APICAL DOMINANCE
Cutting back or pinching out a perennial when it’s newly planted often improves its habit by making it bushier. In effect what you’re doing is breaking what’s known as ‘apical dominance’. This is where the plant hormone auxin allows the leading, more vigorous ‘apical’ bud to stop lateral buds from branching out from the side of the shoot. If the dominant growing tip is removed by pinching out or pruning, the hormone ratio is adjusted to favour cytokinin – a hormone that controls shoot growth and root formation. This spurs lateral buds into growth.