The hardiness of grape wood and bud varies with species and variety. European (vinifera) type vines are the least hardy and require warm to hot summers and mild winters. American (labrusca, aestivalis) and French American hybrid vines will withstand humid summers and cold winters and these are the types grown extensively in Missouri.
A grapevine must undergo many changes in order to survive the winter and grow the following spring. The vine must mature properly at the end of the growing season and must harden to be able to withstand the cold winter temperatures. This hardiness must be maintained throughout the dormant season. In late winter or early spring, the vine will lose hardiness and resume growth when conditions are favorable.
The vine hardens in fall to a certain level that is maintained during the winter Once a variety has reached its particular hardiness level, it will remain inactive and will not resume growth if the weather warms. This is because the plant needs to fulfill its rest period (a certain number of hours of chilling temperatures) before it will begin to grow. Once the vine satisfies its rest, it will then start to grow and lose hardiness if the weather is favorable. This is why a plant will not resume active growth during a warm spell in January but will lose hardiness and begin to grow under favorable conditions in March.
At any given time during this seasonal cycle, different plant parts will vary in hardiness. The primary bud is more susceptible to cold injury than the secondary which is in turn less hardy than the tertiary bud. The wood of the grape is more resistant to injury than are the buds.