Conventional vineyard floor management is shown in a vineyard in the eastern United States.

In cool-climate vineyards, labor (calculated as hours per acre) employed for canopy-management operations such as shoot positioning and thinning, cluster thinning, leaf removal and hedging, is estimated to be much higher than for floor-management practices. Excluding harvest and pruning, canopy management takes approximately 32% of total labor hours compared to 11% for floor management.¹ Canopy-management practices are critical for optimizing crop load, improving microclimate conditions in the fruiting zone and reducing disease pressure on the leaves and fruit.

However, we tend to forget that floor management also has profound implications for the vineyard ecosystem, productivity and, indirectly, wine quality.² The main goals of vineyard floor management span from weed control, soil conservation, soil nutrient and water management, to biodiversity improvement.² Among many factors, the best floor-management strategy for a given vineyard site depends on the age of the vine, growing region, soil type and production goals of the grower.³ Environmental regulations and public perceptions may also influence growers’ choices toward a specific floor-management practice.²

The conventional floor-management practice for mature vineyards in the eastern United States and other temperate regions around the world is a cover-cropped inter-row combined with a vegetation-free area directly beneath the vines to reduce competition for soil resources (i.e., water and nutrients). The under-trellis area is kept bare using herbicides and, in some cases, by soil cultivation (see photo above).

Several studies have been conducted in the eastern United States during the past 10 years to test if and which under-trellis cover crop species could be used as an environmentally responsible means to suppress the use of herbicide and as a proactive measure to reduce excessive vine growth through competition with the grapevine root system for soil resources.

Cover crops as an herbicide alternative
Herbicides are the most widely used pesticides in the United States because they effectively suppress weed growth and are easy to apply.⁴ However, repeated herbicide use increases the risks of resistance development.⁵ Also, bare soil left exposed after herbicide use or tilling is susceptible to erosion, soil structure degradation and crusting, as well as increased water runoff and leaching of nitrates and pesticides.⁴

One alternative option to the under-trellis herbicide-treated strip is to establish cover crops directly beneath the vines (intra-row) in addition to between the rows (inter-row), thereby creating a complete floor cover (see photo at right). Under-trellis cover crops could serve multiple purposes, depending on growers’ needs and the cover crops used.

Cover crop species have different degrees of competition with vines for soil resources such as water and nutrients. For example, annual cover crops tend to be less competitive for soil nitrogen resources than perennial plants because of their shorter growth cycle and less root development. Furthermore, fine fescues such as Festuca ovina tend to be less competitive than other perennial grasses such as tall fescues (Festuca arundinacea Shreb).⁶

Planting annual cover crops such as buckwheat (Fagopyrum esculentum) or annual ryegrass (Lolium multiflorum) under the vines may eliminate the need for herbicide with little impact on vine size or fruit composition. Most of the work on under-trellis annual cover crops has been conducted in the cool and humid Finger Lakes region.^4,7,8,9 In the northeastern United States, perennial cover crop species are not suitable for the under-trellis area due to the need to mound soil from the under-vine row around the graft union for insulation to protect scion budwood from low winter temperatures. However, annual buckwheat planted in late May in mature vineyards in upstate New York (at a seeding rate approximately equal to 350 pounds per acre) was found to compete extremely well with weed pressure without compromising vine size^8,10 or fruit composition⁷.

Cover crops to reduce excessive vine growth and increase crop load
Despite the presence of cover crops between the rows (the inter-row area), grapevines can still exhibit excessive vegetative growth in regions with deep, fertile soils and ample precipitation during the growing season. Large and dense canopies with heavily shaded fruit may contribute to reduced fruit and wine quality as well as an increase in disease pressure.¹¹ In addition, excessively vigorous grapevines are more expensive to train and manage. Main and later shoot thinning, multiple passes of fruit-zone leaf removal and hedging are costly management practices¹ often required to improve the light environment for the clusters.

Cool season perennial grasses have been planted by researchers under the trellis in vineyards located in Virginia, North Carolina, Long Island, N.Y., and Pennsylvania to impose competition for moisture and nutrients early during the growing season, when grapevines grow vigorously in the eastern United States.^6,12,13

Compared to an herbicide-treated soil strip, complete vineyard floor cover cropping reduced excessive vigor of Cabernet Sauvignon vines in the wet and humid region of North Carolina over a six-year-period.⁶ Among the perennial grasses tested, Festuca arundinacea cv. Elite II (a turf-type tall fescue) and cv. KY-31 (a forage-type tall fescue), were the most effective in reducing vine vigor and increasing light available to the fruit with minimum impact on crop yield.⁶

Creeping red fescue (Festuca rubra) planted under Cabernet Sauvignon vines in the fall of the second year of vineyard establishment reduced vine size in a favorable way (an average of 26%) and increased sunlight fruit exposure by 35% over a seven-year-period compared to herbicide-treated strip.10

It is important to note that this study was conducted by Dr. Tony Wolf at Virginia Tech’s AHS Jr. Agricultural 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 vineyards without knowing the history and vigor potential of the site.

Interestingly, the growth-suppressive 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 Cornell University research plot in the Finger Lakes region of New York resulted in considerably diminished vine size, up to 54% (see bottom photo at left).^10,15 Furthermore, chicory effectively suppressed weed pressure underneath 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 effect of the under-trellis cover crops often translated to an increase in crop load (i.e., Ravaz index) toward the “optimal” values. 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 associated with the use of under-trellis cover crops depends on, among other factors, the seasonal weather conditions, soil resources available and nutritional requirements 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 competition. In most of the studies conducted in upstate New York, North Carolina and Virginia, vine water status (stem water potential) never reached what we define as a “stressful” value, indicating that under-trellis cover crops were not overly competitive with grapevines for soil moisture under the specific weather conditions of those regions and during the years studied.^{4,6,9,10,11,15} However, 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 nitrogen^6,12,15 levels relative to the under-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 concentration of YAN in musts may lead to sluggish or stuck fermentations and negatively impact aroma of wines.¹⁷ While soil-applied nitrogen 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, leguminous cover crops tend to lack persistence—they often need to be reseeded every two to three years, and they don’t compete well with weeds.^9,13

Another under-trellis management option is to leave native vegetation growing under the vines instead of planting cover crops. At several of Vanden Heuvel‘s research sites in upstate New York, native vegetation was allowed to grow under the vines. Compared to an herbicide-treated strip, native vegetation had a variable impact on vine size across sites, from nil effect at one site⁸ up to 57% reduction in pruning weight in a young vineyard.⁹ Site characteristics, plant material and age—in addition to diverse weed populations at the research sites— may explain the variable impact of native vegetation on reducing vine size.

Growers in Long Island, N.Y., have been experimenting with under-trellis mowing of native vegetation several times throughout the season. Costs of under-trellis mowing compare well to the herbicide regime (approximately $120 per acre).¹³ Alice Wise, senior viticulture research and extension associate with Cornell Cooperative Extension of Suffolk County, N.Y., pointed out that, while more growers are interested in under-trellis mowing, a deterrent for its adoption may be the price of suitable mowers, which range “from $3,500 for a single head to more than $15,000 for two mowing heads with a row middle mower” (see top photo on page 80). Wise also noted that “under-vine mowing is a viable option for under-vine management, though it is best suited to mature, laser-planted vineyards because of the risk of trunk damage to young vines and crooked trunks.”¹³

Impact of under-trellis cover crops on root distribution and size
Research on under-trellis cover crops focused more on the manifestations of cover crop/vine competition than on the mechanisms. Understanding how cover crops impact the distribution of grapevine roots and other morphological and physiological root traits may help to reduce uncertainty of vine response to cover cropping.

We found that vines managed with under-trellis cover crops for a short term (three years) and long term (seven years) had a deeper distribution of fine roots, which are responsible for the majority of water and nutrient uptake, compared to those managed with an under-trellis bare soil strip.^7,14 A decreased proliferation of fine roots in shallow soil layers (0-20 cm) due to cover crop competition may reduce vine access to nutrients, often more abundant in shallow rather than deep soil strata.¹⁴

We also found that vines managed with under-vine creeping red fescue over a seven-year period had a 45% lower absorptive root biomass than those growing without under-vine grass. Surprisingly, despite the shift in root distribution toward low-fertility deep soil and smaller root system, vines growing with under-vine grass accessed enough resources to maintain aboveground vegetative growth, suggesting that grapevines may be capable of acclimating to cover crop competition.¹⁴ This study, however, was conducted during a humid season (rainfall from grapevine bud burst to harvest = 584 mm). A drier year or a less fertile site may result in more reduced canopy growth.

Other impacts of under-trellis management strategies
Under-trellis management strategies can impact leachate composition.^4,13 For example, dissolved organic carbon leaching was reduced by maintaining a green cover—white clover (Trifolium repens) or native vegetation—under the trellis compared to conventional under-trellis strategies such as herbicide (glyphosate) application or soil cultivation.⁴ Thus, not only herbicide application but also soil cultivation can increase carbon loss from the soil, in addition to increasing its erosivity. Also, total nitrogen concentration was higher in leachate samples collected from under-trellis glyphosate and white clover plots compared to those from native vegetation or cultivation plots.⁴

In several studies, the use of under-trellis cover crops did not significantly impact, either positively or negatively, juice chemistry—including total soluble sugar (Brix), pH and titratable acidity (TA), or total phenolics or anthocyanins.¹² In other cases, an increase in juice Brix or reduction in TA (1 g/L) was attributed to the devigorating effect and increased fruit sunlight exposure brought on by the presence of under-trellis cover crops.^12,15

The cost of establishing and maintaining under trellis cover crops
A partial budget analysis was developed for one of the studies on under-trellis cover crops using information gained from the research trial and grower cost estimates (see table “Partial Budget Analysis Comparing Impact of Undervine Groundcover on Yield and Management Costs, 2011-13”).⁹ In this study, establishing and maintaining white clover or native vegetation was a cheaper under-trellis groundcover option than repeated soil cultivation or glyphosate applications. However, vines maintained with herbicide (glyphosate) generated the highest revenue because of their higher yield. Thus, the yield penalty associated with vines growing with under-trellis cover crops was the cause of reduced grower’s income.⁹

The decrease in yield of cover-cropped vines was exacerbated by their young age; the vines were four years old when cover crops were established under the trellis. Outcomes could definitely change if crop yield is maintained or only slightly reduced, or if other cover crops are used. Compared to white clover (seeding rate: 5 pounds per acre), creeping red fescue can be more expensive to establish because of its higher seeding rates, as much as 220 pounds per acre.¹³ However, fescue can persist for more than four years, while white clover would need to be reseeded every two to three years.¹³

These figures should be used only as an example because there are many variables that could change the outcomes,¹³ and growers interested in experimenting with under-trellis cover crops should develop their own cost analysis. Also, vigor suppression associated with under-trellis cover crops may result in reduction of costly canopy management operations.

For grapegrowers managing vigorous vineyards and interested in reducing pesticide input, complete vineyard floor cover could be a viable option. However, to avoid an undesirable decline in pruning weight, vine nutrient deficiency or water stress, it is recommended that the grower monitor pruning weight and Ravaz index on sentinel vines. It is also important to assess vine nutrient status annually and be prepared to apply fertilizer efficiently, if needed. In a dry season growers should look closely for visual symptoms of vine water stress. To avoid over-devigorating the vines, it may be possible to start with an “aggressive” cover crop and switch after a few years, when vine balance is achieved, to a less competitive cover crop species.

Dr. Michela Centinari is an associate professor of viticulture in the Department of Plant Science at Pennsylvania State University in State College, Pa.

Literature cited
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