Frontenac is a red wine grape prized for its cold hardiness, deep blue-red color and sweet juice. It is a hybrid of the two grape cultivars Vitis vinifera and Vitis riparia, according to the University of Iowa. Frontenac grapes are small to mid-size with a blue-black, thick skin and deep purplish blue interiors, making their juice an unusually deep hue. They are the most commonly grown red grape in Minnesota and may be trained and pruned in keeping with the high cordon system. Pruning thins the buds and shoots on the vines to ensure the greatest number of high quality large grapes in large clusters.
Prune your Frontenac grape vines while dormant in the late winter or early spring, but before April, when the vines are coming back to life and the buds can begin to swell. Refrain from pruning in the first year after planting except to remove dead or diseased canes to focus the plant on root development. Prune back with secateurs to a point of healthy cane tissue and discard the cuttings.
Cut dead or unhealthy canes in the second year back to a point of healthy tissue, but leave all healthy canes in place to keep the focus on root development and not cane replacement. Choose one cane as the main trunk and cut it down to 4 inches below the top support wire. Tie a length of soft twine to the top of this freshly cut cane and tie the other end to the top support wire under light tension to encourage the development of a straight and upright trunk.
Prune canes substantially for the first time in their third year after planting. Preserve two to four of the best, most vigorous canes for long fruiting cordons that attach to the support structure horizontally.
Prune away all but two to four of the most robust shoots on each cordon. Cut away all of the green shoots that grow below the lower cordons with secateurs or pruning shears. Place the cuts nearly flush but just 1/8 inch off the parent cane.
Cut down the length of the remaining canes emanating from the cordons to half the size of the un-pruned cordons so that no less than 8 to 12 buds or leaf nodes but no more than 60 remain on each cane. Too many buds lead to excess vegetative growth and leaf shade that inhibits fruit ripening and root development.
Repeat Steps 3 through 5 each year for mature vines that are four years of age and older to keep the vines producing fruiting canes. Prune back any dead, diseased or otherwise compromised vines each year, burning or destroying any diseased canes.