Using Cover Crops to Make Great Wines

by Jim Law

My never ending quest to make great wine has led me down some unusual paths. For the past several years, I have been working with under vine cover crops with the goal of creating a more balanced vine. I have found that worldwide there is universal agreement on very few winegrowing concepts, but one rare exception is the premise that a balanced vine produces the best potential wines. In this context I define ‘balanced’ as a vine that fills its allotted trellis space with a thin, airy canopy. The vines have no significant lateral growth, and shoot tips stop growing at veraison. The vineyard requires at most one light hedging and very little leaf pulling.

Although, quite frankly, the title of this article is a bit of a ruse in order to draw the reader into an otherwise admittedly boring subject, it is the latest focus of my viticultural efforts at Linden Vineyards. My travels to France and California have confirmed that only small, balanced vines produce great wines. In France the best sites have depleted, restrictive, excessively well-drained soils. In California they achieve vine size and balance through deficit irrigation. In the East we have high capacity soils with water falling at God’s will. This is a problem.

The idea is to use a cover crop to compete with the vine for both water and nutrients in order to make the vines smaller and more balanced. This, in theory, would improve wine quality and reduce vineyard canopy management costs. Improving wine quality and reducing labor costs are a rare win/win concept in winegrowing. Other possible benefits could include drawing water out of the soil in a rainy fall, reduced chemical use, and achieving more biodiversity. Many of us have discovered that in winegrowing, theory and practice don’t often coincide. I would like to comment on my own practical experiences. I must include a disclaimer that the following is primarily anecdotal and only based on three growing seasons. Tony Wolf at Virginia Tech will be doing more focused, controlled work with cover crops as the result of recently secured grant money.

In 2003 I planted three different cover crops under a diversity of vineyard panels with differences in soils, varieties and ages. I was especially encouraged by the way Creeping Red Fescue (CRF) slowed vine vigor and by its low growing and invasive growth habits. In 2004 I enthusiastically planted CRF under 60% of my vines. In some cases, CRF balanced excessively vigorous vines beautifully, but in other cases it ‘choked’ the vines, causing weak growth and nutritional deficiencies. My ‘take away’ message from all of this is: 1) most vine roots are superficial; 2) soil water holding capacity seems to trump all other causes of vigor, including rootstock and variety; and 3) if a vineyard is only marginally sufficient in certain nutrients, cover crops will probably put it over the edge. In 2005 I reluctantly decided to go back to using herbicides, fine-tuning the strips according to perceived vine vigor. In my high vigor blocks, I only put herbicide on a small ring around the base of the vine. In our humid growing conditions, having vegetative growth right next to the vine crown is probably a bad idea. The crown was always moist and it seemed as if this could be a breeding ground for problems. I opted to carefully burn the surrounding CRF with herbicides.

One of my goals was to greatly reduce or eliminate the use of herbicides. Unfortunately, at this time that has not been achievable. Early this year I used Round-up or Gramaxone to burn narrow strips under the vines and I continue to maintain those strips. Moderate vigor blocks have a 12″ strip, lower vigor vines have a wider strip. In addition, we are trying to regulate the growth and competitiveness of the cover crops by both physically mowing (I use an under-the-vine swinging arm mower) and by ‘chemical mowing’ (using low rates of herbicides to kill the weeds growing in the cover crops and to stunt the cover crop’s growth). I have found that especially during the period from bloom until a few weeks pre-veraison, too much cover crop competition can trigger severe nutritional deficiencies. We have been attempting to ‘stun’ the cover crops during this phase, but not kill them, so that by veraison the grasses are back growing and competing with the vines. Interestingly, it seems that CRF is fairly Round-up resistant at low rates. I let the cover crop grow unencumbered from veraison on in order to keep new vine shoot growth in check and also theoretically to transpire out any late season rains that could have a negative impact on fruit quality. My vineyard floor looks like a mess, but I try to keep reminding myself to look up at the canopy, not at the ground.

One of my goals in deciding to work with cover crops was to transition from monoculture (di-culture) if you include the existing Kentucky 31 Fescue in-row cover crop) to a more heterogeneous environment. I did accomplish this to some degree by experimentally sowing low growing ‘wild flowers’ under one row. My studies and readings on biodynamics have influenced my new approach to looking at the vineyard as part of a larger ecosystem. As my vineyard ages, I am seeing mites and insects to go from no problem to more of a major concern. This makes sense to me as certain species of insects such as grape berry moth and grape root borer feel at home in a vine-dominated ecosystem. My hope is that through diversity I can build a more balanced system with predator insects balancing the population of economically damaging mites and insects. At this point I have no idea if this will be the case. In fact there is some anecdotal information that cover crops can increase grape root borer and leaf hopper populations in vineyards.

There is a trend worldwide now towards small vine viticulture and closer vine spacing. After experiencing trunk damage as a result of the dreary 2003 growing season, I have seen that ‘renewal’ viticulture may be a reality, even as far south as Virginia. By using a couple of suckers, small vines are easy to get back into full production, whereas large vines occupying more trellis area require several years. I am now planting 6.5 to 7 feet by 4 feet, but remain nervous about possible canopy crowding. I feel that cover crops will be an important tool in the East for those who pursue closer spacing.

I think that cover cropping will be a very good viticultural tool. I have made good progress in my main goal, which is to reduce vigor and to contribute to the possibility of growing smaller vines, but many questions still remain. What will happen as the cover crops mature and begin to add back organic matter as they decompose? How will drought affect the vine/cover crop relationship? Can we use cover crops to fine-tune vine vigor at different stages during the growing season in a manner reflecting the West Coast’s ability to do the same with irrigation?