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Thornless blackberry bushes are trailing cultivars that should be planted in a row with a wire trellis. The primocanes of thornless blackberry bushes grow along the ground for two years after you plant them, similar to a vine. Plant thornless blackberries in the spring, but don't plan on harvesting a crop until the second year. The planting year is a time for the bush to become established and will require little to no pruning or training.
Plant your thornless blackberry bushes in an area with full sun and good drainage. You'll need between 4 and 10 feet of space between each plant. Do not plant your thornless blackberry bushes where tomatoes, eggplant, peppers, potatoes, strawberries or other caneberries have grown within three years.
Test your soil for pH levels. Blackberries prefer fertile loam soil, with pH levels between 5.5 and 7. Add lime to increase the pH level of your soil. If you have clay or sandy soil, improve the planting and growing condition for your thornless blackberry bushes by tilling the area and adding in organic planting material.
Make a trellis for your thornless blackberry bush with 12-guage wire and wooden posts. You'll need two heavy end posts, planted securely into the ground, and lighter posts spaced 20 feet apart along the row. Stretch one strand of wire between the posts 5 feet from the ground and another wire 18 inches below the first wire. When the planted blackberry cane reaches 4 feet tall, tie it to the wire with stretchy fabric. You will also tie the primocanes to the wire when they are long enough.
Dig a shallow hole that is slightly larger than the root spread of your blackberry bush and only as deep as the roots are placed in the grower's pot. Remove any dead or damaged roots before you place the plant in the hole. Spread the roots so that they will grow in every direction. Cover the roots with soil, and tamp it down firmly to remove the air pockets. Water the plant well.
Cut the cane back to 6 inches tall. Your nursery may have already done this for you.
Make sure that your blackberry bushes receive at least 1 inch of water every week until the plant becomes dormant in the fall.