Concord grapes (Vitis labrusca 'Concord') form on stems that grow the previous year--"year old wood," as gardeners call it. Spur pruning year-old stems back to one or two live buds limits the amount of fruit produced, ensuring the grape clusters develop their best shape, size and flavor. Once the stems are three years old, conduct an annual spur pruning on the grapevines in early spring.
Examine the vine stem structure on the Concord grape plant. The main stem trunk emerges from the soil and supports arms with lots of twiggy stems.
Cut away any dead arms and vines with your pruners, making a crisp, clean cutting motion through stems with the hand pruners. Make the cut 1/2 inch above a dormant bud or junction to a lower, live branch. Cut dead trunks at their base, 2 to 4 inches from the ground, so the entire branching system can be pulled away and removed. Snip off any support ties holding dead stems to the trellis or other support structure.
Prune all weak or scrawny side shoots from all the arms growing off the main trunks of the vine. Make the cuts 1/4 to 1/2 inch above their point of attachment to the arms.
Retain the strongest side shoots on the arms so that their spacing ranges between 6 to 12 inches apart. Remove any shoots that fall between the selected and appropriately spaced strongest shoots so the entire arm bears only the healthiest-looking shoots.
Reduce the length of all the strong shoots on the arms so each bears only two buds. This creates the "spur". Make the pruning cut 1 1/2 inches above the second bud, as counted from the main branch outwards to shoot ends.
Gently tie any pruned vines or canes to the support structure with twine if you feel they need training or for added strength for the upcoming growing season.
Repeat all steps again one year later. The only difference in all subsequent year's pruning is the lowermost shoot is pruned off each spur and the next lowest two new buds form the new spur for the growing season.