Like other vine berries, boysenberries need regular trimming or pruning to stay healthy and productive. Do this twice a year in every year except the first year of planting. The rules for pruning are the same, regardless of whether you have the thorny or thornless variety of boysenberry. Once you know what to look for with your boysenberry plants, they will be easy to prune.
Identify the difference between the two types of vine on your boysenberry. Primocanes will sprout this year. They do not produce fruit in the first year. Next year, those primocanes will turn into floracanes, which will produce both fruit and flowers. It is important to note the differences, because you will treat them differently.
Prune floracanes all the way down to the ground directly after they have completed fruiting in the summer. Do not be afraid to cut too much off; this is actually the healthiest thing for the plant.
Cut the primocanes back to a length of 6 to 8 feet after you have tied them to poles, trellises, or whatever other training device you are using for your boysenberry vines. Wait until late winter in your area to move to the next round of pruning in step 4.
Prune all types of canes back to somewhere between 4 and 5 1/2 feet sometime during late winter. Do not do this any earlier or you will endanger the health of your boysenberry plants. Completely cull any canes that look weak and spindly compared to the majority of the canes.