SATI Beyond The Bunch 2nd Quarter 2024

EUNICE AVENANT

One application discussed below is manipulating the harvest window. Growers wishing to shift their harvest window should remember that plastic covers are part of a broader management strategy. Each grower must determine appropriate pruning and other interventions for their conditions and cultivars.

Covering earlier Deploying plastic covers during dormancy will raise vineyard temperatures and can advance bud break and subsequent ripening. Changing the harvest window may afford an advantage in specific markets or lighten the logistical challenge of harvesting many hectares of the same cultivar. Earlier covering gives more advancement than later covering. Italian researchers report that covering vineyards up to 50 days before the natural bud break will bring the harvest date forward by as much as 40 days. They consider this strategy most suitable for early-maturing and seedless cultivars. In South African studies, covering vineyards 35 or more days before natural bud break resulted in a 10–28-day advancement of the harvest date. In a commercial trial, plastic installed at bud break advanced the harvest by seven days. Heating calls for vineyards to be fully enclosed with plastic designed to trap heat. Chill requirements must be satisfied before grapevines are covered. Raising temperatures is only effective when low temperatures keep plants dormant – using plastics to advance bud break will have little effect in warm conditions. Be aware that too-high temperatures following bud break will partly reverse the advancing effect while possibly compromising berry size and colour. Growers should monitor temperatures and open the sides of enclosed structures as needed to avoid overheating. Plastic covers deployed within a week of bud break can boost growth and improve berry size and quality in white cultivars by creating softer growing conditions. Colour development in red and black cultivars can suffer when these are covered early. Early covering will help mitigate but not entirely address frost risk.

Covering later South African growers mainly use plastic covers to protect their table grapes from rain. Vines are usually covered at about véraison. Rain protection plastics have different radiometric properties than heat-trapping plastics – more on this in the next article. Rain covers can be elevated above or rest on the canopy and are usually open at the sides and over the work row to allow air circulation. Poor circulation contributes to dangerously high temperatures and humidity while potentially encouraging decay-causing microbes. Late covering can also prolong the harvest window of some cultivars by protecting bunches hanging on the vine. Italian researchers report this works best for late-ripening cultivars with large berries and strong skins. They note that maintaining berry hydration during prolonged hanging is critical and suggest applying a little extra irrigation and nutrition. Bunches must be monitored for disease. In a South African study, plastic protection of vines allowed grapes to be harvested 3–6 weeks later than usual. The covers didn’t delay ripening but protected grapes from rain and other adverse conditions so that the harvest could be delayed without increasing risks. Logistics partly determines the timing of covering. Growers with many hectares at risk may start covering earlier rather than face scrambling later in the season when labour is stretched thin. Growers with few hectares could let the weather forecast guide the decision to deploy covers. Keep in mind that shading delays sugar develop ment. This effect may be negligible if grapevines are covered late and light is carefully managed, but most growers can expect harvest delays of 2–3 days.

BEYOND THE BUNCH • 6 • QUARTER 2 • 2024