The scion variety (that above the graft) is what will determine your hardiness as far as bud wood goes. Having a grafted vine makes the vine more vulnerable to cold injury at the site of the graft which is why mounding over graft unions in cooler growing zones is necessary. If a grafted vine is killed back to the graft or below any shoots the roots produce will simply be rootstock, a non fruit bearing vine. Rootstock has no positive impact on cold hardiness but can make a vine more susceptible to injury.

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