Pruning Philosophy Spring 2020

GRAPE PRUNING                         New Mexico State University                       Winter 2019 - Spring 2020

Gill Giese, Extension Viticulture Specialist           ggiese@nmsu.edu                 

            Pruning is arguably the most important cultural practice a grape grower performs. Pruning has a dwarfing (pruned vines are physically smaller) and invigorating (the vigor of remaining growing points is increased) effect on the vine. When wood is removed, the roots that previously supported the top growth will have an abundance of hormones and carbohydrate reserves. The roots will also have an increased water and mineral absorbance capacity relative to the remaining top growth. This increased capacity or ‘excess supply’ stimulates the remaining shoots to grow and expand at a rapid rate. The rate of growth over time is referred to as “vigor”. Vigor is not the total amount of growth, or overall vine/plant size. To summarize thus far: the above ground portion of pruned vines are physically smaller and the remaining shoots grow more rapidly (increased vigor) compared to shoots on an unpruned vine.

A term often confused with vigor is “vine capacity”. Vine capacity relates to vine size, and is a separate concept. Capacity is the total amount of fruit/crop a given vine can support to a desired degree of maturity. Vine productivity and sustainability depends on vine capacity: first, for the vine’s physiological requirements and second, for the economic viability of the vineyard enterprise. At pruning, the following is determined: type, position and number of buds that will produce fruit. Bud number, that impacts crop load, is set at pruning and can impact berry composition i.e. fruit ‘quality’. Thus, pruning plays a crucial role in vine health and productivity.   

            Purpose of pruning: achieve the desired amount and quality of fruit over a number of years.

1.      Set the appropriate fruit load to the size/capacity of each vine

2.      Shape and train the vine, integrating the vine to a given trellis system. Direct vine growth into a well-spaced canopy that allows air, sunlight and spray materials into the canopy, to contact the foliage and fruit at the optimum time and degree during the growing season. Within this goal, one should keep in mind not just the current upcoming fruiting season but subsequent seasons. Ideally, this multiple year outlook informs how you shape or set-up the vine for the long term.

Balanced vine growth begins with pruning: How many buds to leave?

1.      Weigh pruned, one-year old wood. Leave 30 buds for the first lb. removed and 10 buds for every lb. thereafter. This is called the 30+10 formula, and is applied to Concord and other American varieties. Another formula, leaves 20 buds per 1 lb. of wood removed and is the ‘formula’ for many vinifera or European wine grapes. These ‘formulas’ provide a starting place and are not definitive…they are an aid to matching pruned wood with the fruit crop. Balanced pruning is a technique that uses: cane weight (one-year old wood), node counting and a pruning ‘formula’ for estimating vine capacity. Dr. Nelson Shaulis of Cornell University developed this technique in the middle of the 20^th century. It assumes the selection of well-exposed canes with fruitful buds. Each pruning formula, or nodes per pound of canes, is driven by growth and fruiting characteristics of the variety. Vine capacity varies between trellis/training systems and between adjoining vines in a row. The intent of balanced pruning and/or a ‘formula’, is to avoid over or under-pruning vines of differing capacity. Balanced pruning is the first step in achieving the annual desired quality level with maintained or improved vine capacity for the following year’s crop (Coombe and Dry, 1992).

Example bud number formulas:

Variety                                    Formula

Cabernet Sauvignon    20+20                          Concord                      30+10

Cabernet Franc            20+20                          Niagara                        40+10

Chardonnay                20+20

Riesling                       20+20

Seyval                           5+10

Vidal                           15+10

Other Hybrids             20+10

2.      Leaf area: fruit ratio, a measure of a vine’s capacity:

The leaf area to fruit ratio is defined as: Pn (photosynthetically active) leaf area measured in square centimeters (cm²) to mass, measured in grams (g). This ratio depends on the trellis system used. The vertical shoot positioned (VSP) trellis generates a leaf area:fruit ratio of about 0.8 to 1.2 m² leaf area per kg fruit or about 10 cm² of leaf area per 2g of fruit, or about 5-6 mature leaves per each medium sized cluster (Figure 1). Divided canopies can be a bit more efficient, where 0.5 to 0.8 m²/kg are needed to mature fruit (Kliewer and Dokoozlian, 2005). Efficiency here is defined as the least amount of leaf area that is required to mature a given amount of fruit.  

Figure 1. The relationship of leaf area to crop weight. Examples of three trellis systems: A1 = vertical, B1 = lyre and C1 = Geneva double curtain, (Kliewer and Dokoozlian, 2005).

3.      If vines have 3.5 lbs. of prunings/vine, one should leave about 42 buds as a default, 6 buds/ft., or 2-3 spurs per foot. What if one must choose between total number of buds/vine and achieving the desired number of shoots per foot? In this case, the number of buds per foot takes precedence over the total number of buds per vine. Avoid a shoot density per foot that results in a dense and congested canopy, that leads to increased disease pressure and poor light exposure of fruit clusters.

4.      Wood:fruit ratio should be approximately 5-6 lbs. of fruit per 1 lb. of wood removed.

Factors that impact pruning…or, pruning depends on:

1.      Climate: Is your climate continental or maritime? If continental (BTW: all of New Mexico has a continental climate), there is an increased chance of winter and/or frost injury due to wide swings in night and day temperatures and no dampening effect from large bodies of water. In this case consider:

-          Leaving more buds at pruning time and shoot and crop thin later in the season

-          Prune late, to delay budburst and aid avoidance of frost injury in the spring

-          “kicker canes” are ‘extra’ canes left on the vine, that can enter budburst relatively late. Kicker canes can be used to replace spur borne buds killed by frost. Kicker canes are typically replaced each year and help dissipate excess vine vigor or be positioned to serve as a replacement trunk if needed.   

2.      Soil Fertility:
      -   deep soil = vigorous growth, prune light (leave many buds) to increase rate
          of early shoot growth
      -   poor soil = prune severely (leave few buds) to stimulate vigor in remaining
          buds

3.    Variety: pay attention to number of: bunches/shoot, flowers/cluster, berry weight,
                      and bud position
            -   some varieties have sterile basal buds, example: Nebbiolo, some have many
                basal buds that will not be counted at pruning time that can burst and grow
                later leading to a congested canopy.  

-   apical dominance? Phenomenon most evident in the spring, where the buds
    at cane tips   
    break bud first, and subsequently, buds located closer to cordon or trunk will
    burst.
    One can use this to advantage by leaving longer spurs and after the apical
    buds reach
    bud burst, they are pruned off …this will delay budbreak in remaining buds
    and could help avoid a late frost.

 4.   Economic: winter pruning most economical way to control crop, but is least
                          precise method of crop control
            -   cluster, fruit and shoot thinning during the growing season costs more, but is
                 more precise compared to winter or dormant pruning

Pruning Procedures

                        1.  Timing:

-          Early prune to finish pruning by budburst, this may be dictated by size of vineyard and number of pruners available. Pruning can be done after budburst, but the risk of bud damage is greater because the process can damage of knock off the tender growing buds.

-          Early pruning = early bud break, growth advancement can persist through harvest

-          Double pruning = making two passes through the vineyard.

o   First pass, make rough cuts and remove bulk of the prunings from the trellis

o   Second pass, adjust bud numbers to desired number/vine prior to bud break

-          Delayed pruning, cutting off wood as budburst approaches, development of remaining buds will be delayed

o   The delay can persist through harvest

o   Delayed pruning can reduce Eutypa, Esca and other wood rotting diseases.

      
Figure 2. Left: a cross section of a cordon infected with fungal wood rotting disease, evident in the discoloration. Right: end cut cordon revealing fungal wood rotting infection.

2.   Pruning Degree: 

-          Balanced pruning: number of buds retained based on weight of wood removed at annual dormant pruning

o   SEVERE                < 20 buds left on the vine

o   MODERATE        20 to 70 buds left on vine

o   LIGHT                  > 75 buds left on the vine 

            3.  Cane Selection Criteria:
                        -  varietal color: light brown Vitis vinifera varieties, with brown coloration to
                            the cane tip

                        -  1 cm (3/8 inch) in diameter desired, strive for cane uniformity

                        -  moderate internode length, about 3-4 inches

                        -  round buds instead of flat buds
                        -  canes of different length and diameter, should retain different number of
                            buds
                        -  age or location

4. Vine Age: different training goals for different age vines

-     young vines – establishment of framework based on trunk arrangement

-     mature – framework and buds type placement within your trellis system

Pruning affects most vineyard activities and vine growth parameters such as: crop yield, winter hardiness, insect and disease management and ultimately berry composition and resultant juice and wine quality (Chapman et al. 2004). Although pruning can become intuitive once you have practiced a bit, a winegrower with an eye towards the bottom line should maintain good records to supplement and support her pruning decisions.

REFERENCES

1.      Chapman, D.M., Matthews, M.A., and J.X. Guinard. 2004. Sensory Attributes of Cabernet Sauvignon Wines Made from Vines with Different Crop Yields. Am. J. Enol. Vitic. 55: 325-334.

2.      Coombe, B.G., and P.R. Dry. 1992. Viticulture Volume 2, Practices. Winetitles. Adelaide, Australia

3.      Fisher, H.K. 1999. Factors Affecting Successful Vine Growth, Yield and Quality., Wine Industry Workshop, Geneva NY

4.      Howell, G.S. 2001. Sustainable Grape Productivity and the Growth-Yield Relationship:  A Review.  Am.J. Enol. Vitic. 52:3 165-174.

5.      Kliewer, W. M. and N. Dokoozlian. 2005. Leaf Area/Crop Weight Ratios of Grapevines: Influence on Fruit Composition and Wine Quality. Am. J. Enol. Vitic. 56:170-181.

6.      Pool, R.M. 2004. Vineyard Balance- What Is It?  Can It Be Achieved? Acta.Hort. 640, ISHS p 285-302.

7.      Winkler, A.J., Cook, J.A., Kliewer, W.M., and L.A. Lider. 1974. General Viticulture. University of California Press, Berkeley.