Bud dormancy pattern, chilling requirement, and cold hardiness in Vitis vinifera L. 'Chardonnay' and 'Riesling'

In recent years, new vineyards have been established in southwestern Ontario. The open water of Lake Erie provides some winter protection for Vitis hybrids and winter-hardy Vitis vinifera L. cultivars in this area. However, winter damage is possible when vines are grown distant from the open water or when lakes are frozen. To better understand the risks to winter survival, the dormancy and chilling phenology were studied over three winters from 2013 -2016. Ten dormant canes of two V. vinifera cultivars, 'Chardonnay' and 'Riesling', were collected weekly from 1 September until 30 March from the mature vines in a commercial vineyard located at St. Williams (Ontario). The canes defoliated in early October, and the endodormancy was completed by the end of December. The cumulative chilling hours (0 -7.2 degrees C) from defoliation until the completion of endodormancy were averaged 606 hours for 'Chardonnay' and 665 hours for 'Riesling'. 'Chardonnay' buds were slightly less hardy than 'Riesling' to cold temperatures, with a threshold of about -24 degrees C for 'Chardonnay' and -25 degrees C for 'Riesling'. Most primary buds of both cultivars died after 16 February 2015, and more than half died after 12 February 2014, due to severe low temperatures of -33.1 and -26 degrees C, respectively.