Tips for Under-Vine Cover Crop Adoption
Standard vineyard practice for many grape growers includes leaving a bare zone of soil underneath the vine trellis. A vegetation-free zone under the trellis has been considered essential for maximizing yield of bulk juice and winegrapes. However, bare soil can also promote excessive vigor — particularly for vinifera winegrapes — as well as increasing the potential for soil erosion and leaching of agrichemicals and nutrients.
In many cool-climate wine regions, vinifera growers hedge the vines and pull leaves from the cluster zone multiple times in the growing season in order to reduce the negative impacts of excessive canopy growth. But one of the reasons that vines can grow excessively is that most vineyards maintain a weed-free strip under the trellis using herbicides or cultivation. The bare soil eliminates competition from non-vine plant species for water and nutrients.
But in addition to reducing competition, maintaining bare soil also increases the opportunity for soil erosion and runoff. Planting cover crops directly beneath vines has the potential to mitigate the environmentally detrimental features of herbicide application and cultivation while increasing competition for water and nutrients to reduce excessive vine vigor.
Impact on Vine Size
Our research group looked at the impact of under-vine cover crops as an alternative to bare ground soils. We found that under-vine cover crops can be an effective tool to reduce vigor and promote better-balanced vines, while also reducing management costs compared to maintaining an herbicide strip. Under-vine cover crops also promote healthier soils and reduce soil erosion and nitrogen leaching.
We’ve worked with a range of under-vine cover crops: buckwheat (Fagopyrum esculentum), rosette-forming turnip (Brassica rapa), chicory (Chicorum intybus), annual ryegrass (Lollium multiflorum), white clover (Trifolium repens cv. Dutch White), tillage radish (Raphanus sativus), alfalfa (Medicago sativa), fescue (Festuca arundinacia), and what we call ‘native vegetation’ (i.e., we let the weeds grow). Table 1 shows a summary of the cover crops we’ve used on mature vines and their impact.
Under-vine cover crops have had variable impacts on vine size. The range of impacts of the under-vine covers on vine size suggests that these crops can be used as a tool for maintaining soil cover while maintaining an appropriate vine size. For example, buckwheat had essentially no impact on vine size or yield at multiple sites, while chicory plots had significantly lower pruning weights compared to glyphosate plots.
Cover crops that provided ample competition for vines (such as chicory) likely did so through competition for both nutrients and water. Some of the cover crops reduced petiole nutrient concentrations at veraison — specifically nitrogen — but others had little impact.
The choice of under-vine cover crop impacted the timing of competition for water and nutrients. For example, natural vegetation provided competition throughout the season as senescing weeds were replaced by new species. In contrast, buckwheat emerged early and then senesced by mid-August. These timing differences with respect to cover crop growth should enable growers to fine-tune the timing of competition in their vineyards.
Considerations for Adoption
Age of the vines will have an impact on how they respond to the competition that an under-vine cover crop may provide for water and nutrients. When we initiated our under-vine cover crop treatments when the vineyard was in its fourth year, the impact on vine size and yield was large. But when we let the native vegetation grow under vines in a 17-year-old vineyard, the pruning weight wasn’t affected by the treatments after three years. While our economic analyses show reduced production costs with under-vine cover crops compared to glyphosate, the impact on revenue will depend on whether yield is reduced.
The impacts of an under-vine cover crop on vine growth and yield will differ significantly based on soil composition and water availability. Our experiments were all conducted in the Finger Lakes region of upstate New York. All of the studies on vinifera were conducted in vineyards with deep, rich soils.
In most years, we have ample precipitation and our soils are high in organic matter so excessive vigor can be a challenge in vinifera plantings. A good choice of an under-vine cover crop can potentially alleviate some of that vigor, reducing the need for multiple rounds of hedging and leaf removal.
However, during the 2016 drought, ‘Noiret’ vines growing on shallow soil were in distress as a function of the under-vine cover crops, resulting in poor growth and partial defoliation. If cover crops are providing too much competition during a dry season, they should be killed so the vines can access the water and nutrients required to ripen the fruit.
Adoption of an under-vine cover crop is an annual decision. For example, vines in the ‘Riesling’ vineyard in a study benefitted from the vigor reduction provided by the chicory beneath the vines, which then had to be killed for the vines’ to size right.
Finally, over the course of our experiments, we’ve noted little to no reason to maintain an herbicide strip in mature vineyards growing on moderate to deep soils. Under-vine cover crops can be used are a tool for vigor management, soil health, to decrease the leaching of nutrients in comparison to herbicide use, and to potentially reduce management costs.