By Dr. Richard Smart, “the flying vine-doctor”
This article will continue with the theme of the last one, that is vine balance. Here the emphasis will be on management of vine balance. The last article included my suggestion of how to measure vine balance at winter pruning by recording pruning mass. I have always previously termed this metric pruning weight, as Prof Nelson Shaulis taught me. I now understand that pruning mass is the preferred term.
Better Balance for Better
Yield & Quality
At the one extreme of poor vine balance are over-vigorous vines typically with canopy shading. Such vines are characterised by large leaves, long shoots and many lateral shoots. Without remedial treatment such canopies have shaded fruit which affects berry competition and wine quality, and also reduces fruitfulness and yield. Some diseases such as Oidium and Botrytis are encouraged by dense canopies. This contribution will deal particularly with vines of high vigour, and present management strategies to avoid them.
Vineyards of low vigour are an important cause of low profitability. Growth inhibition can be due to many causes, too numerous to mention here. In recent years Grapevine Trunk Disease has been frequently implicated in poor vine health.
We will discuss the two most important methods of vine balance management, firstly by root competition and secondly by pruning level.
Tools of Grapevine
Balance Management
Using Root Competition
It is more difficult to manage root competition with rainfall-fed vines than for those which are irrigated. One can use volunteer weeds or select a cover crop species which will more efficiently compete with vines. Because vines are generally deep rooted, the most efficient, competing species are those which are also deep rooting. However even species with moderate or shallow root depth can be effective in competition when rainfall amounts are small.
Normally competing species are planted in the inter row space, where they can be mowed or even cultivated if the level of competition is excessive. Recent research from Europe has shown that planting competing species under the vines within the row is much more effective at competition. Such plantings can however be more difficult to manage.
How much stress is desirable? I suggest some simple guidelines here. Shoot growth rate or more exactly the extension activity of the shoot tip is the criterion I prefer to determine the degree of vine stress. Generally we like to see the shoots growing actively early in the season up through flowering and fruit set. Then say about four weeks or so before veraison the shoot tip growth should slow, and then be limited around the time of veraison. Ideally this mild stress should continue until harvest, avoiding severe stress so that leaf health and function are maintained. Such timing can be difficult to manage when rainfall is the water source.
Using Pruning Level
Our aim here is to determine the appropriate bud number for even and adequate shoot growth. My general preference is to have shoots that are 3 to 3 ½ feet long with tapering tips indicating modest water stress during fall. How many buds should we retain to achieve this ideal length?
We understand that if too many buds are retained at winter pruning then the vine reserves which support initial shoot growth become diluted and shoot growth can be somewhat stunted. Conversely, if too few buds are retained then the growth of only a few shoots is very much encouraged. The shoots grow rapidly often with many laterals. Because of less crop on the vine they will continue to grow most of the season.
The correct shoot number per vine is obviously somewhere between these two extremes. There are several ways in which it can be determined. The one I favour was again taught to me by Nelson Shaulis, and relates the pruning mass to bud number retained at winter pruning. Again there are various forms of this formula ; the one I prefer is to retain 20- 30 buds per kilogram (9 to 14 buds per pound) of pruning mass. In cool climates, I suggest 20 buds per kilo (9 per lb) although in warmer climates I suggest 30 (14 per lb). And I judge if it is a warm or cool climate by summer temperatures, obviously not those of the winter.
There is a simple rationale for this approach. The pruning mass is related to the amount of leaf area of the previous growing season, which in turn is related to the vines capacity for growth in the new season. Another way to put it is that we are balancing the bud number retained at pruning to the amount of vine carbohydrate reserves available to promote new growth.
The careful reader will be concerned about the apparent need to measure pruning mass of so many vines prior to pruning. In fact, by recording pruning mass for a range of vine sizes teaches the pruner to estimate pruning mass sufficiently well to make the judgement. This can be reinforced for very experienced pruners by assessing last season’s shoot growth. If it appears balance as I describe here then pruning to about the same bud number as last winter is a good guess. Obviously one might make some allowance for the last growing season.
In practice many vineyards are pruned by unskilled labour and one can but hope that pruning level will be reasonable. I think it a good investment to spend time for experienced pruners to train those less experienced in some simple guidelines.
The Golden Rules of Viticulture
I like to think that some decisions in vineyard management can be reduced to simple concepts or rules. I have developed two which are as follows:
Golden Rule 1: Pruning to 20 to 30 buds per kilogram (9 -14 per lb) of pruning mass.
Golden Rule 2: Aim for 15 shoots per metre of canopy (4-5 shoots per foot).
We need some more definitions here. If the vines are trained to a simple system like vertical shoot positioning, then there is 1 ft of canopy per ft of row. If the canopy is divided, as for saying with the Scott Henry, then we have 2 ft of canopy per ft of row.
Golden rule two relates to the density of the canopy and aims to avoid excessive shade. Provided the shoots are of moderate vigour, a canopy with this shoot spacing should not be shaded. There will be sufficient gaps between the leaves to allow sufficient sunlight for adequate leaf and fruit exposure. There will be limited leaf overlapping, and shading.
Implications for Choice
of Training System
Vineyard vigour is also an at important reason for choice of the trellis system. In principle, the higher is the vigour, then the more buds needs to be retained to accommodate this vigour, and so there are more shoots per vine. There for higher vigour vines have more shoots and so need a larger trellis system to better display them without crowding.
Again there are some simple guidelines. Simple single canopy trellis systems like the popular vertical shoot positioning (VSP) are well suited to low vigour conditions. For example, these are vineyards with less than 0.5 kg of pruning mass per metre row length (1/3 pound pruning mass per foot of row). The most common vineyard balance problem I see is for too-high vigour vines trained to the simple VSP system. I wonder is this also the case in the Mid West.
Medium vigour vineyards are characterised by pruning mass ratios of 0.5 to 1.0 kg per metre, or1/3 to 2/3 lb per foot of row. This vigour classes is quite common for rain fed vineyards. We cane pruning is commonly used, I suggest the simple conversion to Scott Henry trellis. For those preferring spur pruning the smart Dyson can be used. Both of these trellis systems are vertically divided.
High vigour vineyards are characterised by pruning mass ratios of more than 1 kg per metre or more than 2/3 lb pruning mass per foot of canopy. The Geneva Double Curtain GDC is normally recommended here as the pendant shoot growth helps devigorate the vine. This training system is well suited to some hybrid varieties with less erect shoot growth. If the recent experience is hot and sunny summers than caution needs to be exercised regarding excessive fruit exposure. This can be readily overcome by training up one or two shoots along the cordon wire to provide some protection.
Conclusion
After a while the concept of vine balance becomes quite intuitive, and one knows some tell-tale signs of problems at different times of the year. For example, it is very easy to look out for excessive shoot growth around veraison which we know will harm fruit ripening.
A second useful visual cue which I suggest growers learn is the appearance and weight of an ideal shoot. As I have previously indicated such a shoot normally has 15 to 20 nodes of moderate spacing and average shoot diameter. The length is normally around 3 to 4 feet and the most recent growth should be obviously tapering with internode length becoming shorter indicating growth with moderate water stress. When measured in winter such a cane will weigh around 40 to 50 g, or 0.1 lb. Learn to recognise such canes from the pickup in summer or winter and you will be well on the way to growing balanced vineyards.
