by Tamara Scully
In many vineyards, the prevalent practice is to have a grass alley with an herbicide strip under the vines. A focus on enhancing vineyard sustainability, through the implementation of practices that promote soil health, decrease nutrient and chemical leaching and minimize erosion, has led to research trials on the use of under vine cover crops.
Dr. Justine Vanden Heuvel, associate professor of viticulture at Cornell University, spoke about the research being done in the Finger Lakes region of New York during a podcast with The Vineyard Team, an organization devoted to enhancing wine grape sustainability.
The purpose of the herbicide strip is to reduce competition for water and nutrients. In the Northeast region, grape vines are often very large with too much vegetative growth, Vanden Heuvel explained. This excessive vegetative growth reduces the crop load.
“Our vines are really big. Very often we are growing in soils that are high in organic matter, and we have high precipitation,” she said.
With high precipitation rates and fertile soils, growers here would benefit from a decrease in vine growth.
With many vineyards on steep slopes down to the lakes, potentially minimizing runoff and groundwater discharge of chemicals and excess nutrients is yet another benefit which cover crops could provide.
Cover Crop Trials
Cover crop trials to determine if this lush vegetative vine growth could be controlled while simultaneously improving soil health, preventing nutrient and chemical runoff from reaching the Finger Lakes and decreasing soil erosion were conducted on plots of Riesling and Sauvignon Blanc grapes.
“Most of the growers are hilling up over their graft union to protect a scion bud in case of cold weather,” Vanden Heuval said.
Cover crop seeding of annuals is performed after the spring hill removal. Hand seeding under the vines is not practical on a larger scale, and Hans Walter-Peterson, of the Finger Lakes Grape Program, has been working on a modified fertilizer spreader which can mechanically seed under the vines.
Annuals tested for their cover crop potential included chicory, buckwheat, tillage radish, alfalfa, fescue, rosette-forming turnips and native vegetation. The native vegetation is whatever grows when no herbicide is applied.
When using native vegetation, the “amount of competition that is presented varies from year to year and vineyard to vineyard,” Vanden Heuvel said. On Long Island, where soils are sandy, growers are finding that allowing the native weeds to grow and serve as a cover crop brings good results.
In the Cornell trials, all cover crops studied were tested for their impact on vine vigor. Low impact cover crops did not impact vine size, and included buckwheat and the turnips. Tillage radish, fescue and alfalfa had a moderate impact, while chicory and annual ryegrass where found to have a dramatic effect on vine vegetative growth.
During the first year, a chicory cover crop decreased vine vegetative growth, as measured by pruning weight, by 10 percent. By the third year, the impact on vine vegetative growth had increased to 30 percent. For growers with excessive vegetation, planting chicory for a few years and then switching over to a buckwheat cover crop is a promising strategy to decrease excessive vegetation to the desired point, then maintain it.
Groundwater Studies
In order to assess the environmental sustainability of planting an under vine cover crop, Vanden Heuvel collected groundwater samples from vineyard rows planted with white clover, native vegetation, cultivated rows and rows treated with glyphosate. Groundwater was sampled after every rain event, and the amount of dissolved organic carbon was determined. The dissolved carbon comes from soil breakdown. Lower amounts of dissolved carbon mean less soil disturbance. Both the cultivated rows and the herbicide-treated rows had 40 percent more dissolved carbon than the rows planted to either cover crop.
She also determined the amount of nitrogen present in the groundwater. The total amount of nitrogen was highest in the rows treated with glyphosate. Nitrogen in groundwater causes the detrimental eutrophication of waterways.
Interestingly, the white clover cover crops also caused an increase in nitrogen in the groundwater. She hypothesizes that the tilling up and down of the hills was causing some of the clover to break down, releasing nitrogen into the soil.
Researchers also found that the breakdown products of the insecticide Imidacloride (a neonicotinoid) were more prevalent in groundwater discharged from glyphosate-treated rows when compared to any of the other treatments.
Western Growers
Growers in the West are faced with different climates than in the Northeast. Irrigation is often necessary, and reducing irrigation water use in the vineyard is a primary concern. Viticulturist Fritz Westover and Craig Macmillan, Ph.D., utilized a Sustainable Agriculture and Research Education (SARE) grant to study the impact of under vine cover crops on soil moisture depletion in the Paso Robles Groundwater Basin region. There were two research plots, and the trial spanned two years.
The researchers tested a variety of under vine winter-planted cover crops – barley, medic, triticale, brome and peas – and compared results to that of a clean cultivated plot. They took soil samples at 18 and 36 inch depths the next spring and tested for gravimetric soil moisture content. The goal was to determine if the use of cover crops, whose positive effects include decreased erosion, increased water infiltration and enhanced organic matter content, could also reduce the overall water use in vineyards.
Biomass dry matter was measured as well to determine if there were any significant differences in the vegetative growth of the various species of cover crops. Biomass dry matter was not significantly different except for during year one, when the pea cover crops showed slightly increased biomass dry matter. However, the second year of the trial showed no increase in pea biomass compared to the cultivated control or other cover crops.
There was no change to the pruning weight of the vines across any of the treatment trials, indicating that the cover crops were not impacting vine vegetative growth.
The soil moisture content in all cover crop trials showed no significant change from the cultivated control. This indicated that the use of cover crops would not increase the water needed to irrigate the vines. Researchers did expect to see some changes in soil moisture content between species of cover crops; however, that did not occur.
The two-year study was conducted from 2014 – 2016, during which winters were dry. If normal rainfall had occurred, it’s possible that increased growth of cover crops may have had some impact on the soil moisture depletion, or that different cover crop species may have produced differing amounts of biomass.
These conditions, in theory, might cause the vines to require added irrigation, should soil moisture depletion then occur. More study is needed to assess whether or not increased rainfall leading to a presumed increase in cover crop growth would deplete soil moisture at a higher rate than in a cultivated control.
No matter which region a vineyard is in, keeping soil covered prevents soil runoff, decreases the leaching of nutrients and chemicals promotes soil health and enhances soil structure.
“Anything growers can do to decrease the use of bare soils in their vineyards is going to contribute to environmental sustainability,” Vanden Heuvel concluded.