Problems with Hedge Clippers
Hedge clippers cut branches indiscriminately, allowing you to make straight or curved lines to match the shape you're looking for. But pruning blackberries requires a bit more finesse. Blackberry bushes produce two types of canes. The first-year canes are new and rarely develop fruit, while the second-year canes flower and fruit, then die. Removing the second-year canes helps the plant flourish, but coming in with the strong pruning hand of hedge clippers can cut the first-year canes as well, as they intermix with the second-year ones. Removing too much of the first-year canes, such as when you cut all branches with a hedge trimmer to shape the plant, can kill the bush.
Hand pruners and loppers help you pick and choose which canes to cut. Instead of cutting the tips off all canes, choose the dead second-year canes and cut them away. Trim the ends off older first-year canes. Young first-year canes that developed late in the season shouldn't be pruned.
When to Use Hedge Trimmers
Blackberry bushes can become invasive if not managed properly. They grow from seed and develop new shoots from the roots, quickly taking over large areas if not pruned every year. If you no longer want your blackberry bush, cutting it back with hedge trimmers helps remove enough of the surviving first-year canes to kill the plant. Without enough leaves to help feed the plant, it won't survive. After cutting down the canes, you can control new canes that sprout by mowing them with a lawn mower while they're still seedlings.
Basic Pruning Tips
When trimming blackberries, hold your hand pruners so the top blade faces inward toward the center of the plant. This helps you make a clean cut; if the loppers get stuck, they're more likely to damage the part of the branch you're removing rather than the part that's staying. For first-year canes that are turning woody and developing into fruit-bearing second-year canes, cut them back to between 40 and 48 inches long to help them survive the cold season and be ready to flower in the spring. This helps provide the branches support as well; if they get too long, they get weak and can't hold up under the weight of the berries in the summer. Adding a trellis line helps support the second-year branches as they grow throughout the spring and summer.