How to Grow Blackberries
In the South, blackberries hold an honored place not just on our tables but also in our memories. For many of us, one taste of a blackberry cobbler instantly brings back thoughts of childhood summers spent picking berries. Blackberries are some of the first fruits of the season. White flowers in spring are followed by clusters of fruit that turn from green to red to purple before ripening to black.
Planting & Feeding
Blackberries need deep, well-drained soil and regular moisture throughout the growing season. They are best located on slight slopes, and a northern exposure helps keep them dormant until spring freezes are past. Don't put plants where they will be in standing water during the dormant season. Rows of all kinds should be about 10 ft. apart. Plant trailing blackberries 5–8 ft. apart, erect blackberries 2–2½ ft. apart, and semierect blackberries 5–6 ft. apart.
Plant bare-root blackberries in late winter to early spring in the Upper and Middle South (USDA 6-7), and in late fall to early winter in the Lower and Coastal South (USDA 8-9). Set new bare-root plants an inch deeper than they grew at the garden center, their crowns covered with an inch of soil. Plant container-grown plants so that the top of the root ball is even with the soil surface.
Feed blackberries with a complete, balanced fertilizer just before new growth begins (rapidly swelling buds will tip you off).
Pruning & Training
Blackberry roots are perennial, but the canes are biennial: They develop and grow one year, flower and fruit the second, then die. Hence the need to distinguish between first- and second-year canes. ‘Prime Jan' and ‘Prime Jim' are exceptions. They yield two crops per season and have their own pruning regimen.
Trailing and semierect types should be trained fanwise onto a trellis; after harvest, cut to the ground all canes that have fruited, and train remaining canes onto the trellis. Canes of semierect types often become more upright as plants mature.
Erect types don't need support, but tying them to wire helps organize the canes. In midsummer of the first year, cut the canes of all kinds (except ‘Prime Jan' and ‘Prime Jim') to 2½ ft. to force side growth. Late in the dormant season, cut resulting side branches to 12–15 in. After canes bear fruit in the second year, cut them to the ground. Start the process over with new canes growing from the ground.
‘Prime Jan' and ‘Prime Jim' produce their first berries in fall on the top third of each first-year cane, and the second crop the following summer on the bottom two-thirds of each second-year cane. Pruning is done in stages. After the fall harvest, cut off the upper (just harvested) portion of each cane; after the subsequent summer harvest, cut out the remainder of each cane that has fruited.
Blackberries are subject to many pests and diseases, including scale, borers, anthracnose, leaf spot, powdery mildew, rust, and cane blight; for best success, start with healthy plants from a reputable supplier. Also look for disease-resistant selections. Blackberries are also susceptible to verticillium wilt, so don't locate them where potatoes, tomatoes, eggplants, or peppers have grown in the preceding 3 years. To control red-berry mites, spider mites, and whiteflies, apply a dormant spray containing lime sulfur once in winter, then again as buds are about to break. Rosette (also called double blossom) is a serious fungal disease of blackberries in the South. New growth and blossoms of infected plants are deformed, and fruit fails to set. Most selections are susceptible, though ‘Navaho' and ‘Gem' have shown some resistance. Good sanitation is the best control. Removal of infected canes after harvest may help, but severely affected plants require removal of all canes at ground level; the resulting new growth will be free of the fungus. Burn or discard prunings. Keep plantings weed free. Wild blackberries can spread the disease, so be sure to plant well away from them.
A new pest, spotted-wing drosophila (a vinegar fly) has found its way into Southern gardens. It makes tiny holes over sunken areas in ripe fruit, including blackberries. So far, the only control is to completely cover plants with fine netting.