Pruning thornless blackberries (Rubus spp.) increases production and improves plant health. Pruning your blackberry plants makes it easier to pick the fruit, even in thornless varieties. Blackberries have perennial roots, so they come back each year in in U.S. Department of Agriculture plant hardiness zones 4 through 9 but each cane grows for two years and then dies. New canes, called primocanes, won’t flower and produce berries until their second year. Then the second-year canes, called floricanes, fruit and die after harvest.
Growing thornless cultivars, such as ‘Triple Crown’ (Rubus 'Triple Crown,' USDA zones 5 through 8), ‘Navaho’ (Rubus 'Navaho,' USDA zones 6 through 8) and ‘Apache’ (Rubus 'Apache,' USDA zones 5 through 9) means you won’t have to fight through thorn-covered branches to reach berries for harvesting or for pruning your plants.
Pruning Thornless Blackberries
With a pair of handheld pruners and seasonal monitoring of your thornless blackberry plants, keep your blackberries maintained for increased production.
Prepare the Pruners
Clean the blades of a pair of bypass pruners with undiluted household disinfectant or rubbing alcohol before each use. The rubbing alcohol should have at least 70 percent concentration. Soak the cutting blades of the pruners in the cleaner for at least 30 seconds and wipe dry with a clean rag. Cleaning the pruners with disinfectant reduces the chance of spreading diseases to your blackberries.
Pruning During the Growing Season
Tip erect thornless blackberry primocanes when they reach 30 to 36 inches tall during the growing season. To tip the branches, make cuts at 45-degree angles, removing only enough of the terminal, or end, of the branch to reach the desired height.
Cut the tips of trailing thornless primocanes when they reach 6 inches above the highest level of their trellis. Tipping the canes encourages side canes to grow, which is where the fruit production happens the next year. It also prevents excessive sprawling and strengthens the canes to hold the weight of the fruits.
Pruning Blackberries After Harvest
Cut floricanes to the ground after harvest, usually in late summer or early fall. Floricanes will die and can harbor insects and diseases. Removing them also makes it easier to access the rest of the canes for pruning.
Thinning the Blackberries for Production
Thin the blackberries in late winter or early spring. Thin erect thornless blackberries to about six primocanes per linear foot and trailing thornless blackberries to eight to 10 canes per plant. Thinning blackberries involves removing primocanes to the ground making clean, 45-degree cuts. Select the healthiest canes to leave and remove any diseased, twiggy or droopy canes. On trailing varieties, tie the remaining eight to 10 canes to the trellis wires for support.
Tip the Lateral Branches
Perform lateral pruning in winter or early spring. Cut lateral branches, or side branches, on erect thornless varieties to 12 to 14 inches in length. On trailing varieties, prune the laterals to 18 to 24 inches long so they don't crowd the other trellis layers.