Once you have established a thriving thornless blackberry patch, you can propagate new plants by "tip layering" the canes. This process involves burying first-year shoots and waiting until they root. Transplant them the following spring as new thornless blackberry plants.
Choose long stems that have not borne fruit in the middle of September. Look for vibrant stems with no imperfections.
Bend the stem toward the soil. Dig a hole 6 inches deep where the tip of the stem strikes the soil, then bury the tip.
Pat the soil firmly around the buried stem tip. Place several stones over the site to hold the stem in place.
Water the buried stem well.
Check the stem the following spring and you should find roots growing beneath the soil. Cut the buried stem 12 inches above the soil surface with pruning shears.
Dig up the new thornless blackberry plant with a trowel. Do not disturb the new roots.
Transplant the specimen to a sunny location that is at least 4 feet away from other blackberry plants.