By Thomas T. Watson
When I decided, somewhat late in my life (speaking relatively, since I'm anything but fossilized - yet) to complete a college degree, plant biology came easily as my choice for a major. Plants, wild and cultivated, have always been the center of my interest in natural history. Because of that choice and the education that grew from it, I see my garden in a very different way, down to a level of detail that a few years ago I would have had trouble imagining. Simple garden chores become biological fascinations, as it occurs to me why so many of the things gardeners do in their gardens actually work. For example, this year - like every year - I've planted several varieties of basil. It's hard to grow too much of the stuff if you're a gardener who happens to love to cook. And as every gardener should, I pinch the basil back to make it bush out.
Most of you know about pinching back, but even if you are a new gardener, the term is almost self-evident: to grow herbs that look like cultivated plants (instead of sprawling weeds) you pinch off the growing tips and force growth out lower down on the stems. It need not be wasteful; for me, pinching back is really part of my on-going herb harvest. (I've never actually seen any hard and fast rules for proper technique in this regard; I take off the tip down to the first set of full-sized leaves, and that seems to do the trick.) Pinching back does not make basil (or any other herb) taste better, nor will it make the plants any healthier or more resistant to pests. It does, however, make the plants a bit more productive. All those new branches have leaves, and those leaves are of course the reason you grow most pot herbs.
But why does it work? Why does a plant respond as it does to being pinched back? The other day, while pinching (and harvesting) some wonderfully aromatic "Sweet Thai" basil, I suddenly remembered a course I took in plant physiology, and realized I knew the answer.
Think of a plant that naturally forms branching stems as a sort of balancing act in slow green motion. To grow well the plant needs (among other things) to keep its roots pretty well matched with the leafy stuff above ground. If the shoot (above ground) portion of the plant were to outgrow the roots, not enough water or minerals would be available for the growing parts of the plant. An overgrown shoot system also defeats its own purpose by shading its own leaves. A plant needs a way to control how it grows to prevent shading and to avoid overpowering its root system.
At the same time, a plant needs to be ready to respond to damage from browsing animals, hungry insects, or the occasional gardener with a taste for pesto. If the existing growth points at the ends of its original branches are all it will ever produce, and something eats those growing ends, the plant is in serious trouble. To recover from browsing, whether by a deer, a grasshopper, or yours truly, the plant needs to be able to grow new stems that will sprout new leaves.
The system that strikes this balance is a phenomenon plant biologists call apical dominance.
Apical is a descriptive term that comes from the word apex, and simply means the tip, top, or upper end of something. You put your hat on the apex of your body (assuming you're in an upright position). Dominance, of course, means just that: dominant, in charge, calling the shots. So all the fancy phrase means, then, is that the tip of a branch is pretty much in charge of what goes on below it on the stem, as far as growth and development are concerned. And down along those stems, more often than not in the angle formed where the leaves are attached, are dormant buds, waiting to do their thing. Apical dominance keeps those buds dormant.
There is a place in the tip of a branch or stem, where you see new leaves unfurl on growing basil or thyme plants, where the plant cells are dividing like crazy, creating the material of those new leaves. It's called the meristem, and from it comes a hormone called auxin. Auxin is the stuff of apical dominance. Now, before all this scientific jargon makes you crazy, hormones are just signals, a biochemical way of shouting orders and getting things done. We have hormones working in us all the time; this is true of all living things. Adrenaline is a hormone; when someone startles you out of your wits, you've just had a hormone signal flash through your body (giving you a rapid heartbeat and an inclination to slap the inconsiderate so-and-so). Of course, if you have teenagers you know about hormones. Plants just do it in a quieter way - yet another argument for planting gardens. Auxin has many roles in a plant, but in this case it makes apical dominance happen. So long as those auxin signals move out from the growing tips, few - if any - of the dormant buds on the plant will open up and begin to grow.
The auxin produced by the meristem diffuses away from the tip of the stem and into the plant, becoming less concentrated as it goes. Perfume works that way in a large room; the closer you stand to the person wearing the scent, the stronger it is (sometimes compelling you to stand further way, or wish the room were a bit wider). The farther a bud is from the source of the auxin, the weaker the signal it receives. This is why, when you pinch the tips back on a basil plant the new stems usually appear from somewhere nearer the ground than the site of the damage. It also explains why, even if you do not practice pinching back, your herbs do eventually produce at least a few branches on their own. If the stems get long or tall enough, sooner or later the lowest buds don't receive enough of the auxin signal for apical dominance to occur.
So, when I pinch back a basil plant (or prune a rose, for that matter), I've cut off the signal that keeps the rest of the plant from wasting its resources on unnecessary (from the plant's perspective) growth. The plant responds to what it perceives as damage by calling in the reserves, so to speak. It tries to replace the damaged stem. One or more buds on the affected stem - and sometimes elsewhere, depending on the plant in question - break dormancy. In other words, they wake up and start to grow. In an herb such as basil you usually get more than one bud "breaking" for each tip you pinch back, with the result that the herb gets bushier. As time passes the meristems of these new buds and the stems that grow from them gradually build up the concentration of auxin, and any buds (those produced along the new stems) that have not yet gotten themselves into the game get that hormone signal to shut down and wait.
If this first pinching does not give the effect you desire, you eventually pinch back the new growth, and the whole thing happens again. The plant, all the while, is trying to adjust its balancing act. As time passes the response to the pinching of tips diminishes, since too many leafy stems will just block the sun from other leaves on the plant, and plants like all living things will respond to diminishing returns. But when that new balance has been struck you should end up with a neat, tidy little herb that looks like the pictures in the books.
If you've been growing herbs for a while and following the practice of pinching back, you've seen all of the above happen and never given it a second thought. This is just a thing you do with herbs of certain types, as well as a host of other garden plants. But as it is with so many of the things we do to cultivate the plants we grow in our gardens, there's a lot going on that we can't easily see. These matters unseen are the reasons we can do the things we do, the reasons why techniques such as pinching back produce their desired results. You don't need to know any of this stuff to make your garden grow, but if you take the time to ask "Why?" the answers you find are often incredible. We are all, as gardeners, manipulating miracles.
About the Author
Thomas Watson is graduate of the University of Arizona College of Agriculture, where he earned a BS in Agriculture, majoring in plant sciences. When he is not writing he is usually working in the garden, re-landscaping his Tucson home, studying botany and plant identification, or "out there" somewhere, studying the natural history of the Sonoran Desert. This is all, of course, based on the assumption that it isn't baseball season, in which case he'll get all of the above done before the next Diamondbacks game.