra Spase will join the University of California at Davis and Napa County’s Upper Valley Recycling to study the effects of applying mulch to vineyards in the Carneros, Napa and St. Helena areas.

"We're excited about collecting some data that I don't think has ever been collected before," said Skinner.

Candidate vineyards for the study include: Domaine Chandon, Turley Wine Cellars and property managed by David Abreu Vineyard Management, Renteria Vineyard Management and Michael Wolf Vineyard Services.

Because of processing and distribution costs, using mulch may be more expensive than laying down straw, according to Melissa Prange, project coordinator for Upper Valley Recycling.

“But hopefully the costs of using this product will be balanced by the nutrient value of using it,” she added.

Ultimately, the cost effectiveness of the technique will depend on how much compost is needed, how much processing it requires and how far it must be transported, according to Christy Porter, a compost specialist at the California Integrated Waste Management Board now known as CalRecycle. CalRecycle is helping fund the two studies.

Even a limited application of mulch on Napa County vineyards could consume more than the total amount of yard waste generated by the county, according to Prange. About 162,000 tons of clippings would be used to apply a one-inch layer of mulch between the vine rows of 10 percent of the county’s vineyards. In 2000, Napa County is projected to generate about 39,000 tons of yard waste.

Besides testing different depths of application, the group will also study different formulations of mulch. Some batches of yard waste will be ground finer and allowed to decompose longer.

CalRecycle’s $80,000 grant for the two-year project will be matched by the various partners.

Bolstering Berms

A study complementary to the one in Napa will also include Sonoma vineyards and focus on the area beneath the vines themselves.

Soil in vine rows is typically exposed to the elements and therefore susceptible to erosion. But adding a layer of partially composted mulch might help protect the dirt from the impact of raindrops and add beneficial microbes to the area, according to Will Bakx, a soil scientist at Sonoma Compost and a manager for the study. The microbes themselves also might help reduce erosion by forming small beads of soil that are more porous and able to absorb more precipitation, Bakx added.

Bakx’s team will study erosion rates in the abstract and in the field. They’ll plug figures into a computer model called the Revised Universal Soil Loss Equation in order to predict the effects of adding a mulch layer. Crews will also install sediment traps in vineyards so that scientists can collect runoff that’s headed downhill and then weigh the amount of sediment in the water.

Bakx said several vineyard managers who heard of the project “immediately jumped at it. To us, that’s indicative that the project will be welcomed by grape growers at large,” he said.

Exact sites have not yet been chosen, but the vine row experiment will take place on two-acre plots at Iron Horse and Everett Ridge Vineyards in Sonoma County, and on property owned by Beringer Wine Estates and Walsh Vineyard Management in Napa County.

“I’m just real curious to see how it works,” said Martin Mochizuki, vice president and director of viticultural services at Walsh Vineyard Management.

Mochizuki said his company has been struggling to reduce erosion from the berms underneath vines.

“It’s something we’re not doing all that well right now in the present management scheme,” he said. Weeds have helped keep soil in place, but they also compete with the vines for water and nutrients. Adding a mulch could allow vineyardists to dispense with the weeds without increasing erosion.

In order to test the compost application under the most erosive of conditions, the researchers will focus on vines planted across slopes, rather than those that follow the contour of a hill.

“If it’s effective there, it’ll definitely work on contour planting,” said Bakx.

One third of the land in the study will receive a three-inch layer of compost in the first year, or about 100 cubic yards per acre. Another third will also receive another 1.5 inches in the second year. The final third will be a control plot and will receive no mulch.

Bakx said that the use of recycled yard trimmings in Napa and Sonoma would increase by 30 percent if only 5 percent of hillside grape growers in the two counties spread the mulch once every five years.

The project will include researchers from the Resource Conservation Districts and University of California Extensions in Napa and Sonoma. The Sonoma County Farm Bureau and the City of Napa are also partners in the project, which received nearly $100,000 in funding from the CalRecycle.

A Win-Win Technique?

If mulch in hillside vineyards proves to be a cost-effective way to reduce soil erosion, California’s wine industry could become a key player in the state’s recycling strategy.

A state law passed in 1990 requires that California cities and counties halve their contribution to landfills by the end of the year 2000. Yard waste and other organic matter comprises nearly a third of the state’s waste stream, so finding new markets for recycled clippings—rather than new garbage dumps—could go a long way toward meeting the goal, according to IWMB’s Porter.

“We want to change the perception of this material as being a waste into it being a resource,” said Porter.

Farmers of a variety of crops in California—including wine grapes—buy compost in order to suppress weeds and improve soil quality. There’s even some evidence that compost can combat phylloxera, says Porter. But this is the first time anyone has tested whether the material can reduce erosion in vineyards. The CalRecycle is also conducting a study to determine whether mulch in hillside road projects can reduce runoff on those sites.

If there is a stronger market for recycled yard waste, less of it will end up in landfills, according to Porter. But it is still unclear whether the mulch technique will be economical when compared to traditional methods.

“If we knew that, we’d just be telling people to do it,” Porter said. As a result, gathering data on the costs of the procedure will be as important as measuring its biological impact on soil and grapes.

Although the two studies will focus on different parts of the vineyard, researchers plan to collaborate.

“We’ll be able to compare notes and actually provide the grape grower with a broader spectrum of options,” said Bakx.

Finding new ways to control runoff may become even more vital for grape growers as their hillside plantings come under increased scrutiny from citizen activists and government regulators. Critics have charged that soil erosion from hillside vineyards is clogging streams and rivers with sediment and hurting local fish populations.

Responding to environmentalists’ demands, Sonoma County enacted an ordinance in May that sets limits on hillside plantings and requires erosion control measures. The law goes into effect on Oct. 1.

Sonoma’s law is modeled on an ordinance passed by Napa County, and Napa’s regulations may become stricter in the coming year. In December, Napa’s board of supervisors appointed a citizens’ task force to make recommendations on reforming the regulations, which were enacted in 1991. In its first round of meetings this spring, the group agreed there should be tighter enforcement of the current law, but deadlocked on whether the county needed stricter controls on vegetation removal and more stringent erosion control measures. The group is scheduled to reconvene after the fall harvest.

Mitch Tobin covers local government and environmental policy for the Napa Valley Register.

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Last updated: July 19, 2011
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