Each type of bramble has a unique growth pattern that needs an individual pruning technique. To understand how to prune brambles such as raspberries and blackberries, you must first realize that these are biennial producers. Each spring new shoots arise from the crown but they do not flower or bear fruit until the second year. Cut these canes back to the base when they die after fruiting. Unpruned plants develop into a thicket that gives the name bramble true meaning.
There is an exception to this life cycle. Canes of everbearing brambles produce a crop late in the first summer and repeat with another crop in midsummer of the second year before they die.
Pruning Black And Purple Raspberries
Plant black and purple raspberries with one plant per hill. Thin all but seven or eight of the strongest and thickest stems as the new plants grow. Either after harvest or the following spring, prune off close to the ground those canes that have borne fruit. If waiting until spring, prune before the buds begin to swell but after danger of frost has passed.
In the summer top the first-year canes to encourage a strong plant with fruitful side branches. Top new canes of black raspberries when they reach 24 inches tall by cutting or snapping off the top 2 to 4 inches of growth. Do the same to purple raspberries when the canes approach 30 inches tall.
During the following dormant season, cut back the side branches on new fruiting canes to increase berry size. On black raspberries shorten the laterals until they contain 8 to 12 buds or are 6 to 10 inches long. Prune purple raspberries, which are more vigorous, back to 11 to 16 buds. Then cut back any spindly or short laterals. Ideally the plant should have four to five strong fruiting canes remaining.
Pruning Red Raspberries
Use the same procedures described for black and purple raspberries for red raspberries, but eliminate the summer topping. Topping reduces yields of red raspberries. These raspberries normally don’t produce any side branches because the canes aren’t topped to encourage branching.
Grow red raspberries in a hedgerow and don’t let the row spread wider than 18 inches. Thin individual canes by cutting them at ground level so they are spaced 6 to 10 inches apart. Leave the largest diameter canes, which are the most productive. After the harvest remove canes that have borne fruit. During the dormant season, head back canes growing within a wire support by one quarter of their length. If there is no support, keep the canes at about 3 feet tall.
Treat everbearing cultivars similarly, but leave the first-year canes that have just fruited intact in the fall. They will fruit again in summer, after which they can be removed.
When planting blackberries in a hedgerow, restrict upright cultivars to three or four canes per plant and cut or pull out the rest. Allow 12 to 18 shoots per foot with trailing types. Prune blackberries the same as black raspberries with one exception: Leave laterals on second-year canes 12 to 18 inches long.
Thin trailing blackberries to seven or eight canes per plant by removing unwanted canes at ground level. Shorten the canes to about 5 feet long and tie them all to a stake or trellis. Omit the summer topping. On upright-growing blackberries top new shoots back to 30 to 36 inches. Cut or pull out excess sucker shoots during the summer or the plant will develop into an unruly thicket.
Pruning Boysenberries and Loganberries
Like the other bramble fruits, these berries are biennial and require annual pruning. Support the canes on a 36-inch-high wire stretched between two poles. In spring thin all but five of the first-year canes and remove the old fruiting canes. Let the new canes grow on the ground until they exceed 36 inches. Then tie them to the support and head them just above the wire to encourage strong lateral branches. The following spring cut the laterals to 18 inches; they will fruit later in the summer. Remove fruiting canes at ground level after harvest.