Prune Black and Purple Raspberries
Cut back all small canes from each clump in very early spring. Leave up to five of the largest canes on every raspberry plant. Cut the lateral branches from these canes so they are 12 inches long for black raspberry plants and 18 inches long for purple raspberry plants.
Remove the top 4 inches from the shoots when the canes reach between 3 and 4 feet tall (usually occurs in mid-spring). Topping off the canes to create branched canes will encourage lateral growth and you will yield a greater berry harvest. Because the canes will reach the topping-off height at varying times, go through the canes to top them off every week during May and June to ensure you top off all canes.
Cut all canes that fruited off at just above soil level immediately after the canes stop producing. This will ensure that bacteria do not develop among your raspberry canes.
Prune Red Raspberries
Cut back all small canes from each clump in very early spring. Leave the largest canes to continue to grow. If you are growing summer-bearing raspberries, this should result in raspberry canes growing approximately 6 inches apart. If you are growing fall-bearing raspberries (two crops), your raspberry plants should fill a row that is between 1 and 2 feet wide (the canes will not be evenly spaced).
Remove the tips from the canes to remove any diseased or injured areas--2 to 3 inches total. Not only will this remove the diseased portions of the plants, but it will also encourage lateral growth.
Cut all canes that fruited off at just above soil level immediately after the canes stop producing. Summer-bearing raspberries will be finished producing for the growing season. Fall-bearing raspberries will produce a second harvest at the end of the summer on the canes that remain.