Heard it through the grapevine

Linn County Master Gardener, Lisa Slattery, sent the following about pruning grapevines: 

This week's blog is about pruning grapevines, it's the last in a series about pruning small fruits for the home gardener. Grapevines require specific pruning practices in order to train them on a trellis and reap a good harvest of fruit.  

Grapevines produce fruit on the previous season's growth. The vines can produce hundreds of buds and if the vine isn't pruned the plant won't have the energy to ripen the crop or produce enough vegetative growth. To maximize fruit crop, grapevines are trained to a common system called the four-cane (or six-cane) Kniffin system. I'll cover the four-cane system here since that's the easiest for home growers.

Grapevines are trained to grow on a wire trellis with recommended heights of 2 to 3 feet above the ground with a second wire about 5 to 6 feet high. This is the basis of the Kniffin pruning system. Pruning in this manner trains the vine to grow four fruiting canes (two on each side) on the two trellis wires. You will select four canes to grow on the upper wire, two going in each direction. Then select four canes for the lower wire. It can be helpful to tie a colored ribbon on the canes you want to keep. Once main cane selection is done, choose four new short canes or buds (called renewal spurs) that generally grow close to the main cane. These will provide shoots to become next year's canes. Remove the rest of the canes more than one-year-old.

 The main canes and renewal spurs are pruned differently. On both trellis levels of the grapevine, the main canes should be pruned back to eight to twelve buds. If the grapevine is really vigorous, leave 12 buds per cane. Trim back to eight if the vine is less vigorous. The renewal spurs should be pruned back to one or two buds.  

When you're done pruning the ideal number of buds left should be close to 60. This includes both the buds on the fruiting canes and those on the renewal spurs.

 Out of all the small fruits for home gardener grapes are the most specific when it comes to pruning practices. If you need diagrams of the Kniffin pruning system visit the Iowa State University Extension viticulture homepage at

http://viticulture.hort.iastate.edu/  where you'll find this information plus loads more on growing grapes in Iowa.