By Brianna Hoge (February 2018)
Vine balance is defined as the state at which vegetative vigor and fruit load are in equilibrium and can be sustained indefinitely while maintaining healthy canopy growth, adequate fruit production, and high fruit quality. It is a critical concept in professional vineyard management. Balanced vines have increased light in the canopy, resulting in minimized leaf and fruit shading, thus maximizing carbohydrate production for vegetative growth and fruit quality. Optimizing light and temperature can improve color, enhance flavor compounds, decrease pH and potassium content, and reduce vegetative aromas. Balanced canopies also have increased airflow, which helps reduce canopy moisture and improves spray penetration, both of which are crucial to reducing disease risk. All these factors lead to healthy, productive vines capable of producing high quality fruit. Unfortunately, there is no one-size-fits-all recommendation that can be implemented.
Environmental factors impacting balance revolve around characteristics such as soil depth and type and nutrient and water availability. Vineyards with deep fertile soils and relatively unlimited moisture and sunlight are at greater risk of growing excessively vigorous vines. This type of vine produces shoots with large leaves, long nodes, and excessive lateral shoot development. The abundance of vegetation means that the fruit and renewal zones are shaded, resulting in poor bud development for the following year’s crop, inferior fruit quality, higher disease pressure, and poor periderm formation, decreasing cold hardiness. These same vines, with proper management practices will be able to produce a balanced ratio of fruit to canopy allowing for quality fruit production as well as plentiful nutrient storage reserves to promote winter hardiness and sustain post-budbreak growth the following season.
On the contrary, vines planted in soils with limited water and nutrient resources will be able to produce less canopy and will have lower carbon levels. Therefore, they will not be able to sustain as large a crop load. Drought, shallow soils, weed competition, insufficient nutrients, and disease pressure can lead to insufficient vigor- sparse canopy with little or no ability to ripen a crop. Another route to the same problem can be found in overcropping. Excess crop load without enough canopy to support it can cause insufficient photosynthetic capacity, poor fruit maturation, and increased cold susceptibility. The short-term benefits of high yields must be weighed against the negative impact on fruit quality and the long-term effects this stressor may have on vineyard health and longevity.
Rootstock and scion (cultivar) also play a role in vine growth and development. Many rootstocks have beneficial qualities, such as increasing and decreasing vigor of various grape cultivars, which demonstrate a range of vigor on their own. Research has shown that rootstocks have the potential to affect not only growth potential, but fruiting potential, pest resistance, water efficiency, and nutrient uptake, all of which influence vine growth and development (Skinkis and Vance, 2013). This should be taken into account when selecting rootstocks in order to maintain proper vine balance.
Vineyard management practices which affect vine balance include irrigation, fertilization, pruning, thinning, and vineyard floor management. All of these practices, conducted strategically can either enhance or decrease vine balance. High vigor vines may benefit from competition with cover crops for nutrients and water keeping growth in check, while lower vigor vines may be unable to obtain adequate resources for healthy growth and fruit development. Similarly, fertile soils with good water holding capacity are likely to produce high vigor vines, and will not require the fertilization and irrigation inputs a site with low vigor vines and lower nutrient and water availability would. Management practices should be site specific and keep cultivar characteristics in mind in order to be effective in steering vines towards balance.
Balance of Source vs. Sinks
To understand vine balance, we need to understand grapevine physiology and the concept of carbon sources and sinks. During the growing season, carbon is produced through the photosynthetic activity of the canopy (source). Before vegetative growth is adequate to supply the rest of the vine, the carbon reserves stored the previous season function as a source to support early season shoot growth and flower development (sinks). The major carbon sources and sinks within a vine changes over the course of growing and dormant seasons as depicted below.
Remember: Imbalanced vines lead to lessened ability to transition into dormancy via shoot lignification and cold hardiness reduction. Overly vigorous vines continue to grow past véraison and have excess canopy shading and less shoot lignification late in the season. Research suggests that vine hardening is influenced less by crop load than canopy shading caused by excess vegetation, so canopy management in addition to crop load is critical to preventing frost damage (Howell and Shaulis, 1980; Reynolds et al., 1986).
Measuring Vine Balance
Pruning weight and yield reflects the final size of vines given environmental factors and management practices. It should be noted that if crop thinning is done at lag phase, data can be collected at the time of thinning to calculate potential total crop load and compare it to actual crop load at harvest (Skinkis and Vance, 2013). A Ravaz value of 5-10 is considered optimal for Vitis vinifera cultivars in warmer climates. Values at the low end of the range are considered under-cropped or highly vigorous, and there’s a larger canopy size compared to fruit yield. Conversely, values at the high end of the range are considered over-cropped or low vigor and have larger fruit yield compared to canopy size. In either case, vines are unbalanced, resulting in unsustainable vine growth and fruit quality.
Vine balance can be measured several ways, but the two most common are use of the Ravaz index and the leaf area to fruit yield ratio. The Ravaz index, also known as the crop load method, is the most common and practical for commercial growers. It’s calculated using fruit yields at harvest and dormant pruning weights during winter following harvest.
Ravaz value = vine yield / dormant pruning weight
Implementing Good Practices
Canopy management through direct and indirect methods. Direct methods include shoot thinning, leaf removal, and crop thinning; while indirect methods include irrigation, fertilization, and vineyard floor management.
Vineyard floor management, including weed control, and cover cropping can alter vine vigor by changing nutrient and water availability. High vigor vineyards benefit from the use of cover crops, as they reduce vine growth and restrict potential rooting volume. Research conducted in high vigor vineyards showed a reduction in vine vigor and natural yield, producing vines that were more balanced than those in tilled, non-grass cover treatments. For moderate or lower vigor vineyards, certain cover crops may enhance vigor by increasing soil moisture or nutrition. For instance, alternating legumes or grass cover and tillage in alleys can enhance soil nutrient and moisture levels, while providing a more moderate level of competition, for better vigor management.
Crop management through shoot thinning is performed after budbreak and before shoots are 6 inches long. It assists in optimizing fruit production and canopy density. Typically, 3-5 shoots per linear foot of row is recommended. Shoot thinning reduces competition among shoots for carbohydrate and nutrient reserves for growth and development before carbohydrate accumulation begins in spring. For weak vines, leaving fewer shoots can produce better growth. On a vigorous vine, removing too many shoots can lead to increased vegetative growth of remaining shoots and less than ideal yields. The reduction of fruit that occurs as a result of over-thinning in vigorous vines can lead to less than ideal yields and out of balance vines.
Leaf removal around the cluster zone is often conducted to allow for sunlight exposure and airflow. This practice should be done earlier in berry development, when flavonoid compounds are which act as a sort of sunscreen are produced. If conducted late in the season, fruit may become sunburned, as there are lower levels of these protective compounds at or around véraison. Increased airflow, as a result of leaf thinning provides the added benefit of reduced incidence and severity of diseases such as powdery mildew or Botrytis bunch rot. In healthy vines, leaf removal doesn’t greatly affect carbohydrate production when implemented early in the season (shortly after fruit set), and it can enhance the production of secondary metabolites that enhance wine quality.
Crop Level Management by fruit thinning adjusts yields to obtain a balance between canopy growth and crop load, while enhancing fruit quality. Vigorous vines with large canopies are usually capable of ripening more fruit than low vigor vines. Environmental and management practices must also be considered in determining the amount of crop to remove during thinning. For instance, in cooler climates, a greater leaf area to fruit ratio is needed to properly ripen a crop compared to warmer climates. For low vigor vines, light thinning may be sufficient, but if the vine is also unhealthy, heavy thinning is recommended to ensure adequate carbohydrates are being produced to ripen fruit and store for dormant season reserves. It is generally believed that removal of fruit increases canopy growth and fruit quality, but this may differ depending on circumstance (Vance et al., 2013).
When appropriate, thinning can enhance ripening. Timing of crop thinning is very important to maintaining vine balance. While dormant pruning reduces the potential number of clusters developed, cluster thinning may be also be required depending on the level of balance in a given vine. In lower vigor vines, late thinning (at véraison) may result in heavy competition between shoots and fruit for carbohydrates and push vines further out of balance. This type of vine should be thinned between inflorescence and fruit set. In higher vigor vines, waiting to thin until véraison may help keep canopy growth in check and reduce canopy management throughout the season.
If carbon sources are limited, inflorescences and flower number per inflorescence can be reduced, resulting in lower yield the following season. However, crop level alone may not compete for resources enough to reduce fertility. Research has shown that timing of thinning can impact effectiveness. Early thinning resulted in greater bud fertility, while thinning at véraison had no effect. This may be due to the fact that there is less competition for carbon resources early in the season, and latent buds initiate as early as pre-bloom (Howell, 1999; Vance, 2012).
While achieving Vine Balance can be a complex task, working toward this goal will offer both short term reward of maximizing both fruit quality and yield as well as the long term reward of a healthy and productive vineyard that is resistant to injury and disease.