Definition - What does Overwinter mean?
Overwinter in the vineyard is an important period. It is one of the most active times in a vineyard. Following harvest, the vine prepares for winter to survive colder temperatures and in some regions, sub-freezing temperatures lasting from November to March.
To help the vines prepare for overwinter, planting a cover crop and pruning are an important step.
WineFrog explains Overwinter
To survive an overwinter period, vines must prepare themselves so they do not freeze. Once the grapes are harvested, vines can finally "relax" and utilize their energy for something else.
One of the first things a vine will do is a "root flush". This is an action in which new roots are lent out like tiny hairs. These tiny roots retrieve nutrients from the soil while the leaves which still remain continue on photosynthesizing. This root flush allows the vine to stock up with what it needs for overwinter.
During the last few weeks where the vines still have their leaves, with photosynthesis, the vines are able to concentrate sugar and starches which reduce the internal freezing point to as low as 12-degrees Fahrenheit.