Grapevines often produce far too many buds. If all of them are allowed to produce fruit, you will have many clumps but the grapes will be the size of raisins. By properly pruning, you will end up with a smaller number of clumps that have larger grapes. Grapes only develop on parts of the vine that grew the previous summer. Vine sections more than one year old are "non-productive" and should be removed. This old wood is usually thicker and has mature, loose bark than the new growth.
In Iowa, late February to early April are ideal pruning times. The vines usually add so much new growth each summer that they need to be pruned every spring for best production and plant health. To maximize the crops yield, grapevines should be trained to a specific system. The training and pruning of grapevines is often confusing to many gardeners. Before pruning your grapevines you should have an understanding of the growth and fruiting characteristics of those grapevines. ISU Extension has more information about Pruning Grapevines available.
bmr
5/4/2008