Root pruning apple trees to control excessive vigor in 2011
Some guidelines for trying this technique in your orchard.
Dr. Dave Ferree of Ohio State University and Dr. Jim Schupp of Pennsylvania State University have worked out the use of root pruning to help control excessive tree vigor. Root pruning is accomplished by pulling an offset subsoiler blade 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 passes near the trees at 1 to 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” of the soil surface). A target of 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 distant from the tree. A root pruning distance of 3 times the trunk diameter will cause severe response; 4X to 5X, moderate; and 5X to 7X, a mild tree response. The root pruning effects may be evident for 2 years (usually always more than 1 year), but root pruning can be performed every year.
Root pruning should be done starting at apple bloom and up to 2 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 cropload will impact the success of the root pruning operation. In the situation of high tree vigor and a light cropload, the tree will be less responsive to the pruning. Extremely vigorous trees can be double root pruned 30 days apart for an increased effect.
Bloom time root pruning has no effect on fruit set. Root pruning will generally reduce fruit size by perhaps 1/8” but can reduce diameter up to 1/4” 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 N, P and K in leaves. Root pruning enhances return bloom (perhaps doubling return bloom) and reduced fruit dropped 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, but 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.