Correct trellising practices and trellis systems for mechanically pruned grapevines are very important.
The most important aspect of mechanically pruned blocks is for the trunks of the grapevines to be upright. This is more crucial for mechanically pruned grapevines than for grapevines pruned by hand and requires a lot of attention. If trunks are severely crooked, most mechanical pruners will either cut them off or damage them.
Crooked trunks are caused by canopies falling over in the first and second year of growth, when there is only one canopy wire. In areas where much wind prevails, the falling over of canopies is even worse. This causes the trunk to push out in the other direction, resulting in a crooked trunk. It is therefore advisable to plant in the same direction as the prevailing wind. The other main reason for crooked trunks is when grapevines bear grapes on the cordon in the second year. The trunks are not strong enough yet to support the weight of the grapes, allowing the cordon to sag and consequently the trunks become crooked. Once the trunks are crooked and become thicker, they cannot be straightened again without sawing off the grapevines. All practices should therefore be aimed at getting the trunks as straight as possible. Mistakes abound and shortcuts are legion, the result being poor grapevine development with the concomitant problems. On the other hand, there are many recipes for success.
One way of achieving straight trunks is to prune half a “kierie” (vine stem) (approximately knee-high) during the first winter after planting. This gives the grapevine the opportunity to develop a strong trunk, which will not bend easily. The vines will not bear grapes on the cordon and therefore the cordon will not sag. However, a few bunches may occur under the cordon and a small crop may be harvested (Photo 1).
Many producers do not like this method, especially where grapevine growth has been good and grapevine development may be completed in the first year. This means that considerably more grapes may be harvested in the second year. However, this practice makes it difficult to obtain straight trunks, although it is possible with the correct practices and good management. For this method good concrete anchors (Photo 2) should be used. This will prevent the anchors from being pulled out of the ground and head poles from being pulled over when the cordon wire is tightened. A very tight cordon wire can thus be obtained. Ensure that the cordon wire is pulled very tight when the trellis system is erected. Before the start of the second growing season, the wires must again be tightened to support the crop. The canopy should also be topped after set (Photos 3 and 4) so that the shoots remain upright. This also prevents the canopy from falling over. Crop control is applied by removing the top bunch on each shoot, thus leaving one bunch per shoot. By so doing a good crop is harvested in the second year and straight trunks are obtained (Photo 5).
Obviously the distance between poles in the row plays a big role; the distance should not be longer than 6 m. Some producers even plant poles 4.8 m apart (four grapevines between poles). This is especially important when longer poles are used and it is more difficult to obtain straight trunks. Two temporary wires approximately 40 cm below the cordon, bound together in the middle, have also been used successfully to obtain straight trunks (Photos 6 and 7).
An ordinary cordon wire suffices in most instances. There have been a few cases, however, where the cordon wires broke with high production cultivars, such as Colombar. In such cases two cordon wires should be used instead (Photo 8). A second temporary canopy wire can also be used to great effect to keep the canopy upright in the first and second year (Photos 9 and 10). The timing of the topping action is very important to ensure an upright canopy (Photo 11). A good tip is also to attach the cordon wire or wires only once the arms have been developed on the cordon wire. This saves a lot of time, seeing that it is not necessary to work around this wire. Drill the holes for the cordon wire in the vineyard once the poles have been planted. If the holes in the poles are drilled in advance, it is much more difficult to plant the poles, seeing that the holes first have to be aligned, which complicates pulling of the wires if they do not line up exactly.
Most cultivars can be successfully grown using the mechanical pruning system. Cultivars with shoots that are not rigid or with thin shoots, for example Merlot and Petit Verdot, are not suited to mechanical pruning. There should also be sufficient vigour, in view of the fact that shorter shoots are obtained with mechanical pruning. This system should not be used with the objective of very high productions, although yields are usually bigger. Some trellis systems produce considerably higher yields, for example ballerina, gable and Smart-Dyson. Systems for mechanical pruning should be established with a view to facilitating cultivation with less input. Grapes produced on this system are less susceptible to rot as a result of the smaller berries and loose bunches; it works well with cultivars that are sensitive to rot. Good bunch distribution is also obtained. In an irrigation area the quality of the grapes can be managed by irrigation and fertilisation inputs.