Grapevines usually are pruned in the winter, after the coldest weather is past. This timeframe coincides with late February through March for much of the country, although the optimal pruning time varies for each growing zone.
The first winter after planting, the two canes that branch off from the main trunk are pruned so that they have three buds each. Limiting the number of buds allows more energy to flow to the developing trunk. During the second winter, the side canes are pruned to 10 buds each, and an additional four shoots are allowed to develop. During the third winter, the original side canes are pruned away, last year's shoots are pruned to 10 buds each, and four more shoots are allowed to develop. This final pruning process is repeated each subsequent year.