Thornless blackberries are a modern shrub cultivar of the genus Rubus and are closely related to raspberries. Thornless blackberries bloom and fruit in spring and summer on vining canes covered with dark green leaves. They are hardy in USDA zones 4 to 9, thriving in full sun and soil with a slightly acidic to neutral pH.
Water your blackberry shrubs so that the soil surrounding the roots is uniformly moist an inch down throughout the active growing season. Do not allow the plants to dry out in the fall and winter; supplement rainfall with occasional deep watering if the soil feels dry.
Feed your blackberries around their roots with a high-quality, water-soluble fertilizer with a balanced formulation of nitrogen, phosphorus and potassium. Start fertilizer applications in the spring when the blackberry canes are in bloom and repeat in the summer and fall after the fruit harvest. Mulch your blackberry plants with a few inches of compost, well-aged manure or leaf mold to raise the nutrient levels in the soil, prevent moisture loss to evaporation and protect the roots in winter.
Prune your thornless blackberries each year in late summer or fall after fruiting. Cut down the length of older canes to boost berry size and give structure to the shrub. Prune the tips of new canes to force more lateral branches to form.
Protect your blackberries from fruit-poaching birds by covering the shrubs with small-gauge nylon netting. If you see green aphids on the stems and canes, hose them off with a strong stream of water from your garden hose or sprayer attachment. If some of your canes are dying, cut them down to the base with clean sharp secateurs and discard them in the trash, not the compost bin. To control powdery mold or mildew, cut away the infected foliage and canes and thin the shrubs to ensure air circulation. Scale back watering for a few weeks until the problem abates.