Plants grow new buds at the tip of each shoot. These are known as the “apical buds.”
The apical bud is where new plant growth and elongation occur. But that’s not all the apical bud does. The apical bud produces a plant hormone, auxin, that flows through the plant’s vascular system (phloem) down the stem, and inhibits the elongation of axillary buds which would otherwise produce new side shoots from the plant cells.
As the primary leader or the main growing shoot gets longer, the influence of its apical bud on the lower buds diminishes. The closer to the shoot’s tip, the stronger the effect the hormone auxin acts. Further down the shoot, towards its base, the growth of lateral buds and branches are free to develop.
If you prune the primary leader or a main branch, the apical bud is removed, and the hormone signals it is no longer need or being produced. This causes the stem to lose its apical dominance, permitting the side buds to develop and elongate. This kind of pruning is called heading back. (See Perfect Plants’ blog, Basic Pruning for Shrubs and Trees.)
Apical dominance is what ensures that the central leader of a tree (or the longest shoot on a branch) remains the longest. If it were not for the dominance of a single primary shoot, all the other shoots would grow at the same rate and the branch would look like a brush with all its shoots the same length. You want strong apical dominance in some trees!
If you shear off all, or most of a shrub’s apical bud to grow, you will cause the hedge effect to play a role. This of course is desirable if you want to create a formal border or screen. The art of topiary also depends on eliminating apical dominance so that no individual shoots dominate.
When you pinch off the tip of an herbaceous plant’s main shoot, you encourage bushier growth. This results in a more compact plant with shorter and more numerous stems, producing more flowers, and ultimately more fruit buds to grow. This works especially well for basil plants when you want maximum basil!