To plant a tree, we dig a hole, place the root ball of the seedling inside, and back-fill the gaps with soil. It is as simple as that. However, this is only the beginning of a massive, long-term commitment.

The art of planting trees is somewhat like raising a child. Imagine if a youngster was not taught to distinguish right from wrong when little, it is quite possible that they will end up following the wrong path. Similarly, if we fail to care for seedlings after the initial act of planting, it is likely they will develop poor structure. This can doom the tree to a life of poor health and unsociable habits, such as leaning, branch shedding, leaf loss and even outright collapse. Therefore, to avoid poor structural growth, we must nurture our seedlings by carrying out structural pruning.

What is ‘good structure’ when it comes to trees?

Friend A: “I vouch for the tree in photo 1. Its multiple main trunks should help stabilise the tree, as well as provide good shelter for wildlife.” Friend B: “I, on the other hand, think the tree in photo 2 has a better structure – one single main trunk, along with many smaller branches that radiate outwards all around.” What do you think? Who do you think is a better judge?

Speaking from the angle of an arborist, friend B has a better understanding than friend A. Simply, a well-structured tree should possess the following qualities:

1. A single main trunk
2. All branches should be no more than half the diameter of the main trunk and their length should not exceed the height of the main trunk
3. The placement of branches should vertically spiral up, spreading equally from the main trunk in all directions
4. In an urban setting, the size of the crown should occupy at least 60% of the tree’s overall height, although it can be substantially less in a forest
5. Both the main trunk and the branches should decrease in size towards the tip as they grow
6. No obvious structural issues, such as “included bark”, hollows, serious injury, fungal disease or rots, cracks, etc.

The tree in photo 1 has a lot of thick branches that have grown vertically from the same point on the main trunk, creating competition among them for limited space and sunlight. The contact point of the branches to the main trunk carries a huge burden and so will be relatively weak, making it prone to snapping in strong winds. This could lead to serious injuries to passers-by or untold damage to property. Therefore, we can conclude that the tree in photo 2 has a better structure than the tree in photo 1.

Structural pruning

Trees play a critical role in nature by turning carbon dioxide into oxygen, as well as by providing habitat and food for animals. There are many benefits to planting trees in urban areas, such as reducing the impact of global warming, creating a greener environment, providing shade and shelter, and so on. The benefit of planting trees is maximised by proper care, such as reducing the risk of disease or failure. Based on this, we carry out regular structural pruning to ensure our trees have a healthy, strong architecture, which in turn ensures a long and prosperous life.

The logic behind structural pruning is to control the growing structure of a tree by removing certain branches so that the remaining branches receive more water, nutrients, sunlight and space to grow. Put simply, there are five important steps to note in structural pruning:

1. Nurture just one main trunk
2. Wait until the tree has produced a branch at the level at which we want the crown to form
3. Once the tree has produced this lowest structural branch, all branches below it can be pruned and eventually removed; no one branch should be thicker than half the diameter of the main trunk
4. Make sure all other structural branches are straight, spirally arranged and balanced around the main trunk
5. Lastly, remove or prune any branches that are pointing upwards

Photos 3 and 4 demonstrate the “before” and “after” of a S. euonymifolium tree that was structurally pruned at KFBG in 2021. Over the past few years, our team of arborists has worked tirelessly to nurture it to become a structurally sound individual. To ensure it will grow into an ecologically valuable tree that flowers, fruits and shares its wealth of ecosystem services, we will continue caring for it and carrying out more structural pruning when necessary.

Why prune structurally?

Structural pruning can be carried out regardless of the age of the tree. Young seedlings have stems and branches with relatively small diameters, meaning minimum injury after pruning. This also means their structures can be more easily improved. Improving structure in young seedlings is much easier than in large, older trees.  

The diameter of a tree naturally increases as it grows, and so pruning will result in a progressively bigger wound. It therefore becomes more challenging to correct structure and inflict the least possible damage in older trees. Having said that, structural pruning could still be beneficial regardless. Early pruning can definitely attenuate structural problems by reducing competing trunks and other structural hazards, such as included bark; later pruning could help alleviate structural problems by reducing risks posed to pedestrians, vehicles or buildings.

Trees in urban areas are maintained through structural pruning with the aim of reducing risk, increasing aesthetic appeal and promoting a longer lifespan. On the other hand, KFBG’s tree management team is passionate about maximising the ecological integrity of our regenerating secondary forests. Trees with multiple trunks often possess a wider, fuller and denser crown that blocks sunlight from reaching the forest floor, thereby impeding the growth and germination of species that grow in the understorey. On top of that, trees with multiple trunks impose significant limitations on the overall structure and elevation of the forest through myriad constraining interactions with their neighbours and other forest residents.

Through pruning and improving the structure of all the trees in our regenerating secondary woodland, we are nurturing the return of a highly diverse ecosystem that will support many other lifeforms for centuries to come.