Seedless Thompson grapes are one of the most popular grape varieties and are usually the kind of grapes sold in grocery stores. Thompson grape vines are heavy producers, yielding 20 to 30 clusters of pale-green grapes per vine. Because the Thompson grape vine is such a vigorous grower and fruit producer, proper pruning is essential to keeping the vine healthy and productive. Many different pruning methods exist for grape vines, but the "four-cane Kniffen method" is the easiest. Prune your Thompson grape vine in late winter or early spring, or in mid-winter when the canes have hardened, if you're performing heavier pruning.
Select two canes on each side of the main vine that are the healthiest young canes. Tie colored yarn onto the canes to mark them.
Clip back the selected canes if necessary so that they have no more than 20 buds each. Make clean cuts when you're pruning the vine.
Remove the other canes, leaving the selected ones intact on the vine. Cut the other canes back to the main vine, but leave 10 or 12 short spurs, or two buds, on either side of the trunk for growing the following year's canes.
Prune away all watersprouts or "watershoots" growing from the base of the main vine. Cut away any canes that are more than 1-year-old.
Prune away and destroy any diseased fruit, canes or leaves, especially if your grape vine is infected with a fungal disease. Be sure to disinfect your pruning tools before and after pruning diseased vines.