Tag Archives: Trunk

Bonsai Tree Growth Stages

Most bonsai trees progress through stages of development, each with a different objective. In general the progression is thicken trunk -> achieve branch & root structure -> achieve branch, foliage & root ramification -> reduce leaf size -> evolve as branches grow/fall off. The faster we can move through the first few development stages, the faster we will have beautiful, well-proportioned bonsai – harnessing the tree’s natural growth is a way to speed this up. We also want to avoid doing things which slow down a tree’s growth during these phases, as this will mean it takes longer to get the tree we want. Read about how trees grow before starting at #1 below. Also consider what do old trees look like?

1. Trunk

Some bonsai enthusiasts collect mature trees for bonsai specifically so they can start with a thick trunk, following a collection process which minimises damage to the tree. The alternative is growing your tree’s trunk. Once a tree has its roots and foliage reduced in size in a bonsai pot, it won’t generate the energy needed to make significant sapwood additions and its girth will only increase by small increments every year. So you really need to be happy with the trunk size first before you stick it in a tiny pot. But – how big should a bonsai tree’s trunk be?

2A. Branch Structure & Overall Shape

Arranging the branches is what gives you the canopy and overall foliage shape that you’re after and the first step in this process is growing (or developing) the branches you want in the positions they are needed. Growing a branch starts with a new bud, which, unless it’s a flower bud, becomes an extending shoot and eventually a new branch. So firstly you need to work out where new buds will grow on your tree and then deal with the extending shoots as needed to get the required internode length.

You may need to remove some buds and shoots if they don’t help achieve the shape you are looking for – this should be done as soon as possible to avoid wasting the tree’s finite energy reserves. You have a trade-off to make here because leaving more foliage on the tree will provide more energy overall which contributes to its health and ability to recover from interference. However, growing areas of the tree which won’t be part of the future design is a waste of energy. You don’t want to remove so much of the tree’s foliage that it struggles to stay alive or develop the areas that you do want to grow out.

When you are creating your branch structure, often you will need to reposition branches – this is done with a wide range of different tools and techniques. A more advanced technique for adding new branch structure is grafting.

Sometimes the trunk itself or larger branches need a rework, to make them more interesting or to make them look more like old trees – for example adding deadwood or hollowing out the trunk. Usually this is achieved through carving.

2B. Creating a Strong Root System

The trunk thickening and branch structure phases both work best when the tree has lots of energy and so letting it grow in the ground or in a decent sized pot during these phases will get you there quickest. This also allows the roots to keep growing, but you want to understand about the role of roots, and root structure & architecture even if you still have your bonsai in a training pot. Particularly in this case, knowing about how to foster the the rhizosphere will help your tree stay vigorous. To maximise the roots’ exposure to nutrients and water you want to encourage Ramification of Roots (lateral root development).

Eventually it’s time to move the tree into a bonsai pot. This requires cutting back the roots, but as long as the roots are balanced with the foliage in terms of biomass, the tree should be OK. Root growth is usually prioritised outside of times of stem/foliage growth, and above 6-9 degrees C. So repotting might be best conducted at times that meet this criteria. Your growing substrate/medium is an important consideration.

3. Ramifying Branches & Foliage

Ramification is when branches subdivide and branch, giving the impression of age and a full canopy – and a well-ramified tree is a bonsai enthusiast’s goal. There are some techniques for increasing the ramification of branches and foliage. But not as many as there are for root ramification.

This stage also involves ongoing branch selection and reshaping (see 2A above). Another consideration is whether to keep or remove flower buds.

4. Reducing Leaf Size

An end stage in the journey to bonsai perfection is leaf size reduction. In nature, leaf sizes reduce relative to the biomass of the tree as it ages but since bonsai are small this effect doesn’t translate since the biomass never gets large enough. The tried and tested method for reducing deciduous tree leaf size is actually to practice one of the various methods of defoliation. A couple of others are covered here in reducing leaf size.

When to conduct these various activities depends on when the tree can best recover from them – which is a function of the Tree Phenology (or Seasonal Cycles).

5. Evolving Branches

Trees are not static organisms – they obviously continue to grow which is what we harness in the above steps. Part of this is that eventually branches may become too large for the design, or they may fall off (Peter Warren notes that Mulberry are known for this). As bonsai artists we want to have this in mind so that branches are being developed which can take their place in the future. This is an ongoing version of step 2A.

Thickening the Trunk

The first quality of a good bonsai is a thick trunk with movement and mature bark. So what actually contributes to the growth of a tree trunk?

Two processes are involved. The first is the creation of new sapwood. Sapwood is the living wood towards the outside of a trunk which conducts water (Ennos, 2016). Sapwood formed in spring is called ‘earlywood’ and is optimized for water & nutrient transport to help the tree with its growth spurt. Latewood is designed for structural support and carbon storage.

Water and nutrients are conducted from the roots through xylem vessels. The mechanism by which they work is explained in xylem but for the purpose of this section it’s important to understand that the reason why trees add new xylem vessels is because as it adds biomass – new branches and leaves – more water is required. So – the more biomass is added in a given growing period – the more water is needed – the more xylem vessels are added to the trunk. Xylem vessels also become non-functional for reasons explained in embolisms, so trees need to replace them as well as adding to them due to new growth. 

New sapwood (with xylem vessels) is added around the previous sapwood, encircling the tree. How much of the girth of a tree increases each year is determined by the tree’s food supply (Trouet, 2020); this is a combination of the amount of rainfall and the energy from the sun during that year.

This studyref found that “low precipitation at the start or during the growing season was found to be a significant factor limiting radial growth” for a range of urban trees in the UK. According to Trouet, “alternating wet & dry years create wide and narrow rings respectively.” So low water levels lead to small rings and high water levels lead to large ones. The earlywood creates a larger ring than the latewood, since the xylem vessels are larger in earlywood (for water transport) and smaller in latewood (for structural strength) (Ennos, 2016).

What this means for bonsai is that watering your tree well is important while developing its trunk, whilst ensuring you have a well-drained growing medium to avoid creating anoxic conditions (lacking oxygen). If your medium is well-drained and you water thoroughly throughout the tree’s growing season (but particularly during earlywood development), you’ll boost your tree’s girth by creating wide ‘good times’ sapwood rings.

The other factor mentioned is energy from the sun. Energy from the sun is used by the tree in photosynthesis, which converts energy into a form that the tree can use to respire and grow. If there is more sun, more energy is available and the tree is able to create more xylem, buds, leaves and biomass. This isn’t a straightforward linear relationship however, as photosynthesis reaches a saturation point based on a number of limiting factors (more in the post about photosynthesis).

The key point here is that reducing the ability of the tree to capture and convert energy will affect its growth. If you reduce the foliage on your tree or cut it back in spring, you reduce its biomass, it can’t generate as much energy, and doesn’t need as much water, so doesn’t add as many xylem vessels as it would have nor as wide a ring of sapwood. This reduces the trunk thickening you can achieve in a given time period. 

It’s worth noting that the roots of a tree need to be capable of delivering the amount of water that its foliage and branches require. Optimising trunk thickness requires a dense canopy of leaves and branches, matched by roots capable of delivering the amount of water that they need. This is why many bonsai enthusiasts will start a tree off in the ground or in a large pot, allowing growth to drive the trunk size until it’s at the level required.

Attempting to restrict the roots and size of the tree too early (e.g. by putting it in a bonsai pot) will restrict trunk growth by reducing the water available to the tree and reducing the energy it can create by reducing its foliage.

Like people, trees are genetically programmed to have different maximum heights and lifespans. Some trees are slow-growing (such as Yew) and some are fast (such as Eucalyptus) so to an extent the amount of trunk thickening that is possible also depends on the species of tree.

Trees grow most vigorously when they are free from environmental stressors – such as drought, extreme cold, loss of leaves due to high winds, attack by insects or animals.  A stressed tree will grow a narrow ring. BUT stress in the form of wind can foster positive qualities in a trunk. Ennos (2016) says that trees exposed to high winds without a prevailing wind direction grow shorter, with thicker trunks & roots, and adjust their wood cells to spiral around the tree creating a twisting effect. It’s not just the trunk that is affected – apparently this results in smaller leaves and shoots as well. Get your bonsai a wind tunnel!

Another way to thicken a trunk is to grow a ground-level branch, as layers of xylem will be added around this branch as well as the truck, or to have a multi-stem tree, which operates on the same principle. You want to avoid having one too much above the ground though, as it might cause the dreaded reverse taper.

I mentioned two processes involved in secondary thickening – the second process is the effect of an increasing bark layer. In most cases this will be dwarfed by sapwood increases but nevertheless biomass is added as bark via the cork cambium, another secondary meristem on trees. Some trees which retain multiple periderms (layers of cork with their meristems) can develop very thick bark which does contribute to the overall trunk girth as well.

How big should a bonsai trunk be?

It’s a how-long-is-a-piece-of-string question because the trunk on a bonsai doesn’t exist in isolation, it exists relative to the foliage, nebari and pot. Because trees undergo secondary thickening however, their trunks expand with every year. So, older trees have thicker trunks.

For another post I found this data below. It shows mass rather than volume, but you can see that as trees get older and bigger, their mass skews to the trunk, which ends up being 80%+ of the total mass of the tree. Whereas at the beginning of the tree’s life, on the left hand side of the chart, the leaf mass exceeds the stem mass.


So in general if we want to emulate older trees, our bonsai needs to be weighted towards a fat trunk (and main branches). Note also that the root mass doesn’t go below 20% – the main contribution to mass in a root are the big structural roots which are largest within a metre or so of the trunk. So this gives an indication of how big a nebari should be.

But as mentioned above the trunk exists relative to the canopy so what do we know about the ratio between the two? One measure which is used in forestry is the live crown ratio which is used as an indicator of tree health. The live crown ratio is the vertical length of the foliage as a percentage of the total tree height.ref Some studies have measured crown ratios for different species (usually in managed forests):

  • A stand of coast redwoods: between 30%-50%ref
  • Douglas Fir: in the 80% range for 20y old trees, down to the 40% for 40y old trees and up to 60% range for 450y old trees
  • Turkey oak: between 20%-50%

Also interesting is the crown radius to trunk diameter. A study measured this for 22 different species including both angiosperms and gymnosperms and came up with equations that represent the ‘allometric types’ for each species – that is an equation that describes how a tree’s dimensions change over time.ref For example for common beech (Fagus sylvatica) they found (see table 5) that the following equation could be used to calculate the crown radius given a particular trunk diameter:

ln(crown radius) = 0.0111 + 0.4710 x ln(trunk diameter) ; (note crown radius is in m and trunk diameter in cm)

if we have, for example, a 1m wide trunk, you could calculate the crown radius as follows: 0.0111 + 0.4710 x ln(100) = 2.180 so crown radius = e2.180 = 8.85m – this actually then gives a crown diameter of 2 x 8.85 = 17.8m. So an old beech which has achieved a 1m wide trunk could have a nearly 18m crown diameter – which means the trunk is about 5.5% of the width of the crown.

Because I love a bit of excel, I took the data for the rest of the species to work out the trunk/crown diameter ratio for each of them based on a 1m trunk – and here is the answer:

So for most species a 1m trunk will be between 4% and 10% of the width of the canopy. I couldn’t resist looking up Auracaria cunninghamii to see why it was different – it looks like the canopy habit is quite narrow which increases the trunk/canopy ratio#.

If you have Douglas fir, this study found that “the vertical distribution of branch volume shifted toward the upper-crown with increasing tree age”ref The mechanisms at work include self-pruning, branches dying and falling off and then adventitious branches growing in the spaces. As they included a picture you can see it makes quite an obvious difference to the look of the tree.


That’s just one species though – the shape of old trees is going to be to a certain extent genetically determined so different species will have a different mature look in terms of their shape and branch distribution.

Conventional bonsai wisdom says a tree needs to have good taper in order to look old. This means it is thicker at the bottom than at the top. But tree-ring researcher and dendrologist Valerie Trouet in her book Tree Story says otherwise. She says “once height growth has stopped in an older tree, then the upper part of the stem will start to catch up, it’s girth increasing year after year, and the stem will gradually take on a more columnar, rather than tapered, look….the tree’s limbs also continue to thicken; branches and roots of old trees often are quite sizable.”

What we are trying to achieve with bonsai is small trees which look like mature, large ones in nature. So the size of the trunk, whether it has taper or not, needs to be in proportion with the canopy and the roots, and the branches should start anywhere from the 20% to the 50/60% of the total tree height mark and be in proportion to the trunk as in the table above.

There are more attributes which make a tree look old, to learn more check out this post: Old Trees.