By: Kevin Martin, Penn State Extension Educator (Portland, NY)
We are starting to see increases in herbicide management costs. Some of you know all to well that 1-2 applications of herbicide do not provide adequate control of weed competition in vineyards. Complicated tank mixes that cost over $100 per applied acre are not a practice I would consider sustainable. Some growers, though, would disagree.
The cost of materials are not increasing substantially. More frequent applications and a need to apply better materials more often is driving costs up. The majority of herbicides used by growers are off patent these days and available almost exclusively in generic form. A third or even fourth vineyard pass, could be sustainable. The cost of materials and materials selected needs to be looked at comprehensively with the number of passes required to obtain adequate control.
There may be a potential for cover crops to improve the effectiveness of weed control. We can observe this not just in row middle management, but to a lesser extent under trellis management. Cover crops do not offer the potential to reduce herbicide applications in situations where growers are applying between 1 and 3 per year. Rather, they offer an option to improve results without adding an additional pass. This is because cover crops can reduce vine size when row middle competition is undesirable. In 2016 field trials we observed smaller berry size when cover crops were planted in the late summer of 2015 and were not terminated before June 1, 2016. Particularly where hard to control species get established, some growers have added a late summer or fall application to bring their total number of herbicide application to 4-5. In this scenario, the right cover crop mix offers the potential of superior control with one less pass. Cover crops do require some form of termination (usually chemical). By selecting the right species, a low rate of round-up may offer excellent row middle control.
Cover crop mixes being trialed are similar in cost to an herbicide application. Low-end rye grass and radish blends are comparable to many post emergent row middle applications. Higher end seed mixes with oats, more radishes or even buckwheat range between $12 and $15 per seeded acre in materials. Legumes increase costs but potentially reduce fertilizer use. Easy to kill hybrid crimson clover complicates the economic analysis. It may reduce urea applications by 50%, but could be more difficult to grow. Understanding effective seed mixes, their primary benefits and potential secondary benefits will be key to the success of moving cover crops into perennially systems in a cost-effective (saving) way. Regional differences in seed prices also complicated the matter. One of our primary suppliers of cover crop seeds in the Lake Erie Region is Ernst Seed Co. 2016 prices were used to calculate the cost of various seed mixes used in trials.
LERGP, led by Luke Haggerty, is taking an integrated look at cover crops in Concord vineyards. As he observes benefits, I’ll help quantify them. There is a lot we still do not know. While preliminary results show promise for increasing economic sustainability where herbicide program prices are spiraling upward, a few years’ worth of data will allow us to clearly observe measurable benefits in herbicide programs. Right now, it just seems like there are less weeds and more cover crop in row middles that have been seeded.
 Tang, Yijia, Miguel I. Gómez, Gerald B. White. COST OF ESTABLISHMENT AND PRODUCTION OF HYBRID GRAPES IN THE FINGER LAKES REGION OF NEW YORK, 2013. Cornell, Dec. 2014, http://publications.dyson.cornell.edu/outreach/extensionpdf/2014/Cornell-Dyson-eb1411.pdf Accessed 3 Nov. 2016.
 Bowman, Greg, Craig Cramer, and Christopher Shirley. Managing Cover Crops Profitability. 3rd ed.: Sustainable Agriculture Research and Education. SARE, July 2012, http://www.sare.org/Learning-Center/Books/Managing-Cover-Crops-Profitably-3rd-Edition Accessed 3 Nov. 2016. pp. 394.
 Id. At 122 – 124.
By: Dr. Michela Centinari
It is August already, which, for many grape growers in Pennsylvania, means veraison and the beginning of fruit ripening. It seems a good time to comment on the seasonal weather and how it can affect the vines. In July, above average temperatures were recorded in Pennsylvania , and drought conditions varied from ‘none’ to ‘severe drought’ across the state (Figure 1). The regions most affected by drought are North Central, Northwest, and some areas of Northeast PA .
In Figures 2 and 3 I reported the cumulative growing degree days (GDDs) (April to July) and precipitation (March to July) recorded at the two Penn State research and extension stations located in the South Central (Biglerville, Adams County) and Northwest (North East, Erie County) part of the state (http://newa.cornell.edu/). I also included the 2014 and 2015 data so you can compare the heat accumulation (GDDs), precipitation patterns and amount this year with those of the two previous seasons.
When looking at figures 1, 2, and 3, please keep in mind that local weather conditions vary greatly, shower and thunderstorm activity was hit or miss across the state. It is indeed recommended that growers install a weather station at their site to carefully monitor weather conditions and assist with disease control programs.
Compared to 2014 and 2015, this growing season started with lower heat accumulation in some areas of Pennsylvania, such as the Northwest (Figure 2A) and South Central (figure 2B) regions. Higher than average temperatures recorded in July however, pushed GDDs close to or above those of the same period last year. For example, in Erie County, cumulative GDDs were, by the end of July, above those accumulated in 2015 or 2104. In South Central PA GDDs are reaching the 2015 values and they are above those accumulated in 2014 for the same period (April-July).
The hot temperatures recorded in July can accelerate fruit ripening . For example, in Central Pennsylvania, Noiret (Vitis hybrid), which is not one of our earliest varieties, started to turn color last week (i.e., the first week of August), approximately 10 days earlier than last year.
While drought conditions have not been recorded in the Southeast and most of the Southwest regions, it has been dryer than average in the rest of Pennsylvania. For example, in North East (Erie County, Northwest) cumulative precipitation from March to July (13.6²) was 40% and 36% lower as compared to last year (22.6²) and two years ago (21.12²). In Biglerville (Adams County, South Central) cumulative precipitation from March to July (12.7²) was 33% and 31% lower as compared to last year (19.2²) and two years ago (18.4²).
Drought doesn’t always equal water stress
In- and across-season precipitation patterns in the eastern US are unpredictable. In our humid climate, precipitation and the soil water reservoir are usually sufficient to meet (or exceed) vine water requirements through ripening. Even if a drought period occurs, its duration and severity are not usually sufficient to warrant concern about moderate or severe vine water stress. Growers do however need to be aware that non-irrigated grapevines in temperate climates can occasionally face water stress during drought periods in the growing season [3; 4].
Hot temperatures, like those recorded in July, increase evapotranspiration and how much water the vine needs. This could facilitate the occurrence of vine water stress in areas that have been experiencing persistent lack of rain. The risk of water stress, indeed, not only depends on the amount of soil water available (supply), but also on how fast this water is used by the vines (demand) .
Along with seasonal rainfall and winter soil moisture other factors affecting the amount of water available (water supply) to the vines are:
- Soil water holding capacity which is determined by the soil textural properties: heavier soils (loam and clays) hold more water than light sands or gravels. For example, a unit volume of sandy-loam soil can hold about 50% as much water as a clay soil .
- Soil depth: deep soil can hold a greater volume of moisture than shallow soil  allowing grapevines, in the absence of restrictive layers, to develop a more extensive and deeper root system which can access deep resources of water during drought periods.
- Grapevine root system size and rooting depth: In addition to soil characteristics, also the age of the vine will influence root system size and rooting depth. Young vines have restricted root systems and rooting volume for several years, thus they are more sensitive to water stress than mature vines with well-established root systems .
- Presence of competitive plants, as green and actively growing cover crops and weeds in the middle-row and in-row areas.
Water demand is primarily driven by weather conditions (solar radiation, air temperature and humidity). For example, evaporation from an open pan under hot and dry weather (i.e., California) can be around 8-10 inches of water per month, whereas under cool and humid condition, typical of the northeast US can be less than 5 inches . Also the amount of sun-exposed transpiring leaf area and crop load will affect the amount of water used by the vines . For example vines trained to GDC or high-wire cordon tend to have greater sun-exposed leaf area that can capture more sunlight and use more water than those trained to vertical shoot positioning (VSP) . Heavily cropped vine vines also require more water for fruit ripening than vines with a smaller crop .
Vine response to water stress varies with the severity of the stress and the timing of the season it develops
Growth processes (i.e., shoot growth, early berry growth) are more sensitive to water deficit than photosynthesis . Therefore, a mild/slight water stress between fruit-set and veraison can favorably diminish vegetative growth and reduce berry growth leading to smaller berries with potentially higher skin to pulp ratio without compromising photosynthesis and carbohydrates/sugars production . Under moderate to severe water stress conditions, however, photosynthetic activity is reduced possibly leading, early in the season, to poor canopy development and function. Later in the season (after veraison) a reduction in photosynthesis can decrease sugar accumulation in the berries with a negative effect on fruit ripening and flavor development. Further, a reduced storage of carbohydrates and other nutrients in perennial organs may occur. Thus, it is crucial to maintain a healthy and functional canopy after veraison to avoid negative effects on fruit or wine quality and cold hardiness. Furthermore, because after veraison, berry growth is quite resistant to water stress, a post-veraison water deficit is not as effective in reducing berry size as a pre-veraison one .
Growing up in Italy, I remember the old-world “wine dilution theory” that supported the idea that any irrigation after veraison would lead to an increase in berry size (due to water dilution) and a reduction in wine quality . There was not strong scientific evidence, however, supporting this assumption. It was actually found that water doesn’t move into the berry after veraison due to complete or partial lost in xylem functionality  which proved that irrigating the vines after veraison doesn’t actually impact berry size . Thus, nowadays it is recommended to avoid moderate to severe water stress after veraison to ensure vine health and proper ripening and flavor development.
Symptoms of vine water stress:
Since vines change in appearance under water stress conditions it is a good practice to walk through the vineyard and look for sign of water stress, starting with young vines. A comprehensive table that summarized visual symptoms of increasing water stress from mild to severe can be found in the “Wine grape production guide for eastern North America” (page 172) and also available in the July issue of Viticulture Notes  edited by Tony Wolf, professor of viticulture at Virginia Tech University.
Below I summarized some of the visual indicators of vine water status, from ‘well-watered’ to ‘severe drought’ conditions 
Well-watered vines (Figure 4):
- Shoot tips are actively elongating
- Tendrils are turgid and expand well beyond the shoot tip
- Leaves orientation: leaf blades are oriented toward the sun
- Leaf color and temperature: canopy is green and healthy and leaves are cooler than our body temperature
- Berries are turgid
Mild to moderate water-stressed vines:
- Shoot tips are compressed and they are enclosed when the last formed leaves are pushed toward the growing tip (Figure 5A)
- Tendrils are drooping or wilted
- Leaf orientation: leaves are oriented away from the sun
- Leaf color and temperature: leaves (starting from the basal leaves) are grayish-green to light-green and they are warm to touch at mid-day (> 100°F)
- If it occurs around bloom/ fruit-set, berry-set may be reduced
Severe water-stressed vines:
- Shoot growth has stopped and shoot tips are dry or aborted
- Tendrils dried or abscised
- Leaf orientation: leaves may roll and dry
- Leaf color and temperature: leaves (starting from the basal leaves) are yellow with necrotic edges (Figure 5B) and they are very warm (well above 100°F)
- Cluster rachis tip may dry if stress occurs at bloom, fruit-set may be reduced, berries may become flaccid if water stress occurs post-veraison
Water stress in a young planting must be avoided because it can compromise root system establishment and overall vine growth, delay its capability to carry a crop, and reduce cold hardiness. If you notice signs of water stress in young vines and you don’t have a permanent and functioning irrigation system in place, temporary irrigation systems could be used such as a flex tank and hose. It is a very labor intensive operation but it is crucial to ensure the long-term success of your investment. If you notice any sign of severe water stress on your mature vines and you are not able to irrigate them you may want to consider shoot and crop-thinning (especially in heavily cropped vines) to reduce vine demand for water, as well as avoid growth of weeds which can compete with vines for water supply .
- United States Drought Monitor: http://droughtmonitor.unl.edu
- Wolf TK. Viticulture Notes. Vol 31 No. 5. 23 July 2016. Virginia Tech University Cooperative Extension. Available at: http://www.arec.vaes.vt.edu/alson-h-smith/grapes/viticulture/extension/growers/current_VN_newsletter.pdf.
- Hayhoe K, Wake CP, Huntington TG, Luo L, Schwartz MD, Sheffield J, Wood E, Anderson B, Bradbury J, DeGaetano A, Troy TJ and Wolfe D. 2007. Past and future changes in climate and hydrological indicators in the US Northeast. Climate Dynamics 28, 381–407.
- Schultz HR and Stoll M. 2010. Some critical issues in environmental physiology of grapevines: future challenges and current limitations. Aust. J. Grape Wine Res. 16, 4–24.
- Lakso AN. 2000. Basics of Water Balance in New York Vineyards. 29th NY Wine Industry Workshop, NYS Agric. Exper. Sta., p 94–101.
- Wolf TK. 2008. Wine grape production guide for Eastern North America. Natural Resource, Agriculture, and Engineering Service: Ithaca, NY USA.
- Keller M. 2010. The Science of Grapevines: Anatomy and Physiology. Publisher: Academic Press.
- Hansen M. 2016. Rethinking post-veraison irrigation. Vineyard & Winery Management. July-August, 2016. 60–
- Hoheisel G, Moyer M. Grapevine management under drought conditions. Washington State University Extension. EM4831E. Available at : http://cru.cahe.wsu.edu/CEPublications/em4831e/em4831e.pdf
By: Dr. Michela Centinari
In cool climate vineyards labor (hours/acre) employed for canopy management operations (e.g., shoot positioning and thinning, cluster thinning, leaf removal, hedging) is estimated to be much higher than for floor management practices (approximately 32% vs. 11% of total labor hours, excluding harvest) . 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 . The main goals of vineyard floor management span from weed control, soil conservation, soil nutrient and water management, and biodiversity improvement . 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 . Environmental regulations and public perceptions may also influence growers’ choices toward a specific floor management practice .
The conventional floor management practice for mature vineyards in the eastern U.S. 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 (Figure 1).
Why explore alternative under-trellis management options?
- Seeking environmentally responsible and (potentially) affordable alternatives to the use of herbicides
Herbicides are the most widely used pesticides in the U.S.; they are commonly used in vineyards because they effectively suppress weed growth and are easy to apply . However, repeated herbicide use increases the risks of resistance development . 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 .
- Reducing excessive vine growth and the need for expensive canopy management practices
Despite the presence of cover crops between the rows (inter-row area), grapevines can still exhibit excessive vegetative growth in regions with deep,fertile soils and ample precipitation during the growing season (Figure 2). Large and dense canopies with heavily shaded fruit may contribute to reduced fruit and wine quality as well as an increase in disease pressure . 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 practices  often required to improve clusters light environment.
What are the alternative under-trellis options?
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) (i.e., complete floor cover) (Figure 3). Under-trellis cover crops could serve multiple purposes depending on grower’s needs and the cover crops species used. Cover crops species have different degrees of competition with vines for soil resources (e.g., 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 (e.g., Festuca ovina) tend to be less competitive than other perennial grasses such as tall fescues (Festuca arundinacea Shreb) .
Several studies have been conducted in the eastern U.S. over the last 10 years to test if and which under-trellis cover crops species could be used as an environmentally responsible means to suppress the use of herbicide and as a proactive measure to reduce excessive vine grow through competition with the grapevine root system for soil resources.
Do under-trellis management strategies impact leaching of nutrients and pesticide?
Yes, under-trellis management strategies can impact leachate composition [4;9]. For example dissolved organic carbon (DOC) leaching was reduced by maintaining a green cover under the trellis (white clover or native vegetation) as compared to conventional under-trellis strategies, as herbicide (glyphosate) application or soil cultivation . 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 (N) concentration was higher in leachate samples collected from under-trellis glyphosate and white clover plots as compared to those from native vegetation or cultivation plots.
What cover crops can growers use to suppress excessive vine growth and increase crop load?
Cool season perennial grasses have been planted under the trellis in vineyard located in Virginia, North Carolina (Dr. Tony Wolf lab, Virginia Tech University) and in Long Island (NY) (Alice Wise, Cornell Cooperative Extension) to impose competition for moisture and nutrients early during the growing season, when grapevines grow vigorously (at least in the Eastern U.S.) [8;9;10].
Compared to an herbicide-treated soil strip, complete vineyard floor cover cropping reduced excessive vigor of Cabernet Sauvignon vines in a wet and humid region (NC) over a six-year-period . 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 .
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 (average of 26%) and increased sunlight fruit exposure by 35% over a 7-year-period as compared to herbicide-treated strip .
(Note: This study was conducted at the Virginia Tech’s AHS, Jr. Agricultural Research and Extension Center, in a ‘research’ vineyard with high vigor potential. We don’t suggest using under-vine cover crop in young vineyards without knowing the history and vigor potential of the site.)
Interestingly, the growth suppressive effect of the under-trellis fescue decreases over the years suggesting 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 .
Chicory (Chicorium intybus) annually planted under the trellis of mature, vigorous vineyards in the Finger Lakes region of NY resulted in considerably diminished vine size, up to 35 % (Figure 3) [Dr. Justine Vanden Heuvel lab, Cornell University; 12; 13]. Furthermore, chicory effectively suppressed weed pressure underneath the grapevine canopy.
When vine growth is vigorous, the balance between vegetative and reproductive growth tend to be below the recommended Ravaz index (yield /pruning weight) ratio ranging from 4 to 10 for high-quality wine production . 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 .
Can under-trellis cover crops impact fruit composition?
In several studies the use of under-trellis cover crops did not significantly impact, either positively or negatively, juice chemistry [total soluble sugar (°Brix), pH, titratable acidity (TA)] [4;9;15;16;17] or total phenolics or anthocyanins . 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 [ 10;12].
What causes the reduction in vine size associated with the use of under-trellis cover crops?
It 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 vs nutrient competition. In most of the studies conducted in upstate NY, NC and VA vine water status (stem, pre-dawn water potential) never reached what we define 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;10;12;13]. 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 
At several sites, cool-season grasses depressed grapevine nitrogen [4, 10; 12] levels relative to the under-trellis herbicide strip suggesting that under-trellis cover crops can affect vine nitrogen status and thus vine capacity .
What about leaving native vegetation growing under the trellis instead of planting cover crops?
At several research sites in upstate NY, native vegetation was allowed to grow under the vines (Dr. Vanden Heuvel lab, Cornell University). Compared to an herbicide-treated strip, native vegetation had a variable impact on vine size across sites, from nil effect at one site  up to 57% reduction in pruning weight in a young vineyard . 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 (NY) have been experimenting with under-trellis mowing (i.e., native vegetation mowed several times throughout the season). Costs of under-trellis mowing compare well to the herbicide regime (approximately $120/acre) . A. Wise 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 go from $3,500 for a single head to over $15,000 for two mowing heads with a row middle mower” (Figure 5). 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”.
Can under-trellis cover crops be used to eliminate the use of herbicide while maintaining vine size?
Planting annual cover crops, such 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, upstate NY, at Cornel University by Dr. Vanden Heuvel lab [4; 15; 16; 17]. In the Northeastern U.S. 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 temperature. For example, buckwheat planted in late-May in in mature vineyards in upstate NY (seeding rate is approximately equal to 350 lbs/acre) was found to compete extremely well with weeds pressure without compromising vine size [13; 16] or fruit composition  (Figure 6).
How much will establishing and maintaining under trellis cover crops cost?
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 (Table 1). In this study establishing and maintaining white clover or native vegetation was a cheaper under-trellis groundcover options than repeated soil cultivation or glyphosate applications. However, vines maintained with herbicide (glyphosate) generated the highest revenue because of their higher yield. Thus, yield penalty associated with vines growing with under-trellis cover crops was the cause of reduced grower’s income. 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 lbs/acre), creeping red fescue can be more expensive to establish because of its higher seeding rates, as much as 220 lbs/acre . However, fescue can persist for more than four years, while white clover would need to be reseeded every 2-3 years .These figures should be used only as an example because there are many variables that could change the outcomes 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 grape growers 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 to monitor pruning weight and Ravaz index on sentinel vines. It is also important to annually assess vine nutrient status and be prepared to efficiently apply fertilizer 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 crops species.
- Yeh AD, Gomez MI, White GB. 2014. Cost of Establishment and production of vinifera grapes in the Finger Lakes region of New York-2013. Cornell University Department of Applied Economics, 716 Ithaca, NY. 717.
- Guerra B, Steenwerth K. 2012. Influence of floor management technique on grapevine growth, disease pressure, and juice and wine composition: a review. Am J Enol Vitic 63:149–
- Skinkis P. Overview of Vineyard Floor Management. eXtension.org.
- Karl A. 2015. Impact of under-vine management in a Finger Lakes Cabernet Franc vineyard. MS thesis, Cornell University, Ithaca, NY.
- Holt JS. 1992. History of the identification of herbicide-resistant weeds. Weed Technol. 6:615–620.
- Landry D, Dousset S, Andreux F. 2006. Leaching of oryzalin and diuron thorugh undisturbed vineyard soil columns under outdoor conditions. Chemosphere 62:1736–1747.
- Austin CG, Grove GG, Meyers JM, Wilcox WF. 2011. Powdery mildew severity as a function of canopy density: associated impacts on sunlight penetration and spray coverage. Am J Enol Vitic 62: 23–31.
- Giese G, Velasco-Cruz C, Roberts L, Heitman J, Wolf TK. 2014. Complete vineyard floor cover crops favorably limit grapevine vegetative growth. Scientia Hortic. 170:256-266.
- Wise A. 2015. Innovative undertrellis management for vineyards. Available at: http://mysare.sare.org/sare_project/lne12-322/?page=final.
- Hickey CC, Hatch TA, Stallings J, Wolf TK. 2016 Under-Trellis Cover Crop and Rootstock Alter Growth, Components of Yield, and Fruit Composition of Cabernet Sauvignon. Am J Enol Vitic. doi: 10.5344/ajev.2016.15079.
- Klodd AE, Eissenstat DM, Wolf TK, Centinari M. 2016. Coping with cover crop competition in mature grapevines. Plant and Soil 4:391–400.
- Jordan LM. 2014. Evaluating the effects of using annually established under-vine cover crops in northeastern Riesling vineyards. MS Thesis, Cornell Univ., Ithaca, NY.
- Karl A, Jordan L, Vanden Heuvel JE. 2015. Investigating annual under-vine cover crops as an enviromentally sustainable alternative to herbicides in northeastern vineyards. SARE Final report.
- Kliewer WM, Dokoozlian NK. 2005. Leaf area/crop weight ratios of grapevines: Influence on fruit composition and wine quality. Amer. J. Enol. Vitic. 56:170–181.
- Centinari M, Vanden Heuvel JE, Goebel M, Smith MS, Bauerle TL. 2016. Root-zone management practices impact above and belowground growth in cabernet franc grapevines. Aust J Grape Wine Res. 1: 137–
- Jordan LM, Björkman T, Vanden Heuvel 2016. Annual under-vine cover crops did not impact vine growth or fruit composition of mature cool-climate ‘Riesling’ grapevines. HortTech 26: 36–45.
- Karl A, Merwin IA, Brown MG, Hervieux RA, Vanden Heuvel JE. 2016. Impact of undervine management on vine growth, yield, fruit composition, and wine sensory analyses of Cabernet franc. Am J Enol Vitic. doi: 10.5344/ajev.2016.1506.
By: Annie Klodd
My answer is: Maybe, but we’re not sure why. This was the question that motivated me to study the effectiveness of under-vine perennial cover crops for my Master’s degree thesis. Tremain Hatch, now a viticulture extension associate at Virginia Tech, found that in three consecutive years, creeping red fescue grass planted directly under the vines reduced pruning weight by 15-25% in mature Cabernet Sauvignon/101-14 vines (Hatch et al. 2011). This came with small reductions in yield while maintaining healthy vines.
The potential economic cost of reduced yields might be made up for in the money saved on summer pruning and herbicide application. Reduced vegetative growth may translate to less excess summer vine vigor, and thick turf under the vines could completely eliminate herbicide needs. This method seems to have a lot of potential for promoting healthy vine balance, but before the idea gets too far out there, it is important to know what type of stress the grass is inflicting on the vines. We need to know why the vines exhibit reduced vegetative growth, in order to know when and where this practice will work.
Getting to the root of it (literally):
In order to pinpoint how the grass impacts the vines, we had to turn to the roots, where the real competition takes place. The study took place at Virginia Tech’s Alson H. Smith Agriculture Research and Extension Center, in a mature Cabernet Sauvignon 101-14 vineyard where some vines grow with an under-vine cover crop of creeping red fescue grass, some treated with a conventional herbicide strip. We dug soil cores to one meter deep, directly under the vine trellis. We removed all the roots from this soil, and measured the total length and biomass of the vine roots at every depth. We also analyzed the nutrient availability in the soil and vines, measured soil moisture throughout the summer, and compared yield and pruning weights of the vines in both ground-cover techniques.
This is what I found:
1. Grass roots limit vine root access to essential nutrients
Several years of data in the vineyard revealed lower petiole phosphorus levels in vines grown with the cover crop, compared to vines grown with conventional herbicide application. I also found that the cover crop reduced nitrogen availability in the soil. While these levels were still in a healthy range, I hypothesized that phosphorus and nitrogen limitation might be main factors limiting vine growth in cover-cropped plots.
My results support this hypothesis. The grass cover crop roots dominated the top 6 inches of the topsoil, where the highest concentration of soil nutrients lie. As a result, the vast majority of the vine roots were pushed down into deeper soil, where nutrient concentrations (phosphorus and nitrogen) are typically much lower. The cover-cropped vines also possessed smaller overall roots systems. Root length and biomass both decreased by about 20% compared to the conventionally-grown vines. The smaller grapevine root systems may be related to smaller vine biomass above ground.
In all, it appears that the cover crop limited vine growth by restricting the vine’s access to essential nutrients that are necessary for growth. The grass roots achieved this through a combination of pushing the vine roots away from nutrient-rich soil, and reducing the overall amount of roots that can forage for available nutrients.
2. The cover crop helped retain soil moisture
To our surprise, the perennial cover crop did not cause soil moisture shortage. On the contrary, the cover cropped soil retained higher moisture. This is likely because the grass created a thick mulch layer on the ground, showing evaporation. We also performed an experiment to test whether the cover crop forced the vine roots to forage deeper for water, into the deep clay soil where water extraction takes more energy. This was not the case, however – even though the cover crop pushed the vine roots toward deeper soil, the vines in both treatments still took up their water from around 1.5 feet deep.
Will the results hold up in Pennsylvania vineyards?
Our study took place over one growing season, in one vineyard in northern Virginia. While our northern Virginia vineyard has a fairly similar climate to southern PA, every vineyard is different. And, as we all know well, every growing season is different in the mid-Atlantic. But, the results of this study do suggest an important, central trend that I believe can help determine whether perennial cover crops are appropriate in a given location. In my study, the grapevine roots responded to competition with other plants by avoiding them, and moving to deeper soil. Outside of my research, I have also observed this in vineyards in which weeds or inter-row cover crops kept the grape roots out of shallow soil. Based on these observations, I can make an informed hypothesis: Across soil environments and climates, the grapevine roots are still likely to move into deeper soil in response to cover crop roots.
This is where the micro-environments of individual vineyards come into play: if grape roots are forced into deep soil in vineyards where the soil is unfavorable, the results may be an exaggerated from what we found this year. For example, if the soil already nutrient-deprived, the phosphorus decreases we observed may be exaggerated to levels that are unhealthy for the vine. On a different note, although we did not observe cover crop-induced water stress during this humid growing season, a particularly dry season may lead to stiff competition for water between the vines and grass roots. In this case, the grass may deplete limited water instead of helping the soil retain it.
That being said, I do think that perennial cover crops have strong potential to positively impact the efficiency, vine balance, and soil health of Pennsylvania vineyards. I hope to see them become a larger part of the mid-Atlantic industry in coming years.
Annie is a M.S. graduate student in Plant Biology under the direction of Dr. David Eissenstat and Dr. Michela Centinari. A special thank you goes to Dr. Tony Wolf and his team at Virginia Tech University for us of the Virginia Tech vineyard, his time, and his assistance with field measurements.