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Plant Science: The Reason Topping or Pinching Works

This gardener is pruning is tree for a better yield. This gardener is pruning is tree for a better yield.


If you ever looked into how to prevent legginess in your plants, you may have been advised to "pinch" or "top" the topmost bud on the main stem in order to make the plant grow bushier.

After you do that, the newly beheaded stem will suddenly have two or more swollen buds right beneath where you pinched.

The buds eventually give way to strong, leafy branches. This technique is indeed called pinching or topping and has been used by horticulturists for centuries to promote bushier, more compact plants that produce more flowers and ultimately fruit.

This reaction to pruning is due to a phenomenon called apical dominance. From the moment that a seedling pushes its way through its growing medium and up against the forces of gravity, the apical point on its stem, the bud or buds that are forming at its terminus, are compelling the plant to grow upward.

Not all plants exhibit apical dominance but most vegetable plants and trees do. This is likely an evolutionary adaptation that has roots in a plant’s ability to photosynthesize.

This type of dominance is observed mostly in branched plants with one or more stems. As the plant matures, one bud or one whorl of buds will dominate over the buds found on lateral branches. They will receive the majority of the plant's energy and spur its growth towards the sun.

The hormone responsible for apical dominance is the same that is responsible for phototropism, a hormone called auxin. In the tip of every shoot and root, as well as under woody fiber and bark, is a kind of tissue known as meristem tissue. This is where auxin is produced and it is also the only place where plant cells divide or from where a plant truly grows.

But auxin itself is produced in the greatest quantities in the primary apical bud and diffused from there throughout the rest of the plant. The auxin manufactured in lateral buds is of lower concentrations and is stunted by the crowing bud or buds.

When the apical meristem tissue is severed, the high levels of auxin concentrated in the terminal bud no longer exist and the auxin levels of the competing lateral buds then spike. Since they are no longer suppressed by the apical auxin dominance of the main bud, their auxin production takes over and causes new branches further down the plant's stem to grow.

Not all plants exhibit apical dominance but most vegetable plants and trees do. This is likely an evolutionary adaptation that has roots in a plant’s ability to photosynthesize. If a plant focuses its energy on growing as tall as it can, it can then be in a better position to receive light physically and also competitively against other seedlings struggling in the same space for the very same thing.

If a tree, for example, has a strong apical dominance and also a strong constitution, it can grow upwards towards the sun quickly and shade other seedlings in order to prevent them from reaching the sun themselves.

Some plants like branched flowering annuals, perennials, fruit trees, herbs, and vegetable plants benefit from pinching and topping. Other plants, particularly trees and conifers, rely on apical dominance to stay healthy, straight, and robust, and topping them will only lead to disfigurement and weakness. Topping the right plants can lead to greater yields if applied properly, so do your homework to find out if this type of pruning could benefit you and your crop!

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This guy knows how to pinch a plant.
Last modified on Tuesday, 15 October 2013 22:54

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