Apical Dominance - the key to all pruning and tree training
Apical dominance is the control exerted by the apical (topmost) bud over the growth of the buds below it. The apical bud produces a plant growth hormone (auxin) that flows downward inside the shoot, suppressing the growth of lateral buds below the apical bud on the current years growth. The growth (branch extension) of fruit trees can be decreased or increased by various types of pruning, and by bending branches to control apical dominance. Some trees are very strongly apically dominant (apples and pears) while others are weakly apically dominant (peaches).
Shoot angle: Apical dominance is strongest in vertical shoots. If you bend a vertical shoot to 45 degrees or select for branches growing at this angle when pruning, apical dominance will be weaker. If you bend a shoot to horizontal, dominance is temporarily eliminated. Branch bending is commonly used when training fruiting apple and pear trees to produce the short, weak branches that develop into fruit spurs.
Removing the apical bud or any cut in one-year old wood: This cut eliminates apical dominance producing a predictable response. Some of the lateral buds just below the cut (usually within 4” – 6”) will grow into shoots. This technique is used to grow branches in specific locations, particularly when training new trees.
The above diagram demonstrates the range of responses that usually result from a no cut, 1/4 removal, 1/2 removal and 3/4 removal. The more aggressive the cut the stronger the response. All these cuts are into a one year old shoot that starts out with no lateral branches. Note that the response to removal of 1/2 or 3/4 results in the formation of a group of branches with in 3″ – 6″ of the cut.