Gardening With Allen: Principles of pruning: Desired results dictate how plant trimmed
By Allen Wilson for The Columbian
Published: August 20, 2019, 6:01am
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I need help in pruning my plants. It seems like the more I prune my plants the faster and thicker they grow. How can I reduce the size of my plants and still have a nice, open, natural shape?
The most important pruning principle I have learned is what type of growth results from different kinds of pruning cuts.
When a branch is shortened by cutting the end off and leaving a part of the branch, most plants respond by producing three or more shoots on the remainder of the branch. This response is based upon the principle of terminal dominance. The top bud sends auxin (a plant growth hormone) down the shoot, which inhibits lower buds from growing. When this top bud is removed, the inhibition is removed and several lower buds grow. This causes fast, thick regrowth.
Whenever shears or power clippers are used 95 percent of the cuts are of this type. This results in three or more times as many shoots. After the second shearing we multiply three times three and get nine times as many shoots. Shearing three times results in 27 or more times as many shoots. This is the kind of dense growth we want in hedges, so we normally shear them. That is why shearing should be avoided if we want to retain the natural shape and thickness of the plant.
When a branch is shortened just above a side branch, the growing tip in that side branch becomes dominant and sends an auxin signal down the branch, inhibiting the development of lower shoots. As a result we get a one-for-one response. The plant grows one new branch where one was removed, resulting in the same thickness or density of growth as before. If we want resulting regrowth to be the same as before pruning, this is the primary kind of cut we should make.
When a branch is removed entirely, the plant has less than one resulting shoot for each one that is pruned. Thinning cuts make a plant more open or less dense. This is the type of cut used to open up a plant which has become too dense. It is used to restore sheared plants to their normal thickness.
Most pruning cuts on established fruit trees are thinning cuts because we want the maximum light to reach the fruiting branches.
This principle applies when less than 25 percent of the branch surface is removed. When more than 25 percent of the branches are removed latent bud growth is stimulated. Then one or more shoots can grow from the point where the branch was removed.
To summarize, if we want growth to be thicker (such as hedges) we shear plants, removing only part of the branch. If we want to keep the same thickness and natural shape, we make most cuts just above a side branch. If we want the shrub to be more open, we remove entire branches back to their origin.
