Issue link: http://winesandvines.uberflip.com/i/732978
78 WINES&VINES October 2016 GRAPEGROWING WINE EAST J. VANDEN HEUVEL / CORNELL UNIVERSITY Wolf at Virginia Tech's AHS Jr. Ag- ricultural Research and Extension Center in a research vineyard with high vigor potential. He does not suggest that an under-vine cover crop should be used in young vine- yards without knowing the history and vigor potential of the site. Interestingly, the growth-sup- pressive effect of the under-trellis fescue decreased over the years, which suggests that the vines may be able to adapt to the presence of under-trellis cover crops by, for example, relocating absorptive roots to a deeper soil profile. 14 Chicory (Chicorium intybus) annually planted under the trellis of mature, vigorous vineyards in Dr. Justine Vanden Heuvel's Cor- nell University research plot in the Finger Lakes region of New York resulted in considerably di- minished vine size, up to 54% (see bottom photo at left). 10,15 Furthermore, chicory effectively suppressed weed pressure under- neath the grapevine canopy. When vine growth is vigorous, the balance between vegetative and reproductive growth tends to be below the recommended Ravaz index (crop weight/pruning weight) ratio that ranges from four to 10 for high-quality wine production. 16 The devigorating ef- fect of the under-trellis cover crops often translated to an in- crease in crop load (i.e., Ravaz index) toward the "optimal" val- ues. Although the reduction in vegetative growth/vine size is usually greater than that of crop yield, growers should take into consideration a potential yield penalty associated with the use of under-trellis cover crops. 12 The reduction in vine size as- sociated with the use of under- trellis cover crops depends on, among other factors, the seasonal weather conditions, soil resources available and nutritional require- ments of the cover crop. Vines can also compete with cover crops for multiple resources at the same time, making it in some situations very difficult to separate the effect of moisture versus nutrient com- petition. In most of the studies conducted in upstate New York, North Carolina and Virginia, vine water status (stem water poten- tial) never reached what we de- f i n e a s a " s t r e s s f u l " v a l u e , indicating that under-trellis cover crops were not overly competitive with grapevines for soil moisture under the specific weather condi- tions of those regions and during the years studied. 4,6,9,10,11,15 How- ever, during consecutive seasons of summer drought in Long Island, vines with under-vine green growth exhibited more symptoms of water stress than those with an herbicide-treated strip. 13 At several sites, cool-season grasses depressed grapevine nitro- gen 6,12,15 levels relative to the un- der-trellis herbicide strip, which suggests that under-trellis cover crops can diminish vine nitrogen status and thus vine capacity and yeast assimilable nitrogen (YAN) levels in fruit and must. 15 Low con- centration of YAN in musts may lead to sluggish or stuck fermenta- tions and negatively impact aroma of wines. 17 While soil-applied ni- trogen could be used to maintain vine size and crop yield of cover cropped grapevines, target foliar- applied nitrogen near véraison has the potential to increase berry YAN to ensure fermentation success without negative effect on wine aroma development. 18 Establishing a leguminous cover crop under-trellis as a source of nitrogen may improve vine nitrogen status. However, timing and rate of nitrogen release are somewhat unpredictable and are influenced by site and climate conditions as well as management strategies. Furthermore, legumi- nous cover crops tend to lack per- sistence—they often need to be reseeded every two to three years, and they don't compete well with weeds. 9,13 Another under-trellis manage- ment option is to leave native vegetation growing under the vines instead of planting cover crops. At several of Vanden Heu- vel's research sites in upstate New York, native vegetation was al- lowed to grow under the vines. Compared to an herbicide-treated strip, native vegetation had a vari- able impact on vine size across sites, from nil effect at one site 8 up to 57% reduction in pruning weight in a young vineyard. 9 Site characteristics, plant material and age—in addition to diverse weed Overly vigorous Cabernet Franc (above) grows in the northeastern United States. Buckwheat (Fagopyrum esculentum) is planted beneath Cabernet Franc vines. Under-trellis growth of chicory (Chicorium intybus) reduced vegetative growth of the vines at right compared to those applied with glyphosate herbicide (left).