"Bramble" the name given to plants in the genus Rubus, which includes the many forms of raspberries(including red, golden, black, and purple kinds) and blackberries (both upright and trailing types).
As with any plant, pruning reduces the chance of pest invasion and infection, allows better air circulation and light exposure, and so ultimately, more fruit. Pruning most brambles is a necessity simply because they are rampant growers.
How Brambles Grow. It's easier to remember how to prune brambles if you understand how they grow. The plants' underground parts the roots and crown are perennial, but the canes (upright stems) that arise from the crown are biennial, meaning that they live just two years. In its first year, a cane grows vigorously, develops a strong structure, and stores energy. It produces fruit in its second year. After fruiting, the cane declines and dies, though some species called everbearing produce fruit late in their first season and again in their second year.
Since the 2-year old canes die after fruiting, you can cut them back to the ground immediately after harvest. Prune carefully to avoid damaging the bramble's crown with your pruners.
Each category of brambles has a different growth and fruiting habit, so pruning practices vary. At any time of year, you can remove dead, damaged, or diseased branches and canes.
Note: the recommended pruned lengths are for untrellised brambles. If you provide support for your brambles, you can leave longer canes.
Summer-Bearing Red and Golden Raspberries. These brambles produce tall, unbranched canes as well as root suckers (canes that spring from the roots rather than the crown of the plant).
In late winter or early spring before new growth begins, remove canes until there are eight strong canes per three feet of row. If your brambles grow in a bed rather than a row, thin them to six to ten canes per square foot of row. Another way to measure is to leave an average of four to six inches between canes.
In the early spring as growth is beginning, prune back the tips of second-year canes so they stand erect and about 4 to 5 feet high.
Black and Purple Raspebriies. Unlike their red and golden cousins, black and purple raspberry canes branch vigorously, and grow in a vase shape, producing few if any root suckers. If allowed to grow unpruned, the long canes that come in contact with soil form roots, giving rise to new plants.
In spring, when first-year canes reach 18 to 20 inches tall, prune back the tips. This encourages strong branching. The following spring, trim branches back to 8 to 12 inches. When thinning canes, leave five to ten canes per plant, depending on the soil fertility.
Everbearing Raspberries. Despite their name, these varieties also called fall-bearing don't bear fruit constantly. The first year, canes bear fruit at their tips. The following summer, the same canes bear fruit lower down.
After fall harvest the first year, remove the cane tip. After the summer harvest the following year, prune the rest of the cane to the ground.
Remove dead, damaged, or diseased canes any time of year.
Photo by National Gardening Association