Apical dominance is the key reason why we are able to direct the growth of trees to achieve the goals we have for them. The pear shoot in this picture is growing vertically during midsummer. There is a bud at the top or apical end of this shoot. This apical bud is responsible for all the growth you see in the picture. There are several other buds on this branch, they are very small buds attached to the stem just above each leaf. These buds are called lateral buds. The apical bud produces a plant hormone (an auxin) that travels downward inside the stem as though pulled by gravity. This hormone prevents the lateral buds from growing. When trees produce very long shoots (sometimes several feet long) in one year with no side branches, they are strongly apically dominant. If some or all of the lateral buds grow into branches in the same year, the trees are weakly apically dominant. Apples and pears are strongly apically dominant, peaches are weakly apically dominant.
When only the apical bud grows, the result is a long single shoot. If the apical dominance is weak and apical bud plus some of the lateral buds grow, the result is many shorter shoots, a bush shape rather than a long single shoot. The reason for this is the growth energy of the tree is now being shared between several shoots so they are all shorter than when only one shoot is growing.
Most plants start out strongly apically dominant so they can grow up away from the soil. Plants that do not have this ability are the low growing ground covers. Most plants loose their strong apical dominance in the first year or two. Large trees do not loose their strong apical dominance giving them the ability to grow very tall.
Trees and shrubs growing in situations where there are no restrictions on their growth grow into shapes dictated by their genetic potential. Few gardens provide this type of unrestricted growth for large trees. Through pruning and training our trees we can direct their apical dominance to produce some amazing shapes and forms such as espalier and topiary. Directing the apical dominance of our trees can also result in the production of more high quality fruits, nuts and berries. We can also direct our trees to provide more shade, better screening, windbreaks, stabilization of soils and many other services. Future posts to this blog will discuss how to the achieve goals we have for our trees.