Winter hardiness
Winter hardiness is a grapevine's ability to survive cold during its dormant season. It helps determine which varieties can sustain vineyards in places such as Minnesota, Québec, and the Finger Lakes. Hardiness varies with the variety, the tissue being considered, and the vine's changing physiological condition. A published minimum temperature is consequently an observation under specified conditions, not a permanent survival guarantee.
Hardiness changes through the season
As autumn progresses, vines acclimate: their tissues become better able to tolerate freezing temperatures. After the vine's chilling requirement has been met, sufficiently warm weather can begin reversing that protection. This loss of hardiness is called deacclimation.
A sudden freeze after a warm interval can therefore cause damage at a temperature that a fully acclimated vine survived earlier in winter. Gradual cooling may allow some reacclimation. Joseph Fiola's University of Maryland account emphasizes this sequence of weather, which explains why a winter's lowest thermometer reading alone is an incomplete measure of danger.
Surviving roots, injured shoots
Winter injury may kill buds, sections of wood, or an entire trunk while leaving the roots alive. In 2019, University of Minnesota Extension documented severe injury in some vineyards and survival in others, with differences among sites and varieties. Surviving shoots from the base could be trained into replacement trunks. The account also cautioned that the possible contribution of trunk disease required investigation rather than diagnosis from appearance alone.
For own-rooted varieties such as Frontenac and Marquette, renewal from the base can retain the desired grape. On a grafted vine, the origin of a replacement shoot matters: a shoot below the union belongs to the rootstock.
Winter survival and spring frost tolerance are separate questions. Once growth begins, tender green shoots can be damaged by comparatively mild freezing conditions even on a very winter-hardy variety.
Sources
- Joseph Fiola, “Warm Weather and Deacclimation”, University of Maryland Extension, updated 2021.
- Annie Klodd, “Renewing Grapevines from Suckers Following Severe Winter Injury”, University of Minnesota Extension, 2019.
- Madeline Wimmer, “Cold-climate grapes”, University of Minnesota Extension, own-rooted vine discussion.