Most growers train blackberries onto trellises to increase fruit production. Use a simpler method--hill planting--for container grown blackberries. Limiting the number of canes in the hill and pruning to control plant size provides a modest harvest in a small space. This perennial bramble fruits in the second year of growth. The crop depends on success over the winter--probably the trickiest part of container growing blackberries.
Provide a large growing container. A diameter of 24 inches and a depth of 18 inches provides room for roots of this vigorous bramble and gives some extra protection against winter cold. Stout handles make moving the plant to shelter during the worst part of the winter much easier.
Choose a blackberry cultivar suited to local growing conditions, with erect canes and exceptional winter hardiness. Thornless blackberry varieties fit into tight spaces with fewer problems, but the quality and vigor of thornless cultivars may not compare well to the best thorned plants. Since container plantings provide berries only for home use, consider types which offer flavor rather than shipping resistance. Some commercial blackberries were chosen for toughness and shelf life rather than taste.
Plant the blackberry in late winter--ask the supplier for a late shipping date to limit the plant's winter exposure. Place the pot in a sheltered location to limit extreme temperature shifts. Fill the pot with potting soil rich in organic material. Blackberries grow best in deep sandy soil, so avoid dense potting soils or topsoil with high amounts of clay. Set the blackberry root 4 inches deep in the center of the pot and water well.
Fertilize with plant food designed for container growing--easily measured and applied by watering. Water the plant sparingly until weather warms. Soil should be moist but not wet. Check by scraping aside a top layer before watering--out-of-doors the top layer dries quickly, but the deeper soil layers remain moist.
Prune out all but four strong canes in midsummer--clip unwanted canes back to ground level. Cut the four remaining blackberry canes back to 36 inches high to encourage side branches and limit vertical growth. Mulch the pot with a thick layer of peat or sphagnum moss to limit heat stress. Keep the blackberry in full sun and water as needed. Commercial plantings require an inch of water per week during the summer--container plants usually need watering more often.