Grapes should be pruned while dormant. When it comes to pruning grapes, the most common mistake people make is not pruning hard enough. Light pruning doesn’t promote adequate fruiting whereas heavy pruning provides the greatest quality of grapes. Improper pruning of grapevines can make the difference between a good crop and a bad one. When pruning grapes, you’ll want to cut off as much of the old wood as possible. This will encourage the growth of new wood, which is where the fruit is produced.