Blackberries are similar to black raspberries in their pruning requirements. Thornless erect and semierect blackberries are pruned three times a year - spring, summer, and after harvest.
Spring pruning should be done in mid-March. Remove one-fourth of the total number of canes that arise from the crown at ground level. Cut out spindlier canes and ones that have red-necked cane borer swellings. Next, cut back the laterals arising from the main canes to about 16 inches in length.
Summer topping is done when the new canes reach 32 to 42 inches in height; remove the top 3 to 4 inches. Do this only when the canes are dry to avoid spreading disease. This will cause lateral branches to develop, producing a stockier plant and more fruit-bearing wood.
After harvest, the canes that fruited die and should be pruned out at ground level anytime after harvest is complete.
