SATI Beyond The Bunch 2nd Quarter 2024

Temperature

Covering vines at bud break can partly mitigate frost risks and provide favourable conditions for early growth. The danger of overheating under plastic is most significant in very hot regions on very hot days – obviously. Temperatures in the high 40s have been recorded under plastic during trials in the Northern Cape, but the researchers observed no adverse effects on the grapevines. High relative humidity under the plastic likely helped the plants cope. Plastic strips resting directly on the canopy are associated with scorching, especially of leaves that are in contact with or compressed by the plastic. The damage is generally insufficient to affect grapevine performance or berry quality. Growers can avoid overheating – and even achieve cooler vineyards – by choosing the appropriate plastic and designing and managing the system to allow sufficient ventilation.

Not all plastic covers increase vineyard temperatures. The infrared (760–3 000 nm wavelength) component of sunlight causes heating, and plastics can promote more or less of a greenhouse effect depending on how much infrared radiation they transmit. Growers should obtain this information from their plastic supplier. Some plastics are designed to diffuse or scatter the incoming light. Diffuse light is associated with more efficient photosynthesis and lower leaf temperatures. The extent of warming also depends on climate and weather, with a relatively greater effect in cooler and sunnier conditions. Applying plastic covers during dormancy to raise vineyard temperatures is used to accelerate bud break and bring the harvest forward. Vines should be covered once their chill requirement has been satisfied. Covering vines before bud break in warm areas may increase blind buds and reduce berry mass and colour.

Temporary plastic rain covers over Brazilian vineyards.

BEYOND THE BUNCH • 10 • QUARTER 2 • 2024

EUNICE AVENANT