Prepare a planting hole large enough to allow the roots to spread out naturally. Do not prune the roots except to remove damaged ones. Set plants at the same depth they were planted in the nursery. After planting, tamp the soil firmly to remove air pockets around the roots. Water all new plantings well, immediately after planting.
Mixed fertilizers are satisfactory for blackberries.
For best results, apply fertilizer in early spring when growth starts and again in summer just after harvest. For the first year or two, before the root system of the plants develops fully, spread 3 or 4 ounces of fertilizer in a 12-inch radius around the base of each plant. Blackberries require abundant moisture while the berries are growing and ripening. If rainfall is not adequate, provide irrigation water equivalent to 1 inch of rainfall per week. A minimum rate of drip irrigation for mature blackberry plants is 2 gallons of water per day while berries are developing.