There are certain nuggets of good garden advice, so counterintuitive that they seem to make absolutely no sense at all. Why on earth, for example, should we pinch out the growing tip of a perfectly healthy seedling or plant stem as a way of encouraging it to fatten up? “Leggy”, after all, is usually a term of admiration rather than opprobrium. Likewise, why would pruning back perfectly healthy shrubs — essentially cutting away their lovely strong shoots and branches- possibly make them bushier and more floriferous when surely it should result in the exact opposite? In the same vein, why is it a good idea to shear back lavender plants that are only just beginning to fluff up again after a long, dark and dismal winter? And what, oh what, is the Chelsea chop, which sounds to the uninitiated like some form of plant abuse?
The simple answer to all of the above, strange as it may sound, is that pinching out, pruning, and cutting back are all tried-and-trusted gardening techniques, which rely on a plant’s innate urge to react to physical injury, by growing back more strongly than ever.
When you pinch out a cosmos seedling or the young stem of a dahlia, for example, you remove its soft-growing tip. This growing tip is very rich in growth hormones and is responsible for a process known as apical dominance, which directs growth upwards rather than outwards. The seedling or plant’s reaction to this physical injury is to quickly throw out multiple side-shoots positioned lower down its stem at intervals along its leaf axils, an ingenious survival mechanism that plants growing in the wild use as a means of surviving damage from grazing animals or harsh weather. So by deliberately inflicting this very targeted injury, you’re using this
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