Evaluation of hilling efficiency as a method of protection of vine against winter cold in Québec

Extreme minimum temperatures during wintertime sometimes dropping to -35°C can damage the latent buds of hybrid vines whose tolerance to cold temperatures is between -20°C and -30°C and accordingly can compromise their survival. To counter these adverse effects, the majority of winegrowers in Québec cover their vine stocks with 40 to 60 cm of earth taken between the rows. Based on temperature measurements registered within the earth ridges and between the rows it is possible to describe temperature variations near the fruit buds and in the root zone of the Vitis vinifera L. var. Melon. Temperature measurements were carried out during the 1994- 1995 cold season in a vineyard near Sherbrooke, Québec. Five 95 cm long thermocables were installed in the ground, in a straight line and perpendicular to the hilling lines. The thermocables were connected to an automatic data acquisition system. Results show that with hilling, under at least 15 cm of earth, it is possible to conserve the latent buds of the vine at temperatures superior to their cold tolerance threshold as defined by the Leddet exotherms (- 13°C) whether or not in the presence of snow cover. In fact, the minimum temperature reached in the bud zone was never lower than -2°C even when the minimum air temperature reached -33°C. However, because of their higher elevation in relation to the ground, these ridges are more exposed on the one hand to surface climatic elements such as wind and radiation and, on the other hand, this form of micro-topography permits the dissipation of internal ground heat more rapidly than a flat surface. In the Fall, when there is still little snow accumulation on the ground, the upper portion of the ridges remains free of snow and the first centimeters of ground on the surface can present temperature variations in the order of 10°C within 24 hours, while the ground surface between the ridges remains thermally quite stable because of the presence of snow. The thawing periods of January, often accompanied by showers, also affect temperature conditions within these ridges all the way down to the root zone. For example, the January 17, 1995 rain shower (15.6 mm of rain) produced an elevation of 6°C in average temperatures in the bud zone. When the hot air mass responsible for the thawing is replaced by a cold air mass accompanied by snowfall, the earth ridge freshly covered by snow conserves temperatures above the cold tolerance threshold of the buds even if the ambient air temperature drops to -20°C (27 January 1995). Results of this research show that hilling is an efficient method for combating heavy winter colds towards protecting the latent buds of the vine but its impact on the elevation of temperatures at the roots remains minimal because it maintains the root zone temperature 1 to 2°C higher than the same zone without hilling. Given that the tolerance to cold of roots is from -8 to -10°C, this gain of a few degrees nevertheless increases the vine's chances of survival.

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