The biology of pruning plants

If you have been reading my blog for a while — I hope you have! — you probably know that gardening is one of my passions. You probably also know that I like to geek out on the technical aspects of gardening, like the right composition of greens and browns for compost, and, the topic of this blog post, the biology of pruning plants.

If you like gardening, you have probably watched many YouTube videos on pruning the specific plants you liked. Maybe roses, or fruit trees or shrubs you can use for hedges. But have you ever wondered if there was a science behind all that technique? Can we make sense of the basics of plant growth and how it can be affected by pruning? If we can, then it becomes easy to prune new plants.

You just have to apply the rules that you learned to that particular type of plant and you are good to go. In this post, I will try to explain the biology of pruning so that you can, hopefully, prune any plant you encounter with confidence.

The prime directive of plants: pun intended

I hope trekkies enjoyed the pun in the title of this section, but SciFi shows aside, the prime directive of any plant — or any living thing really — is to reproduce. Plants want to make little plants. They want to do this because they want to ensure the survival of their species. I know that want may be too strong of a word here, so let me put it another way.

Natural selection favors living creatures that can reproduce prolifically and out-compete their competitors. Evolution itself selects for living things that can reproduce effectively. What this means is that a plant will do its best to reproduce. Doing all it can to ensure that it can pass on its genes. “ But how does this affect pruning?” I hear you ask.

Well, the main way that plants reproduce is by producing flowers that, when pollinated, produce fruit that contains seeds. The dispersal of these seeds ensures that the plant’s gene pool survives. So a plant will put all it’s energy into producing flowers and, eventually, seed bearing fruits in order to survive.

Preventing bolting: remove those flowers

Depending on what we want from a plant, we can take advantage of this while pruning. For example, if what we want from a plant is foliage — it’s leaves — then we want to reduce flowering as much as possible. This is because once such a plant flowers, it will direct all it’s energy to the flower, neglecting the foliage which we desire.

Some of you may be trying to think of plants where the foliage is more important than flowers — because who doesn’t like flowers, right? Well my friend, there are many plants where the leaves are more important than the flowers. For example, consider herbs like basil, mint, arugula, rosemary. These yummy plants give us pesto and other awesome tasting sauces. The world of gastronomy would be much poorer without them! The part of these plants used in cooking, the part that contains all that yummy flavor, is their leaves.

We want these plants to produce as many leaves as possible. In fact, when they flower, we say that they have bolted and the taste of their leaves deteriorates. They generally become more bitter. For these plants, it is important to delay flowering as much as possible to ensure that we have enough lush leaves to harvest.

Another example of plants where leaves would be more important than flowers are plants that have decorative leaves, like Coleus. Their decorative leaves are much more prized than their, in comparison, plain flowers. Since the prime directive of all plants is to reproduce, these plants will try to produce flowers. However to make sure that they continue to produce leaves, pinch any developing flowers you see.

By pinching the flowers buds before they fully develop, you redirect the energy that the plant would have used to nourish the flower to its leaves. So instead of trying to make seeds, the plant will try to make more leaves. It will do this because the loss of the flower means it has more energy to direct towards leaf growth.

Of course, this is a loosing game in most cases. Eventually the herbs will bolt and produce flowers. All is not lost however, let them produce flowers when it becomes impossible to control. Their small flowers are attractive to beneficial insects, and they will eventually produce seeds that you can harvest and use to plant the next season of herbs.

By understanding the biological imperative of plants to reproduce, and controlling it to maximize foliage, you can have lush herbs and decorative plants for as long as possible. Understanding the biology of pruning can help you understand how to prune your plants.

Grow large flowers: remove it’s brothers and sisters

You can also take advantage of this reproduction imperative to grow large flowers. Think of it this way, a plant will direct most of its energy to flowers when it begins flowering to ensure that it can reproduce. If there are many flowers, this energy will be divided among them. Leading to many small or medium flowers. But if you pinch off most of these flowers and leave only one or two, all that energy will go to them. Since the energy of the plants is concentrated on these few remaining flowers, they will be large show stoppers! As you can see, taking advantage of the prime directive of plants can be very useful.

Apical dominance: the tip is a bully

Another aspect of the biology of pruning you need to understand is apical dominance. Before going any further, let us explain what this means. Apical dominance simply means that the top of a branch is typically where most of the growth in that branch occurs. That is, the branch will elongate from its tips more than it will grow sideways from buds distributed over its length.

I need you to get this part because it will greatly affect how you prune. Please read the paragraph above again if you don’t get it the first time. I will rephrase it again because it is very important. In most plants, a branch will mainly grow along its tip. It’s tip will produce more new growth than any other part of the branch. This is what we call apical dominance. The apex, the tip, of a branch is dominant when it comes to growth.

This is a general rule that most plants follow, in fact palm trees take this to the extreme — they ONLY grow from an apical bud. If that bud is cut off, the plant dies. This is why, in some Bedouin cultures, cutting of the apical bud of a palm tree and presenting it to a visitor is the greatest honor they can bestow. Trees are hard to come by in the desert, and by sacrificing one to honor a guest they show how import he/she is to them. So understanding the biology of pruning won’t only help you prune, it will also help you establish diplomatic relations with foreign cultures! See how important it is?

All joking aside, how is apical dominance maintained? The apical bud typically produces a hormone called auxin. This hormone flows from the tip of the branch and makes all other buds in the branch dormant. Most branches have many buds along their length that can produce new growth, but the auxin of the apical bud inhibits them from doing so.

Making bushier plants: cut off the bully

If you want a bushier plant, with more branches that fill in the spaces, you want to prune that apical bud off. Let us consider basil again. We typically want a bushy basil plant with as many branches as possible producing as many leaves as they can. When you first plant a basil plant it will be rather sparse with few side branches. Let it grow for a while to establish itself, and then cut of the tips of its banches. Soon, each of these branches will begin growing side branches as you have cut off the apical bud that was producing auxin to inhibit their lateral buds. You will have leaves galore!

This concept also affects another aspect of pruning. Don’t cut branch tips if you want to thin out your plant! You will only accomplish the opposite. If you want to thin out a plant, cut a branch where it joins the main stem. If you do this, the place that the branch occupied will become empty, and the rest of the plant will get more energy to grow. But if you cut halfway through a branch, for example, this will only activate the lateral buds in the remainder of the branch and make that area of the plant bushier, not thinner.

Conclusion

So, there you have it, folks! Pruning is more than just a way to shape your plants and keep them tidy. By understanding the underlying biology of pruning, you can harness the power of hormones and growth patterns to create a healthier, more beautiful garden. So go ahead, grab those pruning shears, and let your inner plant whisperer shine!