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Bonsai Tree Growth Stages

Bonsai Tree Growth Stages
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When it comes to developing bonsai trees, there is no shortage of techniques available. As a hobbyist, you'll be exposed to many pieces of advice on how to improve your tree, whether it's from people online or in your local club. The problem is, without a fundamental understanding of bonsai tree growth stages and how these different techniques affect your bonsai, you may be misusing them.

In this blog, we'll take a deep dive into the different stages of bonsai development and how you can use this knowledge to purposefully put into practice all the advice being thrown your way.

Bonsai seeds and saplings

Growing from seed is the natural start of a tree's life. Many people have their first introduction to bonsai through the gift of a seed kit, although it's worth noting that this is not the best way to start a bonsai.

Growing a bonsai from seed takes a very long time, so if you're just starting in bonsai and all you have is a seed kit, I'd recommend you buy a cheap nursery tree or an established bonsai from a specialist bonsai nursery. This way you can enjoy working on a proper tree, while also having the fun of watching your seeds grow into bonsai of their own over the next few decades (yes decades, growing seeds into bonsai is a long process).

Once you have planted your seeds and they have started to grow, you should let them do their thing for at least the first year. Keep your seedlings outside, and don't worry if some of them struggle and die. It's perfectly natural for not all seeds to successfully grow into trees, so don't waste your time nursing a seedling that won't do well long term.

When your tree reaches the sapling stage, up pot it to a bigger growing container and apply some wire. Now is the best time for wiring your bonsai, because you will get some really radical movement into the trunk.

Fertilize your sapling well and let it grow. Once it is two-thirds of the way to your desired girth, you can start styling it further as a bonsai.

Early bonsai development

This is likely the most common stage of bonsai growth that hobbyist will find their trees in. It's really common to find nursery stock material, trees grown from cuttings, or pre-bonsai purchased from bonsai nurseries that need the primary lines and features of the tree to be set.

The primary goals of this stage of bonsai development should be:

  • Potting the tree in a bonsai container and establishing a domesticated root ball
  • Define the trunkline from base to tip
  • Thicken the trunk
  • Develop the 'bones' of the bonsai by growing interesting primary branches
  • Heal large wounds

Your bonsai may have already met some of these goals, but without all of these targets met you will struggle to create a well-balanced and refined bonsai.

There is no particular order of events that needs to happen with your tree - sometimes it's better to develop roots first and style later, while other times styling first can be easier. So long as you have a plan, remain patient, and importantly don't try to do everything at once, your bonsai will do well.

During this phase of bonsai growth, it is really important to let your tree grow. Free growth in spring will allow the plant to build up its energy stores to spend on root growth in the autumn. Letting strong leaders grow out is also the best way to add girth to the trunk and branches.

Since you're encouraging strong growth, you'll also want to fertilize heavily with a high nitrogen fertilizer. Many professionals will advocate this as the only period in bonsai growth where synthetic fertilizers can be used to good effect since they can produce rapid growth and will help speed up the transition to secondary development.

Examples of primary development

These are two Siberian spruce Christmas trees that I rescued from a supermarket (very similar care requirements to Norway spruce). I have treated them differently as an experiment to see which approach yields better results (I'll update this blog in a year or so when I know).

Repotted Siberian spruce bonsai in early development
Siberian Spruce bonsai after first structural setting

The first had its foliage mass left untouched and was potted into a bonsai container appropriate for the size of the tree I want to build.

The second was left in its original container and soil and had its foliage mass reduced and primary lines set with wire.

They have both been fertilized heavily with liquid seaweed fertilizer, although fertilization on the repotted tree started later to avoid burning any emerging root tips.

I'm really happy with the progress of both trees so far. They may not look pretty, but both have made important inroads into the goals of primary bonsai development. Over the next year, the repotted tree will be styled and the styled tree will be repotted, and after that, they'll be transitioning into secondary development.

Development of secondary structure

When your bonsai has been successfully transplanted to a bonsai container and has the majority of its bones set in place, it's time to start working on the secondary structure. Many of the goals we started working on in primary growth will be continued in this stage.

The goals of secondary structural development in bonsai are:

  • Continuing the cultivation of a well-ramified root ball
  • Growing secondary branches
  • Thickening of any primary branches that may still be in early development
  • Continued healing of wounds

As you might have guessed, secondary branches are the branches that grow out of the primary branches. Secondary branches set the scaffolding for the pads of your bonsai, and contribute a huge amount to the visual interest and tone of the composition.

Techniques for growing secondary branches will vary between species, but there is a general concept that holds true - remove the terminal growth tip to promote the growth of interior shoots.

The outermost growth tips on bonsai trees are full of a hormone named auxin. Auxin suppresses the growth of buds behind it on the branch, as a way for the plant to focus energy on growing tall and outcompeting its neighbors.

Since we don't want tall trees in bonsai, removing the terminal bud through pruning or pinching helps to encourage lateral branches to grow. We can then wire these into place to form the basis of foliage pads.

In deciduous bonsai, we can also make use of a technique named partial defoliation, where over 80% of the foliage mass is pruned after the first flush of growth has hardened off. This stimulates the tree to grow more shoots from its interior buds, increasing the number of secondary branches on the bonsai.

New shoot growth after partial defoliation

4 new shoots growing after partial defoliation of a Silver Birch bonsai tree.

Since we are still focusing on growing out sections of the tree during the secondary stage of bonsai growth, it's important to maintain a moderate rate of fertilization. At this point, it's a good idea to move away from synthetics if you've been using them and instead use organic fertilizers.

Organic fertilizers provide a more rounded selection of nutrients for the tree and encourage the growth of healthy microorganisms in the bonsai pot that will help your tree grow and stay healthy.

The refinement stage

Once we have our secondary branches in place, and the bonsai is well established in its container, we can move into the refinement stage of bonsai development.

During this stage our goals are orientated towards slowing down the tree and trying to balance energy:

  • Continue cultivating and maintaining a healthy root ball
  • Balancing foliage growth across the tree
  • Maintaining the shape and style of the tree

Using the bonsai pot for refinement

To refine a bonsai tree, the role of the container is crucial. The small container bonsai trees are grown in acts to constrict elongated root growth and form a finely ramified network of roots. To help this process, we need to repot our bonsai trees as little as possible.

Each time you repot a bonsai, roots are pruned and fresh soil is introduced to the pot. This gives the tree space to grow more freely for a while, so the resultant canopy growth can be a bit coarser.

Many bonsai hobbyists will repot on a schedule, but this usually means trees are getting repotted more frequently than is needed, and they never experience the truly constricted growth that the bonsai pot can give us.

So to make sure you are using the bonsai container to its best potential, only repot your tree if it is required based on these criteria:

  • The root ball is completely full of roots and water no longer percolates through the root ball
  • Soil has started to decompose and break down, causing the root ball to hold too much water

Of course, you will need to repot your tree at some point to refresh the roots and add some strength back to the plant, but it is perfectly normal for a coniferous bonsai to go 8 years or so without needing to be repotted. Deciduous trees grow faster, so will likely need repotted more often than your conifers.

Balancing energy and maintaining the bonsai form

When trees grow, they will normally favor certain branches or shoots that are in good positions. These include shoots that are orientated vertically, or have greater sun exposure.

If we allow this to go unmanaged in our bonsai, certain parts of the tree will outgrow the rest. At some point, there will be a shift in the tree's allocation of resources where it sheds weak branches that it finds are not contributing enough to the function of the tree.

When we are in the refinement stage of bonsai development, we will have a tree that has already been styled and will have a defined canopy and shape. Our goal here is to balance the energy allocation across all growing tips of the tree and to maintain the shape of the bonsai.

Techniques we can use to achieve this will again depend on the species. The major differences will lie between conifers and deciduous trees.

For many conifers, such as pines and spruce, we can pinch new growth in the spring once it has grown past the silhouette of the canopy. This achieves both of our goals - it maintains the shape of the bonsai, and by halting the growth of the strongest leaders we redirect energy allocation to weaker shoots.

Junipers should not be pinched in spring, as the removal of auxin from all growth tips can disrupt hormonal allocation to such a degree that you may see random branch dieback in the tree. Instead, junipers should be allowed to grow in spring and then pruned back after growth has hardened off, usually in early summer. The silhouette of the bonsai should again be used as a guide for where to prune, and weaker branches should be pruned less to balance strength.

Where to prune a juniper bonsai for refinement

Red line indicates where this juniper was pruned to stop elongation and promote growth of interior tips.

When it comes to deciduous trees, there are differences based on sub-species. Maple trees, such as Japanese maples (Acer palmatum) or Field maples (Acer campestre) can be pinched, removing the central leader of a bud as it starts to open. This increases the number of fine branches on the tree and helps to balance energy.

Other deciduous trees, particularly those with small leaves such as Cotoneaster, are not suitable for pinching and can simply be managed by pruning new shoots back to 2 buds once the leaves have hardened off.

There are many more nuances when working with particular species, but hopefully this overview gives you a solid starting point for working with your trees and understanding the general concepts.

Health and bonsai growth

It's important to understand that no matter where your bonsai is in its growth and development, there needs to be a focus on maintaining the health of the tree. There are a few things you can look out for in your bonsai to use as guides on the overall health of the plant.

Firstly, if your bonsai tree is healthy with will produce strong elongating growth in the spring. If your tree is producing very little growth, it's probably struggling with an issue somewhere.

Secondly, evaluate the color of your bonsai's foliage. All trees have differing foliage characteristics so discoloration can look different depending on the plant you are dealing with, but a general sign of poor health can be dulling or yellowing of the foliage.

Color alone isn't enough to diagnose what the actual problem is, but it should alert you to the possibility of a health issue and prompt you to investigate.

For example, a dulling or greying of the foliage could indicate a spider mite infestation, whereas yellowing could indicate a fungal issue, poor root health, overwatering or a micronutrient deficiency.

If you suspect a health issue in your bonsai, you need to address it before you can continue developing the tree. The techniques used in bonsai involve manipulating the energy systems of the tree. If you do this in a plant that is fighting a disease, it will make it harder for the tree to fight the infection.

You need to use some judgment here, but sometimes the safest decision is to sacrifice some refinement for a year to allow the tree to build strength and health again.

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