Thornless blackberries grow less aggressively than the thorned varieties, but still need regular pruning. Otherwise, they become a tangled, unproductive mass. Before you attempt any pruning, consult a local county extension office for information about your specific variety. Fall-bearing blackberries produce berries on new wood (primocanes). They are the simplest to prune because you can mow the entire patch down after harvest. Summer-bearing berries produce fruit on 2-year-old wood (floricanes). They require more care to protect the canes from winter damage. Prune these types to remove old wood after harvest, but leave new canes for the following year's harvest.
Prune young blackberry plants back to 6 inches above the soil in late winter. This hard pruning will encourage stocky, vigorous new growth.
Cut off the tips of new canes during the summer so they stand 30 to 36 inches high. Cutting the tips encourages strong, thicker canes and lateral branches that require less trellising and support.
Allow the blackberry thicket to grow 12 inches wide. Remove any additional suckers that grow up at ground level. Thornless blackberries have a tendency to sucker and left untended, your thicket will quickly become so wide that it is hard to maintain.
Prune out old, diseased or dead canes after harvest and destroy them to prevent the spread of disease. Mow fall-bearing types down to the ground or cut them with loppers. In late spring, prune the lateral branches back to 12 inches. This pruning encourages larger, better quality fruit, according to the West Virginia University Extension Service.
A 2- to 3-inch wood chip or straw mulch conserves moisture and protects the plants from winter temperatures.