Root pruning helps reduce excessive vigor

Using root pruning to help control excessive tree vigor has been researched by Dave Ferree of Ohio State University and Jim Schupp of Pennsylvania State University. Root pruning is accomplished by pulling an offset subsoiler blade or a large coulter along the tree row. The blade cuts roots as it passes the trees. The blade should be set at a cutting depth of near 12 inches and pass near the trees at 1-4 feet from the tree on both sides of the row. Generally, pruning deeper has no additive effect on the trees – most major roots are within 12 inches of the soil surface. About 60 percent of the total tree roots need to be cut to have a significant effect on the trees. A lesser percent of root pruning will have less effect on the vigor control of the trees.

Use the trunk diameter as a guide to estimate root pruning distance from the tree. A root pruning distance of three times the trunk diameter will cause severe response; four to five times will cause a moderate tree response and five to seven times will cause a mild tree response. The root pruning effects may be evident for two years, usually always more than one year, but root pruning can be performed every year.

Root pruning should be done starting at apple bloom and up to two weeks after full bloom. Earlier timing will stress trees sooner with greater tree response and later timing will give less stress and, therefore, less tree response and vigor control.

Tree vigor and crop load will impact the success of the root pruning operation. In the situation of high tree vigor and a light crop load, the tree will be less responsive to the pruning. Extremely vigorous trees can be double root-pruned 30 days apart for an increased effect.

According to Michigan State University Extension, bloom time root pruning has no effect on fruit set. Root pruning will generally reduce fruit size by perhaps 0.125 inches, but can reduce diameter up to 0.25 inches in very hot dry years or with very aggressive pruning. Yields will be reduced from the reducing in fruit diameter, but this is typically not extreme. Vegetative shoot growth, branching and trunk diameter will be reduced.

Root pruning increases light penetration into trees, improves spur quality and increases fruit color. Root-pruned trees tend to have lower nitrogen (N), phosphorus (P) and potassium (K) in leaves. Root pruning enhances return bloom (perhaps doubling return bloom), and reduced fruit drop near harvest has been reported. Use root pruning on excessively vigorous trees, excessively large fruited varieties and varieties/blocks that resist blooming or setting fruit.

In some cases, trees can lean after root pruning, and with dwarf trees the trellis is important. However, leaning generally is not noticeable. For dwarf trees and in light soil, supplemental irrigation may be needed.

Effects of root pruning include:

• Reduced tree vigor.
• Reduced shoot growth.
• Reduced harvest fruit drop.
• Reduced fruit size.
• Reduced trunk diameter.
• Increased light penetration into trees.
• Increased spur quality.
• Increased fruit red color.
• Increased fruit firmness.
• Increased return bloom.
• Increased need for irrigation.
• Increased root suckers at the pruning cut site.
• May increase fruit set.
• Lowered leaf N, P and K.
• May increase tree leaning.

— Phil Schwallier and Amy Irish-Brown, Michigan State University

Source: Michigan State University Extension