Prune blackberry bushes by determining which canes need to be pruned, using loppers to cut out spent floricanes and discarding all of the pruned plant material. By removing spent floricanes, more room is available for new primocanes to grow during the following year.
Blackberries are considered biennial fruiting plants. Vegetative shoots called primocanes are produced in the first year of growth, while floricanes are the fruit-producing canes. Left unpruned, primocanes can grow several feet in length. Floricanes die once fruit is harvested from them, so remove them promptly thereafter.
Determine which canes need to be pruned. During the growing season, tip back developing primocanes to 48 inches in late June and July. Once lateral shoots develop, tip these back to 18 inches in the spring of the following year to avoid injury during winter. Never prune the floricanes before the fruiting season unless they are damaged or diseased. When the fruiting season is over, cut out spent floricanes at the root crown with loppers.
Thorny and thornless blackberry bushes are generally pruned in the same manner. Hand pruners and loppers are both effective tools to remove spent canes. Sharp tools make smoother cuts and cause less stress to the plants.