Pinching and Pruning to Increase Yield
Metus dictum at tempor commodo
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.
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.
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.
Prudent Pruning
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.
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.
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.
