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Have you ever wondered why most plants have a dominant shoot that rises higher above the others? It’s all about raging hormones. You can use this to your advantage if you want to modify the shape of a plant through pruning.
“Apical dominance” is the term scientists use to describe why plants reach for the sky. Apical dominance is the result of auxin (AWK-sin), a hormone that’s produced in the tips of plants’ growing shoots or at the high point of their stems. Auxin travels down each stem and sets off a chain reaction that puts the brakes, to some degree, on growth of side shoots, giving the uppermost growing point (the apical point) of the stem the upper hand in growth.
Side shoots mostly arise from buds along a stem, and whether a bud grows out into a shoot depends on how close the bud is to the source of auxin. The closer it is to the source, the greater its inhibition will be, although the exact degree of inhibition at a certain distance depends on the particular plant. ‘Mammoth Russian’ is a sunflower cultivar that grows as a single stem capped by a large flowering disk. This is an extreme example of apical dominance, with no side shoots at all. At the other extreme is one of the shrubby species of willow that sprouts side branches freely all along its growing shoots.
Pinching back shoots on tomato plants encourages them to become stockier and bushier.
Photo by Getty Images/Helin Loik-Tomson
Even within a single species, plants vary in their tendency to express apical dominance. Fuchsia cultivars ‘Beacon Delight’ and ‘Blue Ribbon’ express strong apical dominance, so they’re easier to train upright into miniature trees (called “standards”) than are trailing cultivars, such as ‘Basket Girl’ and ‘Blue Satin,’ which have weak apical dominance. The latter are more at home with their branches sprawling over the edges of hanging baskets. Only with the help of a stake and diligent pruning of side branches can ‘Basket Girl’ be coaxed to take on the shape of a miniature tree.
Stop That Shoot!
Sometimes, I want to thwart apical dominance — to get a stem to grow side branches, for instance. One way to do this is by removing, if temporarily, the source of auxin by pruning. The effect is only temporary because a new uppermost bud will soon establish itself as apically dominant.
For the least possible pruning, I’ll just pinch out the soft growing point of a shoot with my thumbnail. This quick and simple check to auxin flow not only causes growth to falter briefly, but also awakens lateral buds that were dormant. I pinch back the main shoots of coleus plants to make them grow bushier, and this also happens coincidentally when I harvest basil.
Another reason I’ll pinch out a shoot tip is to slow growth. For example, in late summer, I pinch out the tips of my tomato plants to redirect their enthusiasm for stem growth into ripening fruit. (By then, tomato plants are growing like weeds, so the effect of the pinch is short-lived, and requires repetition to be effective.) The sprouts of my Brussels sprouts plants swell more quickly in response to my thumbnail’s work on the top of the plant’s upright stem. I also pinch out the tips of upward-turning side branches on my young apple tree that threaten the dominance of the single leading shoot (the “central leader”) that I’ve selected for the growing tree.
Whether done by humans, insects, or diseases, pinching elicits a relatively quick plant response. Experiments with pea seedlings have shown that lower buds are stirred into activity in as few as four hours after apical bud removal.
Cutting back a larger portion of stem differs from pinching in the degree of response. The more drastically you cut a stem back, the fewer side shoots will awaken, but the more vigorously each side shoot will grow. This type of cut is called a “heading cut,” and plant response depends on the degree of heading. The more inherently vigorous a young stem is before it’s headed back, the more vigorously it will respond. Inherent vigor is greater the younger and more vertically oriented a stem is.
I have a long privet hedge that needs to be sheared every few weeks through summer to maintain the shape and size I want. Shearing the hedge is, essentially, making hundreds – no, thousands – of slight heading cuts. The plants respond just the way I hope, with dense growth of many short shoots.
Trimming a hedge sparks side shoot development, leading to a denser plant.
Photo by Adobe Stock/Martina
At the other extreme is the pruning I give my butterfly bushes. I lop each bush’s stems almost back to ground level late each winter to coax forth each summer’s long, graceful flowering stems.
On fruit trees, my pruning cuts vary in severity, with the goal of coaxing some — but not too much — new growth that will bear fruit and replace decrepit older growth. The degree to which I prune depends on the kind of fruit tree. The apple trees, for instance, get a relatively light pruning because they continue to bear well on branches even a decade old. I go at my peach tree more aggressively because peach trees bear only on 1-year-old stems, so each year they need enough new stems coaxed for a good crop the following year. Plum bearing habits and pruning lie somewhere between these two extremes.
Harvesting the tops of basil plants also drives them to resprout, giving you ever more basil.
Photo by Getty Images/LiudmylaSupynska
I’ve used pruning (i.e., harvesting) to get multiple heads of cabbage from a single plant. A cabbage head is actually a stem that has been foreshortened, with side buds and their attendant leaves close together, one above the next. So lopping off the head during harvest is, essentially, a heading cut. Harvesting releases side buds from apical dominance so that some of them can then sprout to form new heads. I leave cut cabbage stumps in place after harvest, and then let three or four side sprouts grow, snapping off the rest to strike a reasonable balance between the number of new heads and their eventual size.
Overpruned and Overgrown
Many a homeowner deals with an overgrown tree — one that has grown to block a window, for example — by irreverently hacking back the tops of branches that are blocking that window. Said homeowner then bemoans the dense, vigorous sprouts that regrow and quickly block the window again. Thinning cuts would be more effective; they’re cuts that totally remove offending branches. With no buds left to regrow, the plant’s response is nothing, nada, zip — near the cut, at least.
Thinning cuts are crucial on plants that are prone to fungal diseases, such as roses.
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I use thinning cuts for congested plants. When too many stems clog up the interior of a tree or shrub, removing some of them right to their beginnings opens up the space without eliciting regrowth at the cuts. Not only do thinning cuts not result in regrowth, but they can also be conveniently made closer to ground level, at the origins of the offending stems — perfect for dealing with a tree blocking a window.
A downside to both types of cut is that they stunt plants, to some degree. Stems contain some stored energy, and, if it’s a time of year when the stem is clothed with leaves, pruning will also lop away some of the plant’s energy-producing factory. Of course, you might want to keep a plant small, and pruning is often necessary for plant health or production regardless of the desired plant size.
Select healthy, flexible stems to arch over and tie; old, rigid growth may take years to train.
Photo by Adobe Stock/Hermann
Because auxin is produced at the highest point of a stem, an elegant way to thwart or redirect apical dominance without having to prune off any part of a shoot is to bend it down. If you bend a dampened stem in an arch, a bud at or near the high point of the arch will become top dog and sprout into, usually, the most vigorous shoot. On the other hand, if you lower the tip of a stem without creating an arch, then each bud along the stem will be only incrementally higher than the one before it. In this case, the uppermost bud will express weak apical dominance, and many buds lower on the stem will be released from inhibition to make relatively weak growth. The more vertically oriented the shoot, the fewer buds will awaken, and the more each bud will grow, especially those nearer the tip of the stem. The more horizontal the branch, the more buds will awaken, and the less each bud will grow. So I can regulate the amount of growth and side-branching by adjusting the branch angle, and changing it, if necessary, as I watch my plant grow.
Horizontal branch training all but eliminates auxin’s influence, creating many short side shoots.
Photo by Getty Images/daseaford
Branch bending is used to the extreme to create espaliers en arcure, in which all stems on a plant are bent over. The single, upright stem of a new tree is bent in an arch. The vigorous stem sprouting from the high point of that arch is bent in the opposite direction, and so on, the arches alternating their directions as the young tree grows. A row of dwarf apple trees can even be woven together by tying each successive tier of bent branches to the arches on neighboring trees.
Espalier "en arcure" makes for a graceful branch pattern as well as heavy fruit set.
Illustration by Vicki Herzfeld Arlein
Pruning Fruit Trees for Production
Pruning and branch bending are especially useful with fruit trees during their training years, and again when they begin to bear fruit.
Apical dominance comes prominently into play when training a fruit tree to a central leader form, much like a Christmas tree. A single stem, the leader, grows upright, with side branches of decreasing length, with increasing height along the leader.
Shortening the leader of a young fruit tree will encourage the growth of side branches that become future limbs. A young fruit tree, whether I propagate it myself or purchase it, is often nothing more than a single vertical stem: the trunk-to-be. A heading cut that removes one-quarter to one-third of a young whip’s length produces the desired 3 to 4 side branches of moderate vigor. The bud just below my pruning cut typically grows most upright and vigorously, continuing skyward as an elongation of the leader, which I’ll shorten back again a year later to encourage more side branches.
One caution: Fruits forming high on a young, developing central leader could weigh it down, bend it over, and thwart its apical dominance. Then, a new stem might attempt to take that role, which would make for a poorly trained tree. I re-establish the original leader’s role by removing or reducing the number of offending fruits, or by staking the stem upright.
A tension exists between shoot growth and fruiting in plants, with more of one associated with less of the other. Bringing a side branch to or near a horizontal position while keeping it straight does more than awaken growth in many buds along that stem; the resulting weak growth has a tendency to develop into flower and fruit buds, rather than buds that would grow into shoots. Bending side branches can coax apple trees and pear trees into earlier production, and can lead to much heavier production in a smaller space — perfect for gardeners with limited space to devote to trees.
Lee Reich has a doctorate in horticulture, has written several gardening books, and maintains a blog about his “farmden” in New Paltz, New York. This is an excerpt from his book The Ever Curious Gardener, available in the MOTHER EARTH NEWS Store.