Rootstocks are commonly propagated by layering to form large stool beds. The "mother plant" is set out in the field. The plant may either be mounded, or it may be trench layered. In mounding, the plant is cut off close to the ground. As the new shoots emerge from the mother plant, they are partially covered with soil or well-decomposed sawdust. The sawdust is renewed periodically during the growing season to prevent the base of the shoot from exposure to sunlight. The mounding causes the base of the new shoots to develop roots. In the fall, the rootstocks are undercut, and the individual shoots (now called rootstock liners) are removed to be graded and bundled according to size. The mother plant may remain productive for up to 20 or more years; each successive fall, the "daughter" rootstock liners are removed. One variation of this method is to set the mother plant horizontally in a shallow trench, and the subsequent side shoots that develop serve as the new mother plants.
Answer provided by Dr. Robert Crassweller, Penn State University.