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© G.L. Creasy and L.L. Creasy 2009. Grapes(Creasy and Creasy) 105
● Regulate shoot number/crop load.
● Regulate vegetative growth.
A key concept in thinking about pruning is balance. In the case of most grape growers, the balance is in preparing the vines to produce an economic crop, but also so that they will not spend so much of their energy ripening that crop at the expense of their ability to grow in the following season (a consideration in the cropping of any perennial plant). Refer to Chapter 3 for a discussion about the carbohydrate balance of the vine and how it is related to management.
As the pruner leaves more nodes on a vine, they leave not only more growing points (shoots), but also potential fruit (all the clusters on those shoots). Thus, pruning has a significant influence on the ability of a vine to grow and to produce a crop.
Let’s look at this in more detail. If we approach a vine in the dormant season, we see canes that were the shoots from the previous season. On each of those canes are a number of nodes. At each of those nodes is a compound bud that contains three preformed shoots. Each of these shoots is capable of growing and each may, or may not, have flower cluster primordia on them. In most cases, only the largest of those preformed shoots will grow in the spring.
So, if a vine was not pruned, it would seem like a very large bush, with shoots coming out at all angles and from almost everywhere on the old vine, each potentially carrying fruit (see Fig. 6.2).
Fig. 6.1. A partially pruned row of grapevines in Oregon, USA. Note how much material is removed in the process.
Now if we prune that vine back to just a few nodes, in the spring we will have a much more limited number of shoots coming out, as well as having a much more limited amount of crop ⫺ ideally this would result in the same number of shoots as nodes left, with each shoot having some crop on it. In this case, we have pruned off a lot of the potential vegetative and reproductive growth of the vine. In the spring, it will have available a much smaller leaf area with which to photosynthesize and make carbohydrates and, perhaps more importantly, it has a very limited fruit load. The result of this will be rampant growth of those shoots, the pushing of many latent buds (left over from previously removed canes or shoots) and also the growth of other preformed shoots in the compound bud (see Fig. 6.3).
Some of the most widely used guidelines relating to pruning decisions are published in the book Sunlight into Wine(Smart and Robinson, 1991), where vine balance can be estimated through examining the fruit weight to pruning weight ratio, the number of shoots per metre of canopy and the average shoot weight.
In the case of the fruit weight to pruning weight, a value of 5⫺10:1 is said to represent a vine that is in balance ⫺ that is, the amount of fruit it is producing is appropriate to the amount of canopy the vine has. This concept is not new, having probably been first proposed by Ravaz in 1930. This principle Fig. 6.2. Unpruned vines in Napa Valley, California, USA, showing the many nodes left and the shoots growing at an early time of the season. These vines were due to be pulled out due to phylloxera infestation, and hence there was no need to prune them.
is explained by experiments that looked at a range of crop loads and leaf areas on vines and came to the conclusion that approximately 5⫺14cm2of leaf area was needed to ripen each 1 g of fruit (see Fig. 6.4), depending on the trellising system and environment (Kliewer and Antcliff, 1970; Kliewer and Dokoozlian, 2005). As there is a relationship between shoot growth and leaf area, it is easy to make the jump from this to looking at pruning weight, which is the summation of shoot growth over the season.
Another way of looking at this is through the calculation of approximate dry matter production of a vine: (fruit weight ⫻ 0.25) + (pruning weight ⫻ 0.55), where the numbers that are used as multipliers are roughly equal to the proportionate dry weight of the fruit and dormant canes, respectively. This value can be thought of as a measure of the capacity of a vine (Winkler et al., 1974), which is related to its ability to grow or produce grapes. So, in fact, the index is an estimate of the above-ground dry matter production by the vine for the season just past, which is a concept that we introduced with vine capacity in Chapter 3.
Fig. 6.3. The head of a vine showing the growth of many (all but one of the visible shoots) unexpected (non-count) shoots. These lead to high canopy density, which delays fruit development and encourages disease.
Some knowledge of vine balance can then be used to make actual pruning decisions. In pruning we will refer to count nodes and non-count nodes. Count nodes are those that are left on purpose during pruning andwhich we expect to produce one fruitful shoot (i.e. for the count shoot refer to Fig. 2.4). In practice, the first count node on a cane is not the first visible node, as the compressed nodes at the base of the cane do not normally push during budbreak. The shoot that arises from a count node is referred to as a count shoot and, in normal circumstances, this will be the primary shoot, which bears the most potential fruit.
A non-count node is any node that is not a count node. For example, the compressed nodes at the base of a cane are non-count nodes, as are those that are embedded in the trunk, head or cordon of a vine (latent buds). Similarly, shoots that arise from non-count nodes are termed non-count shoots (see Fig. 2.4). As well, secondary or tertiary shoots that may arise from a count node are also considered non-count shoots, as they are not anticipated to burst in the spring.
Types of pruning
The principal methods of pruning are cane and spur (also known as cordon pruning). With cane pruning, the fruiting shoots for the next season come from a length of the previous season’s shoot, which is laid down and wrapped on the fruiting wire of the trellising system (see Fig. 6.5).
22 20 18 16 14 12 10
0 2 4 6 8 10 12 14 16 18 20 22 Leaf area (cm2)/g fruit
Total soluble solids (°Brix)
Fig. 6.4. Effect of a range of leaf areas/1 g fruit on fruit Brix after 4 years of treatment application to ‘Thompson Seedless’ vines. No appreciable increase of Brix is obtained at the very high values (from Fig. 1, Kliewer and Dokoozlian, 2005, with permission).
Cane pruning typically uses up to four canes of 8 to 15 nodes in length on the vine, depending on trellis, vine spacing and cropping aims. In most cases, one or two replacement spurs are also left below the head, which is to facilitate cane choice the following year. Figure 6.5 shows the head in the centre of a bilaterally trained vine, but the head can be to one side on unilaterally trained vines (typically used for close vine plantings, where there is only one cane tied down).
The goal is to choose canes that are (i) in a good location to retain the shape of the vine; (ii) well-exposed in the previous season, to give maximum fruitfulness; (iii) have good periderm formation; and (iv) not excessively thick or thin. The latter two parameters are associated with the cane’s ability to withstand cold while dormant and increased fruitfulness in the following growing season.
Advantages of cane pruning are that vine fruitfulness is maximized and the number of potential non-count nodes on the vine is minimized, because there is less permanent wood on the vine and therefore less surface area from which latent buds can arise. As some cultivars are inherently less fruitful at the basal node positions on the shoot (Swartwout, 1925; Winkler, 1926; Buttrose, 1969), cane pruning minimizes the relative impact of this.
Disadvantages of cane pruning include (i) blind budding (places along the cane where no shoot develops), especially with long canes; (ii) limitations to between-vine spacing (in that the distance between vines is restricted to no more than twice the length of a typical shoot during the growing season); and (iii) uneven shoot growth along the cane. The latter phenomenon has been called the End Point Principle by David Jackson (1997), where shoots at the end of a cane tend to grow more vigorously than those in the mid-portion of the Fig. 6.5. Examples of cane-pruned vines.
cane (see Fig. 6.6). Extreme examples of this arise when there is no shoot development in the middle parts of the canes, but there is at the ends and bases (see Fig. 6.7), which also demonstrates the Trunk Proximity Principle, where shoots tend to develop more vigorously near the base of the canes (Jackson, 1997). Cane pruning can also be more difficult when training workers to make
Fig. 6.6. Example of a cane showing signs of the End Point Principle, which states that growth of shoots tends to be more vigorous at the ends of canes compared with that at the mid-sections.
Fig. 6.7. This vine is showing an extreme case of both the End Point Principle and Trunk Proximity Principle: there is vigorous growth at the beginning and end of canes, but none in between.
the best decisions, resulting in loss of some of the advantages of cane pruning in the first place.
Spur pruning does away with continued use of canes by establishing a permanent arm, or cordon, from a cane (see Fig. 6.8). Note that, in establishing a cordon, shoots arising from the downward-facing bud on the original cane are removed, leaving every alternate and upper node positions present.
Downward-pointing shoots do not grow so vigorously (Kliewer et al., 1989) and, if shoots from both nodes were used, it would enlarge the fruiting zone, making some management practices more difficult. To counteract the loss of shoots from these positions, canes arising from the remaining nodes are cut back to (typically) two-node spurs. Note that, when training a cane that is destined to become a cordon, it should not be wrapped as tightly as if the vine is cane pruned every year. Too many revolutions around the wire can result in girdling of the cordon as it increases in girth (see Fig. 6.9).
Advantages of spur pruning are that it is easier to teach unskilled people how to do it and it is possible to partly mechanize the process using cutting bars (see Fig. 6.10), which means that the crew that goes in afterwards has much less brush-pulling to do. Spur pruning also results in the vine having more permanent vine material, which has been implicated in fruit quality (Howell, 2001). Disadvantages are (i) vine fruitfulness is limited (because the basal nodes on canes are typically not as fruitful); (ii) spurs can die out over time due to injury, disease or other problems; (iii) the fruiting zone can creep upwards through the seasons if there is a tendency for pruners to choose the distal nodes
Fig. 6.8. Example of a spur-trained vine, showing two node spurs.
on each of the spurs (this can result in very uneven or high fruiting zones); and (iv) the cordon needs to be renewed periodically due to damage, disease, etc.
Other factors to be aware of in spur pruning relate to the canes that were used to set up the cordons. Spur spacing depends on the internode length of the original cane, so initial cane choice is important. If internodes are too long, then spurs will be spaced too far apart and cropping will be limited. If they are too close together, there will be shoot shading, dense canopies and a host of other problems that are related (see the Goals and tools for canopy management section, on page 139). For most situations, keeping replacement spurs near the head of the vine is vital, because it is from those that renewal canes/cordons can be sourced. To avoid the upwardly creeping fruiting zone, pruners must be aware that shoots from the lower of the two nodes need to have preference over the upper. The low potential fruitfulness of spur-pruned vines can be overcome somewhat by altering the length of spurs retained, though this does have significant effects on the resulting canopy unless remedial measures (such as shoot thinning and leaf removal) are taken during the growing season.
No matter what the pruning system, there remain some tips to smooth the pruning process. If movable foliage wires are used in the trellising system, be sure to move these down before budburst as then they will catch the shoots as they are raised. If any trellis posts or wires have broken, your only chance to fix them is at pruning, before any tying down of canes has to take place.
Fig. 6.9. Cordon where the wire has become embedded into the plant. Eventually, the performance of the cordon will decline due to restriction of nutrient flow and, in extreme cases, the entire cordon may die.
Pruning decisions
The basis for pruning grapevines is that each vine has a capacity to support growth (of shoots and roots) and ripen a crop. Each dormant season, an appropriate balance between vegetative production and yield must be found.
Pruning is your first step in managing crop load and vine size, and so an important process that requires thought and reasoning. If too many nodes are retained on the vines, time-consuming canopy and crop management practices such as shoot thinning, cluster thinning and leaf removal may need to be done during the growing season. A vine in balance with site considerations, its crop Fig. 6.10. Example of a barrel pruner, which is used to remove unwanted cane material from spur-pruned VSP-type trellis systems quickly and easily. The two sets of rotating wheels cut the canes into small sections, which then fall from the trellis (image reproduced with permission from Hort Engineering Solutions, part of the Ormond Nurseries Group of Companies, New Zealand).
load and management decisions will have moderate growth that fills the trellis and ripens a commercial crop.
Vines have evolved without something coming in and removing 90% of their season’s growth each year. Vines produce far more nodes, each with a compound bud capable of producing three shoots, than they can support with the kind of growth needed to fill out typical trellis systems. However, if a vine is left with a large number of nodes, it will reduce the number of viable growing points, effectively pruning itself. Any growth that has not hardened off and formed an adequate periderm will not survive freezing temperatures and will be lost during the winter. In the spring, a smaller percentage of buds at those remaining nodes will produce shoots, i.e. percentage budbreak is reduced (see Table 6.1). This (along with other factors relating to the cluster) is why, if twice as many nodes are left at pruning, there won’t necessarily be a doubling of crop (Christensenet al., 1994). An increase of 100% in node number results in a relatively small 60% increase in shoot number and only a 22% increase in yield. Part of this is due to the decreased percentage budbreak, but also to factors related to cluster weight and berry number.
Note that budbreak on the 60-node vines is greater than 100%, because some nodes produce more than one shoot (two or three of the preformed shoots in the compound bud push instead of one) and because there will be a greater number of non-count shoots arising from the permanent parts of the vine (from latent buds). This is related back to vine capacity, where a vine has the capacity to push a certain number of buds each year and, if they are pruned to below that level, a greater number of shoots than nodes retained will be produced. If more buds than that level of capacity can support are left, fewer buds will break, percentage budbreak will decrease and growth of those shoots that do push will be weaker.
Matching pruning to vine capacity
Each vine in a vineyard may have a slightly different capacity due to the microclimate it experiences. Experienced pruners recognize this when they first look at a vine and judge its growth during the previous season. If there has
Table 6.1. Yield components for ‘Sultana’ raisin production given three different vine pruning severities (data adapted and reproduced with permission, Christensen et al., 1994).
Yield component 60 nodes/vine 120 nodes/vine Change (%)
Shoots/vine 67 108 +60
Budbreak (%) 113 90 ⫺20
Yield (kg/vine) 5.31 6.49 +22
been a lot of vegetative growth, more nodes will be left than there were the previous season, which should result in smaller shoots with less vigorous growth and greater crop load per vine. If the vine has put on very limited shoot growth, fewer nodes will be retained to encourage more growth from those canes and reduce the crop load. Crop load in both cases will assist in balancing the vines: a large vine’s increased crop load moderates vegetative vigour, and a small vine’s reduced crop load puts more of the vine’s resources toward vegetative growth (roots, shoots and trunk). However, for very weak vines, fruit should be removed altogether.
Experience has traditionally been the measure to balancing vine growth and pruning level. However, in the 1940s, scientists in New York quantified this by producing a formula that matches the number of nodes retained to vine capacity. This method is known as Balanced Pruning (Shaulis et al., 1953), and can be a useful tool in learning the relationship between vine size and node numbers that should be left on the vine.
In Balanced Pruning, the number of nodes to be left on the vine is estimated by weighing most of the vine’s canes produced in that year (more nodes than needed are left on the vine so that some can be trimmed off to reach the desired number after the measurements) and using that value in an equation. In this case, the capacity of the vine is estimated by just pruning weight. For ‘Concord’ in New York State, USA the formula is 30 + 10x, where x is the weight of prunings in pounds (1 pound is approximately 0.45 kg) (Jordan et al., 1981). The first value is the number of nodes to be left for the first pound of prunings, and the second, the number of nodes to be left for each pound after that. For example, if the vine has been pre-pruned and there is 1.5 lb of canes, 30 + (10*0.5) or 35 nodes would be left on the vine. If the vine is larger and there were 3 lb of prunings, 30 + (10*2) or 50 nodes would be left on the vine. Thus vines that put on a lot of growth that season will have a large number of nodes left, vigour of the individual shoots will be less and the greater amount of crop on the vine will also slow growth, resulting in a smaller vine with reduced capacity the next time it comes to pruning. For a small vine, balanced pruning allows its capacity to grow from season to season. This formula was designed for use with mature vines (note that the minimum node number is 30): for very weak or diseased vines, the formula should not be used.
Both the location and variety were specified with the formula; vines growing in different areas and for different purposes may have a different formula. For instance, in New York where Balanced Pruning was developed for use with ‘Concord’ grapes, the formula was 30 + 10x (x in pounds). Under Australian conditions (and with metric units), the general recommendation of 30 to 40 nodes/kg of pruning weight has been suggested (Smart and Robinson, 1991). Since trellising (divided versus single) and vine and row spacing vary considerably from place to place, it is more informative to look at weight of prunings per 1 m of vine canopy rather than on a per vine basis, which allows for more direct comparison of values from place to place.