Grape Pruning Tips

The point of pruning is to have control over the vines so that they remain productive for many years. Unattended, like in the wild, grapevines tend to become unruly.  They have the ability to grow and spread to cover a sizeable area.  They can produce more vegetation that is needed and is right for them to develop into mature plants.  Under such circumstances and without management, fruiting will be poor and grape quality inferior for the coming year. 

How you would prune your grapes is determined by the training system you have chosen at the beginning of the planting season. There are training systems that by the nature of their physical structure lend themselves to mechanical pruning; while there are others which need more skilled labor, but are not suitable for mechanical pruning.  

The point to remember is that an unregulated vine produces huge crops. If you have not done any pruning the previous year, you would have left buds on the vines and this means a bumper crop of grapes for the following year. However, having been allowed to bear so much fruit, the vines can’t generate enough energy to bring those fruits to the ripening stage.  Fruits produced this way will have poor quality, and not worth using at all. 

For one year old canes that grow along the cordon, pruning is done so that they are reduced to either three to five node spurs as fruiting wood, or one-node renewal spurs as vegetative wood. The diameter of the cut end of the spur should be at least pencil size. Vegetative shoots are produced from renewal spurs and are used in the next year’s fruiting wood.  It takes around three years for grapevines to be considered as mature and fully productive.

By late February through March, dormant pruning should have been completed.   For the previous summer’s growth (all one-year-old wood) pruning is done so as to leave three to five nodes per spurs, which should be evenly spaced along the cordon.

The number of buds to retain for fruiting is dependent on how much vegetative growth there was the previous year.  There are several approaches to determine the number of fruiting buds. At least 85 to 90 percent of the one-year- old wood will be removed during dormant pruning.  This is done so that grapevines are able to maintain their shape, have the fruit load distributed correctly along the cordons and keep the fruit quality high. Approximately 40 to 50 buds should be left on three-year-old, or older, vines.

To control crop load, flower cluster thinning is done.  It is the manual removal of some of the flower clusters on each shoot. This should be done when the clusters first appear. Cluster thinning is effective in increasing the vigor of very weak vines, and in reducing excessive crops on vigorous ones.

Timely reduction in crop load will result in larger, earlier ripening fruits that have higher sugar content than over-cropped vines. It is a common practice to leave one cluster per shoot.

Some growers use growth regulators to control fruit size. Gibberellic acid is a natural plant hormone usually produced by seeds. Most seedless grapes respond to gibberellic acid with an increase in berry size. Some cultivars, however, react negatively to the hormone. On the Canadice, for example, the increased berry size can result in excessive compactness and berry cracking. Concord seedless also have a negative reaction. Gibberellic acid is applied at mid bloom and at fruit set, usually about 7-10 days after bloom, to reduce berry number and increase berry size.

A reduction in bunch rot in seeded cultivars with very compact clusters can be achieved with gibberellic acid by reducing cluster compactness.  For such types of cultivars, the acid is applied when shoots are 4 to 6 inches long. Care must be taken when applying gibberellic acid because some seeded cultivars vary in their response.  Misapplication can result in increased winter damage and reduced bud fruitfulness for the following year.