Domestic blackberry vines are deciduous and will continue to produce fruit for 15 to 20 years in a temperate climate. Erect varieties of blackberry plants produce long, upright canes. Trailing varieties spread laterally along a fence or trellis. Blackberries fruit on second-year canes, according to the University of Florida IFAS Extension Program. The first year, non-fruiting canes are called primocanes; in the second year, when the canes produce fruit, they are called floricanes. After fruiting, the floricanes die out and are replaced by new growth. They need occasional trimming.
Prune out the thinner, weaker stalks early in the growing season with pruning shears, leaving five or six healthy canes on each plant. Remove any dead or diseased-looking canes when you're pruning. Cut the stalks as close to the ground as possible.
Trim the tips off the primocanes to encourage division and increased fruit production. Using a sharp pair of shears, clip off the top 1 inch of each cane. Trim the tips of erect varieties when the cane is 30 to 36 inches tall; trim trailing varieties when the canes are 46 inches long.
Remove the floricanes after the harvest; these second-year canes will not reproduce and will eventually shrivel and die on the plant. Cut out the floricanes close to the ground to improve the plant's health and appearance.
Thin and remove dead or diseased canes at any time of year. When trimming out diseased canes, sanitize your tools between each cut to avoid spreading diseases.