Sixteen years ago, Vredendal farmer, Wilfred Stephan, suffered severe damage to his table grape crop due to birds. The loss prompted him to invest in what was then a brand-new technique in the Western Cape: netting. He soon discovered that the nets not only offered protection against birds, but created a more favourable microclimate for table grape production, and also served as a windbreak.
“The nets resulted in our export packouts increasing from about 4 000 cartons/ ha to about 5 000 cartons/ha, as well as a significant increase in berry sizes,” Wilfred recalls. “We also saved on labour as the vines no longer had to be tied up to protect them from the wind.”
Over the years, Wilfred has significantly improved his netting system. One of the problems had been the vineyards’ tendency to grow into the nets.
“We started with a hut structure, with the nets forming triangular roofs over the vineyards, because we thought it would offer better protection against the wind than a flat structure. Later we found that the netting was too low – the vines and shoots tended to grow into the net, causing it to tear,” he explains.
Wilfred then converted to a high, flat structure where the nets are spanned at a height of 3,6m and the vine canopy is kept at 1,6m. This was much cheaper than having to erect hut-shaped poles high enough to prevent the plants from growing into it. It was also found that the wind did not pose a problem to the flat structure after all.
Wilfred uses wires and cables for the roofs of his structures, where previously he had used wood.“Wooden roofs tend to collapse when there’s too much pressure on them, so they’re unsuitable for a flat roof net structure. Wires and cables are better because they’re more flexible,” he explains.
Using cables and wires was also about 30% cheaper than using wooden frames.
The supporting poles that keep the infrastructure in place are no longer planted in the soil, but left on top of it. This, says Wilfred, saves considerably on labour as the farms’ soil consists primarily of durisol (dorbank), which is very difficult to work.
Securing the system
Strong anchors are required to keep the infrastructure in place. Each consists of a 4m-long, 125mm-diameter wooden corner pole planted 600mm deep. It is held in place by 16mm builder’s steel cable and 6mm connecting steel cable. To anchor the cables, Wilfred fills old pesticide containers with concrete and plants them about 1,4m into the soil.
“A major benefit of using cable instead of wire in the anchors is that it doesn’t stretch, so the chances are slim that the infrastructure will collapse. Wire tends to stretch over time as the vineyard becomes older and, in effect, heavier,” he explains.
Traditional vineyards have a pole to support the trellis every 6m or so, but Wilfred’s system uses a pole every 4m. Every second pole in the row is used to carry the weight of the net; these poles each have a diameter of 80mm and are 3,6m in height. The alternate poles are smaller, with a diameter of 65mm and a height of 2,1m, and function solely to support the trellis.
Two wires are used to keep the cordon in place: one goes across the working row at a height of 2m, while the other is fastened to the poles at a height of 1,6m and turned over the top wire, forming a slight V shape. This flat structure is ideal for table grape production, according to Wilfred.
Vineyards are manipulated through pruning so that all the grapes develop along the first 50cm of the vine.
“In any other system you’d have to pick all over the place. Here, the grapes are in one area and easy to reach. Picking is more than 10% faster than if the vineyards were higher and the workers also had to pick overhead.”
Wilfred uses only knitted nets, as other materials tend to run from even a small tear. He has experimented with four net colours – white, red, blue and transparent – but found no significant difference in production and none in cost. In the end, he settled for white nets as they appeared to let through the most light.
Due to the difficulty of maintaining crop quality, many farmers are no longer growing the Flame Seedless cultivar. In contrast, Wilfred is expanding his production of these sought-after grapes.
“The general perception is that the grapes crack during warm wet seasons, but I think it has more to do with extreme fluctuations in temperatures,” he says.
To manage the temperature underneath the net, Wilfred has added a dripper line to the top of the trellis roof. The drippers are spaced 100cm apart in the same way as those used on the vineyard floor inside the rows. The water temperature in the dripper lines is usually about 17°C. When the ambient temperature falls below 12°C, the overhead dripper is turned on to raise the temperature.
In summer, when the ambient temperature rises above 35°C, the drippers are used to cool down the vineyard. This ‘water treatment’ is normally applied for 10 minutes every hour until the ambient temperature has returned to optimal.
The use of overhead dripper lines has resulted in vines developing root systems that stretch up to the middle between the rows, making them more resistant to heat stress.
“Usually when you have vines under drip irrigation, the roots are concentrated in the area where the water is dripped. By using another dripper line between the rows, we’ve encouraged the roots to spread over a wider area. This means the vines are less sensitive to water stress than normal and they can source nutrition over a wider area. The ideal with any type of fruit production is always to have a good balance between growth above and below the soil.”
Wilfred has introduced two additional production techniques. He covers the lowest section of each young vine with a plastic bag to allow contact herbicide to be applied in the row without damaging the vines. And he leaves weeds such as tumble weeds in the working rows of young vineyards as they help to prevent sand from blowing onto the vines. Sand can cause severe damage to young vines, according to him.
Email Wilfred Stephan at [email protected]