When you prune a shrub, you are directing its growth. If you want a twig to branch out, cut off the tip and the new branches will develop from the hitherto dormant buds lower down. How does this happen? As shrubs grow they send out new branches and increase their length and thickness. Buds form at the tips and sides of existing branches toward the end of the growing season, usually midsummer. Each bud is packed with everything it needs to develop into a flower, or a leaf, or a new twig. Slice a bud open and you will find miniaturized plant parts arranged around a central point called a meristem.
As tip buds grow, they build new tissues behind them, lengthening the branch behind. Other buds form along this branch. Some are fated to develop into flowers or new lateral twigs, but most become new leaves. However, not all buds start growing at once. The tip bud controls all the others to some degree. This is called apical dominance. The meristem in the center of each tip bud contains plant hormones that temporarily suppress growth of all the other buds. Eventually, as the branch elongates, the hormones dissipate, which allows the other buds to develop into leaves, lateral twigs, and flowers. If you prune off the tip bud, you are hastening this natural process. The other buds are free to grow.
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