A trailing blackberry's canes grow along the ground like vines. As these vines grow, they may need to be pruned for any number of reasons. Old canes must be removed annually for the health of the plant. And occasionally, younger blackberry canes or vines should be removed to increase fruit production or stimulate growth. Regular pruning with a pair of sharp, sterilized pruning shears is key to the health of your blackberry and just as important as watering and fertilizing.
Clean the blackberry plant in late winter or early spring before the season's new growth begins. Prune any damaged canes back to the ground. Prune the weakest, smallest vines on your blackberry plant until you are left with four to eight of the strongest vines.
Prune an underproducing blackberry plant's vines back to 24 inches in height in late winter before new growth begins. Next season, the canes will grow back thicker, branch out and create more wood on which to produce more fruit. If the canes are less than 2 feet in height, simply prune off the top inch or so of wood on the vine.
Prune fruiting canes to the ground as soon as they are done fruiting for the season. Blackberries only fruit on each cane once.