Planting Depth

Apple rootstocks are usually propagated clonally in stool beds and have been selected because they root well. The roots are induced on aboveground parts of a stem. When apple trees are budded high on rootstocks and planted with a union at the ground, the underground portion of the rootstock has the ability to form roots.

When the union is planted 2 or more inches aboveground, the exposed portion is unable to form normal roots and may form root initials or burr knots instead. Burr knots are areas of rootstocks where roots try to form but cannot because they are not in a medium conducive to root growth. Rootstocks with burr knots do not enlarge radially in a normal fashion because phloem and xylem cells do not develop all the way around the tree. Several species of tree borers may enter burr-knot areas. Thus, apple rootstocks should be planted with the union at ground level. When mechanical tree planters are used, trees may be set too high. However, trees can be set 1 or 2 inches deeper and manually adjusted to the proper depth. Research suggests that the height of the union also will influence the amount of dwarfing induced by a rootstock. The more of the rootstock shank that is exposed the greater the dwarfing. A general rule of thumb is that for every 4 inches of exposed rootstock shank below the graft union you can expect 10 percent more dwarfing.