Vine to Wine
Question 4: Why are grapevines trellised? Also, describe three trellis types and explain their suitability to either a region or variety.
In their simplest form, vineyard trellis systems provide made-man support for a grapevine and its fruit. But it is their design that is crucial in managing the way in which a grapevine’s leaves and fruit are exposed to sunlight. Trellising directly affects the ripeness, colour and flavour characteristics of the grape berry by how much direct sunlight it receives.
There is an almost infinite variety of vine training systems, since the perennial grapevine, one of the world’s oldest cultivated crops, is easily trained and has succumbed to the whims of vignerons since ancient times. Even the Egyptians and Phoenicians discovered that different training techniques could promote more abundant and fruitful yields (Johnson, 1989). In some cases, vines are not trellised at all, but rather pruned into a goblet shape, such as the Old World self-supported training system, ‘Gobelet.’ However, the necessary monitoring of canopy management, especially vine vigour and disease, became the principal reasons for adopting more elaborate training and trellis systems in the
New World. Trellis systems aim to balance the many factors that influence fruit quality such as number of shoots per vine and their placement on the trellis wires, number of leaves per shoot and their orientation to the sun, number of grape clusters per shoot and their shading within the canopy. Significant effort is now also made to match trellis systems to site-specific factors influencing vine growth potential such as climate, growing region, soil type and rootstock. Hence New World practices explore new and varied techniques of trellising to enhance their fruit and wine. The terms ‘trellising’ and ‘vine training’ are often used interchangeably, however the trellis refers to the actual posts and wires of the architectural structure the grapevine is attached to, whereas vine training refers to the practice of trellising and pruning in order to dictate and control a grapevines’ canopy. Vine training systems often take on the name of the particular type of trellising involved (Cox, 1999). Primarily, the grapevine trellis consists of a stake driven into the soil beside a vine to which the vine trunk or shoots are tied (Robinson, 1994). Nowadays, wires are used to support vines and foliage, and posts are installed at
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intervals along the row. There are multiple end assembly designs all of which are firmly anchored into the ground, in order to support the wire when the weight of the grapes, foliage and wind creates strain on the trellis (Robinson, 1994).
Trellising is the most important tool in managing canopy microclimate to achieve the best quality fruit. It is essentially employed to open up the vine canopy, in order to increase fruitfulness and hence, higher yields.
The trellis system finds equilibrium between sunlight and foliage to facilitate photosynthesis in conjunction with improved air circulation.
Excessive shading can impede grape ripening and promote disease, while excessive exposure to sunlight causes sunburn and heat stress. Excessive shading results in the reduction of successive bud formation, bud-break, fruit set, as well as the size and quantity of grape clusters (Robinson,
1994). The increased airflow heavily reduces the risk of fungal disease and rot, and favourably influences juice components such as potassium.
Trellising also reduces potential vine diseases and pests by keeping the fruiting zone above ground. Phylloxera is one such vine pest. Since the louse’s epidemic of the 19th century, many vines have been grafted on phylloxera resistant rootstock. However, the canopy of the grafted vine is still highly susceptible to phylloxera and thus the both fruit and foliage must be above ground to prevent both mother and daughter vine