October 2008 Issue of Wines & Vines
Capitalizing on Cover Crops
Virginia vineyard finds them practically revolutionary
Virginia is one of the wettest winegrowing regions in the world. Our annual rainfall is often more than 40 inches (1,000 mm) per year and is very evenly distributed in all seasons, including the growing season and harvest. Excessive vine vigor, disease and grape dilution are all constant challenges here because of rain. Cover crops have become a major management tool, mitigating each of these problems.
Achieving a balanced vine is the most important goal of high-end vineyard management. Controlling the growth habits and vigor of vines in an unpredictable climate will always be a moving target. A colleague makes the analogy of two pipes fueling a vine's growth. One pipe feeds water and the other feeds nitrogen. When both pipes flow at capacity, we have serious vigor problems. Soil has the most significant influence in limiting this flow--both in terms of water-holding capacity and nitrogen availability. Cover crops can help adjust both water and nitrogen availability through root competition.
Until 2002, my vineyard was primarily trellised with divided canopy systems such as Lyre or GDC at densities of 600 to 800 vines per acre. For many years I have wanted to try higher density vine spacing, but I was afraid of creating an excessively dense canopy. With the exception of my most vigorous soils, I have now been able to manage balanced vines at densities of 1,555 to 2,074 vines per acre. I attribute most of this success to vine/cover crop competition.
Steep slopes: My best sites are on steep slopes, which have the best drainage. These slopes went unplanted for many years because of the logistical management difficulties and erosion concerns. Permanent cover crops have solved erosion problems and made tractor work easier.
Water evacuation: While Westerners talk irrigation, Easterners are concerned about water evacuation. It is a rare vintage where our vineyards are excessively water stressed. We have too much water. Soils that are well drained and naturally low in water-holding capacities produce our best wines.
It is possible, especially at harvest time, to aid in the evacuation of soil moisture by allowing cover crops to grow. We do very little mowing or weed control after July. The more biomass there is on the vineyard floor, the better. The vineyard floor looks unkempt and aesthetically unattractive, but the vineyard is able to recover more quickly from a 2-inch rainfall in September.
Reduced chemical use: We abandoned pre-emergent herbicides and reduced post-emergence herbicides by 75%. Any applications are done with 4-gallon backpack sprayers. Fungicide use has been slightly reduced, as our canopies are less dense and stop growing earlier in the season. Cover crops are probably altering the insect populations in the vineyard, but we have not yet observed any significant changes for better or worse.
Our cover crops range from specific grass species to random native mixes (a.k.a. weeds). In blocks that need a high level of competition, we like Kentucky 31 Fescue in the row middles. K31 is very competitive and can handle tractor traffic in wet conditions well. Under the vines, Creeping Red Fescue is the grass of choice. It is low-growing and eventually chokes out other weed species. Both are perennials.
When less aggressive competition is desired, annual crab grass is a great tool. It doesn't start growing until June or July, so more balanced vines can get a good strong start to the season. As veraison approaches, the crab grass quickly takes over to help slow down vine growth.
We have experimented with clovers, wildflowers and other grass species, but each has had drawbacks for our situation--too much nitrogen fixation, too tall, not well adapted to our growing season. Native flora (weeds) give us good biodiversity, but are not as competitive or reliable as grasses.
Each vineyard block receives different cover crop rates. Young vines and low-vigor sites have only row-middle grasses with clean under-vine strips of 18 inches or more. Moderately vigorous blocks might have very narrow weed-free strips. High-vigor blocks have nearly 100% cover, although it is recommended in our damp climate to keep at least an 8-inch diameter weed-free perimeter around each trunk. Higher vine density blocks can handle more competition than wider-spaced vines.
Mowing, herbicides and flaming (still experimental) are all used as cover crop control methods. Mowing consists of offset flail mowers in combination with handheld grass trimmers. We tried using mechanized, under-trellis mower attachments. These were unsuccessful on our rocky, uneven, steep slopes.
Contact herbicides are used on young and lower-vigor vineyard blocks. Only backpack sprayers are used, because they give us the best precision. So far our backpack flamer has had little success in weed suppression, but we will continue to experiment in using flami ng for partial cover crop control, scorching the grasses back just enough to stun them, but not kill them.
Seasonal timing is the most difficult aspect of using cover crops as a vineyard management tool. In excessively high-vigor blocks, we simply let the cover crops grow with great abandon. Timing, however, becomes much more involved with our more balanced blocks. In the early days, over-use of cover crops led to severe vine nutritional deficiencies in these blocks. We have now learned about moderation. Rainfall amounts can vary enormously from month to month, and year to year. Unlike irrigation, cover crops are not so easy to control.
Early shoot growth: After bud break, we like to see moderate shoot growth (defined by short internode length), while still retaining healthy shoot tip expansion. In a rainy spring we will let weeds and cover crops grow. We have many early spring, low-growing weeds that are usually allowed to flourish in a wet spring.
We have had problems in some blocks with rampant vine vigor around bloom leading to disastrous fruit set. One of our greatest success stories involved using an aggressive cover crop program in these blocks to throttle back vine vigor. This block is now reasonably well balanced and yields consistently good crop levels.
Post bloom through pre-veraison: This is the period when we spend the greatest amount of time with mowers, grass trimmers, and backpack sprayers. This is when most blocks can easily become over stressed by cover crop competition and show serious nutritional deficiencies (nitrogen and phosphorous in our case). Our goal is to suppress but not kill the cover crops so that they can spring into action at the time when we need them most.
Veraison through harvest: Getting vines to cease vegetative growth at or before veraison is the most elusive quality goal that faces Eastern viticulture. When it rains in July and August, the vines want to keep growing. This is when we stop mowing and applying herbicides and allow the vineyard floor to "go native." This competition not only helps slow vine growth around veraison, it also provides lots of biomass to assist in the evacuation of excessive soil moisture after a significant rain. An additional bonus is that during the winter, the soil is well protected from erosion by cover crop residue.
The use of cover crops in vineyards is not new. After a few millennia of global winegrowing history, it would be difficult to come up with any new techniques in the vineyard. It is doubtful that cover crop use in the Eastern United States will revolutionize winegrowing in the same way as the draining of the Médoc marshes by 17th century Dutch engineers. What is important is the paradigm shift of seeing weeds and unkempt grasses as friends rather than unsightly foes.
Jim Law is the owner of Linden Vineyards in Linden, Va. He first planted grapes at Linden in 1985 and opened the winery in 1988. He teaches seminars on winemaking, grapegrowing and wine appreciation, and has written the column, "A Winegrower's Notes," in Wine East since 2001. He is also the author of The Backyard Vintner: An Enthusiast's Guide to Growing Grapes and Making Wine at Home, which was published in 2005. To comment on this article, e-mail firstname.lastname@example.org.
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